CVE-2026-46113
Description
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
The shadow MMU computes GFNs for direct shadow pages using sp->gfn plus the SPTE index. This assumption breaks for shadow paging if the guest page tables are modified between VM entries (similar to commit aad885e77496, "KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE", 2026-03-27). The flow is as follows:
- a PDE is installed for a 2MB mapping, and a page in that area is accessed. KVM creates a kvm_mmu_page consisting of 512 4KB pages; the kvm_mmu_page is marked by FNAME(fetch) as direct-mapped because the guest's mapping is a huge page (and thus contiguous).
- the PDE mapping is changed from outside the guest.
- the guest accesses another page in the same 2MB area. KVM installs a new leaf SPTE and rmap entry; the SPTE uses the "correct" GFN (i.e. based on the new mapping, as changed in the previous step) but that GFN is outside of the [sp->gfn, sp->gfn + 511] range; therefore the rmap entry cannot be found and removed when the kvm_mmu_page is zapped.
- the memslot that covers the first 2MB mapping is deleted, and the kvm_mmu_page for the now-invalid GPA is zapped. However, rmap_remove() only looks at the [sp->gfn, sp->gfn + 511] range established in step 1, and fails to find the rmap entry that was recorded by step 3.
- any operation that causes an rmap walk for the same page accessed by step 3 then walks a stale rmap and dereferences a freed kvm_mmu_page. This includes dirty logging or MMU notifier invalidations (e.g., from MADV_DONTNEED).
The underlying issue is that KVM's walking of shadow PTEs assumes that if a SPTE is present when KVM wants to install a non-leaf SPTE, then the existing kvm_mmu_page must be for the correct gfn. Because the only way for the gfn to be wrong is if KVM messed up and failed to zap a SPTE... which shouldn't happen, but *actually* only happens in response to a guest write.
That bug dates back literally forever, as even the first version of KVM assumes that the GFN matches and walks into the "wrong" shadow page. However, that was only an imprecision until 2032a93d66fa ("KVM: MMU: Don't allocate gfns page for direct mmu pages") came along.
Fix it by checking for a target gfn mismatch and zapping the existing SPTE. That way the old SP and rmap entries are gone, KVM installs the rmap in the right location, and everyone is happy.
AI Insight
LLM-synthesized narrative grounded in this CVE's description and references.
Use-after-free in KVM x86 shadow paging allows host kernel memory corruption via stale rmap entries.
Vulnerability
A use-after-free vulnerability exists in the Linux kernel's KVM x86 shadow paging implementation. The shadow MMU computes the guest frame number (GFN) for direct shadow pages using sp->gfn plus the SPTE index. This assumption breaks when the guest modifies its page tables between VM entries. Specifically, if a guest first creates a 2MB huge page mapping, triggering KVM to create a direct-mapped shadow page covering 512 4KB pages, and then changes the PDE mapping, a subsequent guest access to another page in the same 2MB area can cause KVM to install a new leaf SPTE with a GFN outside the expected [sp->gfn, sp->gfn + 511] range. Consequently, the rmap entry for that SPTE is not found when the shadow page is later zapped, leading to a use-after-free when an rmap walk occurs (e.g., during dirty logging or MMU notifier invalidations like MADV_DONTNEED). This affects systems using shadow paging (e.g., when nested virtualization or certain features disable hardware EPT). The bug is similar to the issue fixed in commit aad885e77496, which addressed a related problem in the same area [1].
Exploitation
An attacker must be able to modify guest page tables (e.g., a guest user process or a guest kernel component) and induce specific sequences: first create a 2MB huge page mapping, then change the PDE to a different mapping, and access a new page within the same 2MB area. The attacker also needs to trigger a memslot deletion (e.g., hot-unplug memory or similar operation) to zap the shadow page, followed by an rmap walk (dirty logging, MMU notifier callbacks, etc.) to dereference the freed kvm_mmu_page. No special host privileges are required beyond running a VM with shadow paging enabled.
