CVE-2024-35784 btrfs: fix deadlock with fiemap and extent locking
A local attacker can trigger a deadlock in the Linux kernel's btrfs filesystem via fiemap operations, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-35784?
CVE-2024-35784 is a medium-severity vulnerability in Linux kernel, published on 17 May 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix deadlock with fiemap and extent locking
While working on the patchset to remove extent locking I got a lockdep splat with fiemap and pagefaulting with my new extent lock replacement lock.
This deadlock exists with our normal code, we just don't have lockdep annotations with the extent locking so we've never noticed it.
Since we're copying the fiemap extent to user space on every iteration we have the chance of pagefaulting. Because we hold the extent lock for the entire range we could mkwrite into a range in the file that we have mmap'ed. This would deadlock with the following stack trace
[<0>] lock_extent+0x28d/0x2f0 [<0>] btrfs_page_mkwrite+0x273/0x8a0 [<0>] do_page_mkwrite+0x50/0xb0 [<0>] do_fault+0xc1/0x7b0 [<0>] __handle_mm_fault+0x2fa/0x460 [<0>] handle_mm_fault+0xa4/0x330 [<0>] do_user_addr_fault+0x1f4/0x800 [<0>] exc_page_fault+0x7c/0x1e0 [<0>] asm_exc_page_fault+0x26/0x30 [<0>] rep_movs_alternative+0x33/0x70 [<0>] _copy_to_user+0x49/0x70 [<0>] fiemap_fill_next_extent+0xc8/0x120 [<0>] emit_fiemap_extent+0x4d/0xa0 [<0>] extent_fiemap+0x7f8/0xad0 [<0>] btrfs_fiemap+0x49/0x80 [<0>] __x64_sys_ioctl+0x3e1/0xb50 [<0>] do_syscall_64+0x94/0x1a0 [<0>] entry_SYSCALL_64_after_hwframe+0x6e/0x76
I wrote an fstest to reproduce this deadlock without my replacement lock and verified that the deadlock exists with our existing locking.
To fix this simply don't take the extent lock for the entire duration of the fiemap. This is safe in general because we keep track of where we are when we're searching the tree, so if an ordered extent updates in the middle of our fiemap call we'll still emit the correct extents because we know what offset we were on before.
The only place we maintain the lock is searching delalloc. Since the delalloc stuff can change during writeback we want to lock the extent range so we have a consistent view of delalloc at the time we're checking to see if we need to set the delalloc flag.
With this patch applied we no longer deadlock with my testcase.
Weakness: CWE-667
Which versions are affected by CVE-2024-35784?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-35784?
Apply the vendor's patch.
Is CVE-2024-35784 being exploited?
There is no known exploitation of CVE-2024-35784 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-55410, BELL-CVE-2024-35784, DEBIAN-CVE-2024-35784, GHSA-hg9c-4q92-whw7, ROOT-OS-DEBIAN-11-CVE-2024-35784, ROOT-OS-DEBIAN-12-CVE-2024-35784 and 2 more.