ACK/Cmnt: [PATCH][focal/linux-azure, focal/linux-hwe-5.8] btrfs: fix NULL pointer dereference when deleting device by invalid id

[Stefan Bader]

On 04.10.21 14:58, Tim Gardner wrote:
> From: Qu Wenruo <wqu at suse.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1945987
> 
> [BUG]
> 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 \
> 				     /dev/test/scratch2
>   # 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
>    __x64_sys_ioctl+0x83/0xb0
>    do_syscall_64+0x3b/0x90
>    entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> [CAUSE]
> Commit a27a94c2b0c7 ("btrfs: Make btrfs_find_device_by_devspec return
> btrfs_device directly") moves the "missing" device path check into
> btrfs_rm_device().
> 
> 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
> dereference.
> 
> 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.
> 
> [FIX]
> 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 at vger.kernel.org # 5.4+
> Reported-by: butt3rflyh4ck <butterflyhuangxx at gmail.com>
> Reviewed-by: Anand Jain <anand.jain at oracle.com>
> Signed-off-by: Qu Wenruo <wqu at suse.com>
> Reviewed-by: David Sterba <dsterba at suse.com>
> Signed-off-by: David Sterba <dsterba at suse.com>
> (cherry picked from commit e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>
> ---

Not sure how much effort still should go into any 5.8 but since we have some 
cleaning up to be done soon and this looks simple enough lets get a second ack 
quick and I can squeeze it into the run I delayed for the revocation stuff.

-Stefan

>   fs/btrfs/volumes.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 3e3529c600cb7..e882c790292f9 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -2168,7 +2168,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
>   
>   	if (IS_ERR(device)) {
>   		if (PTR_ERR(device) == -ENOENT &&
> -		    strcmp(device_path, "missing") == 0)
> +		    device_path && strcmp(device_path, "missing") == 0)
>   			ret = BTRFS_ERROR_DEV_MISSING_NOT_FOUND;
>   		else
>   			ret = PTR_ERR(device);
> 


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