btrfs: fix race in read_extent_buffer_pages()

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

btrfs: fix race in read_extent_buffer_pages()

There are reports from tree-checker that detects corrupted nodes, without any obvious pattern so possibly an overwrite in memory. After some debugging it turns out there's a race when reading an extent buffer the uptodate status can be missed.

To prevent concurrent reads for the same extent buffer, read_extent_buffer_pages() performs these checks:

/* (1) */ if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags)) return 0;

/* (2) */ if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags)) goto done;

At this point, it seems safe to start the actual read operation. Once that completes, end_bbio_meta_read() does

/* (3) */ set_extent_buffer_uptodate(eb);

/* (4) */ clear_bit(EXTENT_BUFFER_READING, &eb->bflags);

Normally, this is enough to ensure only one read happens, and all other callers wait for it to finish before returning. Unfortunately, there is a racey interleaving:

Thread A | Thread B | Thread C ---------+----------+--------- (1) | | | (1) | (2) | | (3) | | (4) | | | (2) | | | (1)

When this happens, thread B kicks of an unnecessary read. Worse, thread C will see UPTODATE set and return immediately, while the read from thread B is still in progress. This race could result in tree-checker errors like this as the extent buffer is concurrently modified:

BTRFS critical (device dm-0): corrupted node, root=256 block=8550954455682405139 owner mismatch, have 11858205567642294356 expect [256, 18446744073709551360]

Fix it by testing UPTODATE again after setting the READING bit, and if it's been set, skip the unnecessary read.

[ minor update of changelog ]