[PATCH 4.16 130/272] btrfs: fix null pointer deref when target device is missing

[Greg Kroah-Hartman]

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Anand Jain <anand.jain@xxxxxxxxxx>

[ Upstream commit acf18c56fdcb952a06650282192e3b4ca1855c5e ]

The replace target device can be missing when mounted with -o degraded,
but we wont allocate a missing btrfs_device to it. So check the device
before accessing.

BUG: unable to handle kernel NULL pointer dereference at 00000000000000b0
IP: btrfs_destroy_dev_replace_tgtdev+0x43/0xf0 [btrfs]
Call Trace:
btrfs_dev_replace_cancel+0x15f/0x180 [btrfs]
btrfs_ioctl+0x2216/0x2590 [btrfs]
do_vfs_ioctl+0x625/0x650
SyS_ioctl+0x4e/0x80
do_syscall_64+0x5d/0x160
entry_SYSCALL64_slow_path+0x25/0x25

This patch has been moved in front of patch "btrfs: log, when replace,
is canceled by the user" that could reproduce the crash if the system
reboots inside btrfs_dev_replace_start before the
btrfs_dev_replace_finishing call.

 $ mkfs /dev/sda
 $ mount /dev/sda mnt
 $ btrfs replace start /dev/sda /dev/sdb
 <insert reboot>
 $ mount po degraded /dev/sdb mnt
 <crash>

Signed-off-by: Anand Jain <anand.jain@xxxxxxxxxx>
[ added reproducer description from mail ]
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/btrfs/dev-replace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -307,7 +307,7 @@ void btrfs_after_dev_replace_commit(stru
 
 static char* btrfs_dev_name(struct btrfs_device *device)
 {
-	if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
+	if (!device || test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
 		return "<missing disk>";
 	else
 		return rcu_str_deref(device->name);