[PATCH 5.14 04/11] btrfs: fix NULL pointer dereference when deleting device by invalid id

From: Greg Kroah-Hartman
Date: Wed Sep 01 2021 - 08:55:12 EST

From: Qu Wenruo <wqu@xxxxxxxx>

commit e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091 upstream.

It's easy to trigger NULL pointer dereference, just by removing a
non-existing device id:

# mkfs.btrfs -f -m single -d single /dev/test/scratch1 \
# mount /dev/test/scratch1 /mnt/btrfs
# btrfs device remove 3 /mnt/btrfs

Then we have the following kernel NULL pointer dereference:

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 9 PID: 649 Comm: btrfs Not tainted 5.14.0-rc3-custom+ #35
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:btrfs_rm_device+0x4de/0x6b0 [btrfs]
btrfs_ioctl+0x18bb/0x3190 [btrfs]
? lock_is_held_type+0xa5/0x120
? find_held_lock.constprop.0+0x2b/0x80
? do_user_addr_fault+0x201/0x6a0
? lock_release+0xd2/0x2d0
? __x64_sys_ioctl+0x83/0xb0

Commit a27a94c2b0c7 ("btrfs: Make btrfs_find_device_by_devspec return
btrfs_device directly") moves the "missing" device path check into

But btrfs_rm_device() itself can have case where it only receives
@devid, with NULL as @device_path.

In that case, calling strcmp() on NULL will trigger the NULL pointer

Before that commit, we handle the "missing" case inside
btrfs_find_device_by_devspec(), which will not check @device_path at all
if @devid is provided, thus no way to trigger the bug.

Before calling strcmp(), also make sure @device_path is not NULL.

Fixes: a27a94c2b0c7 ("btrfs: Make btrfs_find_device_by_devspec return btrfs_device directly")
CC: stable@xxxxxxxxxxxxxxx # 5.4+
Reported-by: butt3rflyh4ck <butterflyhuangxx@xxxxxxxxx>
Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
fs/btrfs/volumes.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2137,7 +2137,7 @@ int btrfs_rm_device(struct btrfs_fs_info

if (IS_ERR(device)) {
if (PTR_ERR(device) == -ENOENT &&
- strcmp(device_path, "missing") == 0)
+ device_path && strcmp(device_path, "missing") == 0)
ret = PTR_ERR(device);