Impact
Successful exploitation results in a use-after-free of a kvm_mmu_page structure in the host kernel. An attacker can leverage this to corrupt host memory, potentially leading to privilege escalation (from guest to host), denial of service (host crash), or information disclosure. The impact is high as it compromises the isolation between guest and host.
Mitigation
The vulnerability is fixed in Linux kernel commit 738ec97b1855 ("KVM: x86: Fix shadow paging use-after-free due to unexpected GFN") [1]. This commit is included in stable kernel updates released on or after 2026-05-28. Users should update to the latest kernel version from their distribution. If immediate patching is not possible, disabling shadow paging (e.g., by enabling hardware EPT support in the CPU or using nested virtualization features that force EPT) may mitigate the issue. No workaround is available for systems that require shadow paging.
AI Insight generated on May 28, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.
Affected products
1Patches
100cb2af2ea66aKVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 24fbc9ea502a30..892246204435c5 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; struct kmem_cache *mmu_page_header_cache; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
488e386484ecKVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 0dc804149b0f38..774bc26b8235e3 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ struct kmem_cache *mmu_page_header_cache; static struct percpu_counter kvm_total_used_mmu_pages; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1194,19 +1196,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2350,7 +2339,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2428,13 +2418,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
738ec97b1855KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index dad7abb1112b7d..0bd0cb8992c9fd 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; struct kmem_cache *mmu_page_header_cache; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
14d1e55dfd2cKVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index dd06453d5b72c1..729240bc00a269 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; struct kmem_cache *mmu_page_header_cache; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
06c19c967b84KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 2c11819bd216c7..d288c60ae200ba 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ struct kmem_cache *mmu_page_header_cache; static struct percpu_counter kvm_total_used_mmu_pages; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1187,19 +1189,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2342,7 +2331,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2420,13 +2410,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
0cb2af2ea66aKVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 24fbc9ea502a30..892246204435c5 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; struct kmem_cache *mmu_page_header_cache; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
06c19c967b84KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 2c11819bd216c7..d288c60ae200ba 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ struct kmem_cache *mmu_page_header_cache; static struct percpu_counter kvm_total_used_mmu_pages; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1187,19 +1189,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2342,7 +2331,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2420,13 +2410,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
14d1e55dfd2cKVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index dd06453d5b72c1..729240bc00a269 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; struct kmem_cache *mmu_page_header_cache; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
738ec97b1855KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index dad7abb1112b7d..0bd0cb8992c9fd 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; struct kmem_cache *mmu_page_header_cache; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
488e386484ecKVM: x86: Fix shadow paging use-after-free due to unexpected GFN
1 file changed · +14 −22
arch/x86/kvm/mmu/mmu.c+14 −22 modifieddiff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 0dc804149b0f38..774bc26b8235e3 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -182,6 +182,8 @@ struct kmem_cache *mmu_page_header_cache; static struct percpu_counter kvm_total_used_mmu_pages; static void mmu_spte_set(u64 *sptep, u64 spte); +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, + u64 *spte, struct list_head *invalid_list); struct kvm_mmu_role_regs { const unsigned long cr0; @@ -1194,19 +1196,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) rmap_remove(kvm, sptep); } -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) -{ - struct kvm_mmu_page *sp; - - sp = sptep_to_sp(sptep); - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); - - drop_spte(kvm, sptep); - - if (flush) - kvm_flush_remote_tlbs_sptep(kvm, sptep); -} - /* * Write-protect on the specified @sptep, @pt_protect indicates whether * spte write-protection is caused by protecting shadow page table. @@ -2350,7 +2339,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, { union kvm_mmu_page_role role; - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) return ERR_PTR(-EEXIST); role = kvm_mmu_child_role(sptep, direct, access); @@ -2428,13 +2418,16 @@ static void __link_shadow_page(struct kvm *kvm, BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); - /* - * If an SPTE is present already, it must be a leaf and therefore - * a large one. Drop it, and flush the TLB if needed, before - * installing sp. - */ - if (is_shadow_present_pte(*sptep)) - drop_large_spte(kvm, sptep, flush); + if (is_shadow_present_pte(*sptep)) { + struct kvm_mmu_page *parent_sp; + LIST_HEAD(invalid_list); + + parent_sp = sptep_to_sp(sptep); + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); + + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); + } spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); -- cgit 1.3-korg
Vulnerability mechanics
Root cause
"Missing GFN validation when reusing an existing non-leaf shadow page SPTE, causing rmap entries to be installed outside the expected GFN range and later orphaned, leading to use-after-free on rmap walks."
