CVE-2022-50089

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

btrfs: ensure pages are unlocked on cow_file_range() failure

There is a hung_task report on zoned btrfs like below.

https://github.com/naota/linux/issues/59

[726.328648] INFO: task rocksdb:high0:11085 blocked for more than 241 seconds. [726.329839] Not tainted 5.16.0-rc1+ #1 [726.330484] echo 0 > /proc/sys/kernel/hung_task_timeout_secs disables this message. [726.331603] task:rocksdb:high0 state:D stack: 0 pid:11085 ppid: 11082 flags:0x00000000 [726.331608] Call Trace: [726.331611] [726.331614] __schedule+0x2e5/0x9d0 [726.331622] schedule+0x58/0xd0 [726.331626] io_schedule+0x3f/0x70 [726.331629] __folio_lock+0x125/0x200 [726.331634] ? find_get_entries+0x1bc/0x240 [726.331638] ? filemap_invalidate_unlock_two+0x40/0x40 [726.331642] truncate_inode_pages_range+0x5b2/0x770 [726.331649] truncate_inode_pages_final+0x44/0x50 [726.331653] btrfs_evict_inode+0x67/0x480 [726.331658] evict+0xd0/0x180 [726.331661] iput+0x13f/0x200 [726.331664] do_unlinkat+0x1c0/0x2b0 [726.331668] __x64_sys_unlink+0x23/0x30 [726.331670] do_syscall_64+0x3b/0xc0 [726.331674] entry_SYSCALL_64_after_hwframe+0x44/0xae [726.331677] RIP: 0033:0x7fb9490a171b [726.331681] RSP: 002b:00007fb943ffac68 EFLAGS: 00000246 ORIG_RAX: 0000000000000057 [726.331684] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb9490a171b [726.331686] RDX: 00007fb943ffb040 RSI: 000055a6bbe6ec20 RDI: 00007fb94400d300 [726.331687] RBP: 00007fb943ffad00 R08: 0000000000000000 R09: 0000000000000000 [726.331688] R10: 0000000000000031 R11: 0000000000000246 R12: 00007fb943ffb000 [726.331690] R13: 00007fb943ffb040 R14: 0000000000000000 R15: 00007fb943ffd260 [726.331693]

While we debug the issue, we found running fstests generic/551 on 5GB non-zoned null_blk device in the emulated zoned mode also had a similar hung issue.

Also, we can reproduce the same symptom with an error injected cow_file_range() setup.

The hang occurs when cow_file_range() fails in the middle of allocation. cow_file_range() called from do_allocation_zoned() can split the give region ([start, end]) for allocation depending on current block group usages. When btrfs can allocate bytes for one part of the split regions but fails for the other region (e.g. because of -ENOSPC), we return the error leaving the pages in the succeeded regions locked. Technically, this occurs only when @unlock == 0. Otherwise, we unlock the pages in an allocated region after creating an ordered extent.

Considering the callers of cow_file_range(unlock=0) wont write out the pages, we can unlock the pages on error exit from cow_file_range(). So, we can ensure all the pages except @locked_page are unlocked on error case.

In summary, cow_file_range now behaves like this:

• page_started == 1 (return value)
  • All the pages are unlocked. IO is started.
• unlock == 1
  • All the pages except @locked_page are unlocked in any case
• unlock == 0
  • On success, all the pages are locked for writing out them
  • On failure, all the pages except @locked_page are unlocked

Affected Software

References

• https://git.kernel.org/stable/c/9535ec371d741fa037e37eddc0a5b25ba82d0027
• https://git.kernel.org/stable/c/9ce7466f372d83054c7494f6b3e4b9abaf3f0355
• https://git.kernel.org/stable/c/b367f125c80fa838eae49e3b138dc67dfc9f46ef
• https://git.kernel.org/stable/c/e160aa87c87a9c4e0c8d1430235f715a3a91e2cd