CVE-2024-35798 Information

Description

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 ]

Reference

https://git.kernel.org/stable/c/0427c8ef8bbb7f304de42ef51d69c960e165e052 https://git.kernel.org/stable/c/3a25878a3378adce5d846300c9570f15aa7f7a80 https://git.kernel.org/stable/c/2885d54af2c2e1d910e20d5c8045bae40e02fbc1 https://git.kernel.org/stable/c/ef1e68236b9153c27cb7cf29ead0c532870d4215