CVE-2026-64593

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

btrfs: do not trim a device which is not writeable

[BUG] There is a bug report that btrfs/242 can randomly fail with the following NULL pointer dereference:

run fstests btrfs/242 at 2026-06-01 10:25:08 BTRFS: device fsid d4d7f234-487c-4787-88e4-47a8b68c9874 devid 1 transid 9 /dev/sdc (8:32) scanned by mount (122609) BTRFS info (device sdc): first mount of filesystem d4d7f234-487c-4787-88e4-47a8b68c9874 BTRFS info (device sdc): using crc32c checksum algorithm BTRFS warning (device sdc): devid 2 uuid fbe72d72-3272-482d-80fb-ab88ed398192 is missing BTRFS warning (device sdc): devid 2 uuid fbe72d72-3272-482d-80fb-ab88ed398192 is missing BTRFS info (device sdc): allowing degraded mounts BTRFS info (device sdc): turning on async discard BTRFS info (device sdc): enabling free space tree Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018 user pgtable: 4k pages, 48-bit VAs, pgdp=000000013fd6b000 CPU: 4 UID: 0 PID: 122625 Comm: fstrim Not tainted 7.0.10-2-default #1 PREEMPT(full) openSUSE Tumbleweed e9a5f6b24978fba3bf015a992f865837fdfff3dd Hardware name: QEMU KVM Virtual Machine, BIOS edk2-20250812-19.fc42 08/12/2025 pstate: 01400005 (nzcv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=–) pc : btrfs_trim_fs+0x34c/0xa00 [btrfs] lr : btrfs_trim_fs+0x1f0/0xa00 [btrfs] Call trace: btrfs_trim_fs+0x34c/0xa00 [btrfs f02c1d570ceea621c69d302ba75dd61868083840] (P) btrfs_ioctl_fitrim+0xe8/0x178 [btrfs f02c1d570ceea621c69d302ba75dd61868083840] btrfs_ioctl+0xdd4/0x2bd8 [btrfs f02c1d570ceea621c69d302ba75dd61868083840] __arm64_sys_ioctl+0xac/0x108 invoke_syscall.constprop.0+0x5c/0xd0 el0_svc_common.constprop.0+0x40/0xf0 do_el0_svc+0x24/0x40 el0_svc+0x40/0x1d0 el0t_64_sync_handler+0xa0/0xe8 el0t_64_sync+0x1b0/0x1b8 Code: 17ffff83 f94017e0 f9002be0 f9402ea0 (f9400c00) —[ end trace 0000000000000000 ]—

Also the reporter is very kind to test the following ASSERT() added to btrfs_trim_free_extents_throttle():

ASSERT(device->bdev,
       devid=%llu path=%s dev_state=0x%lxn,
       device->devid, btrfs_dev_name(device), device->dev_state);

And it shows the following output:

assertion failed: device->bdev, in extent-tree.c:6630 (devid=2 path=/dev/sdd dev_state=0x82)

Which means the device->bdev is NULL, and the dev_state is BTRFS_DEV_STATE_IN_FS_METADATA | BTRFS_DEV_STATE_ITEM_FOUND, without BTRFS_DEV_STATE_WRITEABLE flag set.

[CAUSE] The pc points to the following call chain:

btrfs_trim_fs() |- btrfs_trim_free_extents() |- btrfs_trim_free_extents_throttle() |- bdev_max_discard_sectors(device->bdev)

So the NULL pointer dereference is caused by device->bdev being NULL.

This looks impossible by a quick glance, as just before calling btrfs_trim_free_extents_throttle(), we have skipped any device that has BTRFS_DEV_STATE_MISSING flag set.

However in this particular case, there is a window where the missing device is later re-scanned, causing btrfs to remove the BTRFS_DEV_STATE_MISSING flag:

btrfs_control_ioctl() |- btrfs_scan_one_device() |- device_list_add() |- rcu_assign_pointer(device->name, name); | This updates the missing devices path to the new good path. | |- clear_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state) This removes the BTRFS_DEV_STATE_MISSING flag.

This allows the missing device to re-appear and clear the BTRFS_DEV_STATE_MISSING flag. However the device still does not have the BTRFS_DEV_STATE_WRITEABLE flag set, nor is its bdev pointer updated.

The bdev pointer remains NULL, triggering the crash later.

[FIX] This is a big de-synchronization between BTRFS_DEV_STATE_MISSING and device->bdev pointer, and shows a gap in btrfss re-appearing-device handling.

The proper handling of re-appearing device will need quite some extra work, which is out of the context of this small —truncated—

Affected Software

References

• https://git.kernel.org/stable/c/02c903fc6fc7e16c5d1f22d18784f1208acf43e3
• https://git.kernel.org/stable/c/1b1937eb08f51319bf71575484cde2b8c517aedc
• https://git.kernel.org/stable/c/210af872eafa0cf572a84cb303c0f9d2914c1226
• https://git.kernel.org/stable/c/3d8fa4b828a86b33c60858e58aaab6df273ede05
• https://git.kernel.org/stable/c/7a64521802997257b144e6edfb4e278dbeb972dd
• https://git.kernel.org/stable/c/9c894159c5b8adc84072e3af0e55b0473a69564e
• https://git.kernel.org/stable/c/b4af31b898a948e29861cb0bae734058f9a49d9b
• https://git.kernel.org/stable/c/f41ae7e6664f3c4361129728f2c4d5f3ed995251