fs/ntfs3: Fix null-ptr-deref on inode->i_op in ntfs_lookup()

In the Linux kernel, the following vulnerability has been resolved:

fs/ntfs3: Fix null-ptr-deref on inode->i_op in ntfs_lookup()

Syzbot reported a null-ptr-deref bug:

ntfs3: loop0: Different NTFS' sector size (1024) and media sector size (512) ntfs3: loop0: Mark volume as dirty due to NTFS errors general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f] RIP: 0010:d_flags_for_inode fs/dcache.c:1980 [inline] RIP: 0010:__d_add+0x5ce/0x800 fs/dcache.c:2796 Call Trace:

d_splice_alias+0x122/0x3b0 fs/dcache.c:3191 lookup_open fs/namei.c:3391 [inline] open_last_lookups fs/namei.c:3481 [inline] path_openat+0x10e6/0x2df0 fs/namei.c:3688 do_filp_open+0x264/0x4f0 fs/namei.c:3718 do_sys_openat2+0x124/0x4e0 fs/open.c:1310 do_sys_open fs/open.c:1326 [inline] __do_sys_open fs/open.c:1334 [inline] __se_sys_open fs/open.c:1330 [inline] __x64_sys_open+0x221/0x270 fs/open.c:1330 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x63/0xcd

If the MFT record of ntfs inode is not a base record, inode->i_op can be NULL. And a null-ptr-deref may happen:

ntfs_lookup() dir_search_u() # inode->i_op is set to NULL d_splice_alias() __d_add() d_flags_for_inode() # inode->i_op->get_link null-ptr-deref

Fix this by adding a Check on inode->i_op before calling the d_splice_alias() function.