Attack vector
An attacker who can modify guest page tables between VM entries can trigger a use-after-free in KVM's shadow paging. The flow begins when a PDE is installed for a 2MB mapping and a page in that area is accessed, causing KVM to create a direct-mapped `kvm_mmu_page` covering 512 4KB pages. The attacker then changes the PDE mapping from outside the guest. When the guest accesses another page in the same 2MB area, KVM installs a new leaf SPTE with a GFN based on the new mapping, but that GFN falls outside the original `[sp->gfn, sp->gfn + 511]` range. Consequently, when the memslot is deleted and the `kvm_mmu_page` is zapped, `rmap_remove()` fails to find the rmap entry recorded for the new GFN. Any subsequent rmap walk (e.g., dirty logging, MMU notifier invalidations like `MADV_DONTNEED`) then traverses a stale rmap and dereferences a freed `kvm_mmu_page` [patch_id=2898597].
Affected code
The vulnerability is in `arch/x86/kvm/mmu/mmu.c`, specifically in the functions `kvm_mmu_get_child_sp()` and `__link_shadow_page()`. The shadow MMU computes GFNs for direct shadow pages using `sp->gfn` plus the SPTE index, which breaks when guest page tables are modified between VM entries [patch_id=2898597].
What the fix does
The patch makes two changes in `arch/x86/kvm/mmu/mmu.c`. First, in `kvm_mmu_get_child_sp()`, it adds a GFN equality check (`spte_to_child_sp(*sptep)->gfn == gfn`) so that an existing non-leaf SPTE is only reused if its GFN matches the target GFN; on mismatch the function no longer returns `-EEXIST`, allowing the old SPTE to be replaced [patch_id=2898597]. Second, in `__link_shadow_page()`, the old `drop_large_spte()` helper (which only handled large leaf SPTEs) is replaced with a call to `mmu_page_zap_pte()` followed by `kvm_mmu_remote_flush_or_zap()`. This properly zaps any present SPTE — including non-leaf SPTEs with a mismatched GFN — and removes the associated rmap entries, preventing the stale rmap that leads to use-after-free [patch_id=2898597].
Preconditions
- configThe host must be running a KVM/x86 kernel with shadow paging enabled (i.e., nested virtualization or EPT disabled).
- authThe attacker must be able to modify guest page table entries (PDEs) between VM entries, e.g., as a guest administrator or through a nested hypervisor.
- inputThe attacker must trigger a sequence of guest memory accesses that cause KVM to create a direct-mapped shadow page for a 2MB huge page, then change the PDE mapping, then access a different page in the same 2MB region.
Generated on May 28, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.
References
5- git.kernel.org/stable/c/06c19c967b845b63172601fe459667d973b7e6b7nvd
- git.kernel.org/stable/c/0cb2af2ea66ad8ff195c156ea690f11216285bdfnvd
- git.kernel.org/stable/c/14d1e55dfd2cf4711bff164a6aaaddb783552134nvd
- git.kernel.org/stable/c/488e386484ec8c0e558be6e156edf34ed9f4d5c8nvd
- git.kernel.org/stable/c/738ec97b1855df6c08fe2369f798fa0b972e556bnvd
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