Linux Kernel btrfs Trim Path Calls blkdev_issue_discard on a Device Marked Read-Only During Degraded Mount, Dereferencing a NULL Device Pointer and Panicking

Status: NEW  |  Advisory ID: CVE-2026-64593

Key Details

What to Know

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%lx\n",

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 device's 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 btrfs's 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--- (NVD)

References