CVE-2024-35798

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 ]