CVE-2025-21658 Information

Description

In the Linux kernel the following vulnerability has been resolved:

btrfs: avoid NULL pointer dereference if no valid extent tree

[BUG] Syzbot reported a crash with the following call trace:

BTRFS info (device loop0): scrub: started on devid 1 BUG: kernel NULL pointer dereference address: 0000000000000208 PF: supervisor read access in kernel mode PF: error_code(0x0000) - not-present page PGD 106e70067 P4D 106e70067 PUD 107143067 PMD 0 Oops: Oops: 0000 [1] PREEMPT SMP NOPTI CPU: 1 UID: 0 PID: 689 Comm: repro Kdump: loaded Tainted: G O 6.13.0-rc4-custom+ 206 Tainted: [O]=OOT_MODULE Hardware name: QEMU Standard PC (Q35 + ICH9 2009) BIOS unknown 02/02/2022 RIP: 0010:find_first_extent_item+0x26/0x1f0 [btrfs] Call Trace: scrub_find_fill_first_stripe+0x13d/0x3b0 [btrfs] scrub_simple_mirror+0x175/0x260 [btrfs] scrub_stripe+0x5d4/0x6c0 [btrfs] scrub_chunk+0xbb/0x170 [btrfs] scrub_enumerate_chunks+0x2f4/0x5f0 [btrfs] btrfs_scrub_dev+0x240/0x600 [btrfs] btrfs_ioctl+0x1dc8/0x2fa0 [btrfs] ? do_sys_openat2+0xa5/0xf0 __x64_sys_ioctl+0x97/0xc0 do_syscall_64+0x4f/0x120 entry_SYSCALL_64_after_hwframe+0x76/0x7e

[CAUSE] The reproducer is using a corrupted image where extent tree root is corrupted thus forcing to use escue=allro\ mount option to mount the image.

Then it triggered a scrub but since scrub relies on extent tree to find where the data/metadata extents are scrub_find_fill_first_stripe() relies on an non-empty extent root.

But unfortunately scrub_find_fill_first_stripe() doesn’t really expect an NULL pointer for extent root it use extent_root to grab fs_info and triggered a NULL pointer dereference.

[FIX] Add an extra check for a valid extent root at the beginning of scrub_find_fill_first_stripe().

The new error path is introduced by 42437a6386ff (trfs: introduce mount option rescue=ignorebadroots) but that’s pretty old and later commit b979547513ff (trfs: scrub: introduce helper to find and fill sector info for a scrub_stripe) changed how we do scrub.

So for kernels older than 6.6 the fix will need manual backport.

Reference

https://git.kernel.org/stable/c/24b85a8b0310e0144da9ab30be42e87e6476638a https://git.kernel.org/stable/c/6aecd91a5c5b68939cf4169e32bc49f3cd2dd329 https://git.kernel.org/stable/c/aee5f69f3e6cd82bfefaca1b70b40b6cd8f3f784