[PATCH] dm: fix dax_dev NULL dereference

[Pankaj Gupta <pagupta@redhat.com>]

  'Murphy Zhou' reports[1] hitting the panic when running xfstests 
  generic/108 on pmem ramdisk. In his words:

   This test is simulating partial disk error when calling fsync():
   create a lvm vg which consists of 2 disks:
   one scsi_debug disk; one other disk I specified, pmem ramdisk in this case.
   create lv in this vg and write to it, make sure writing across 2 disks;
   offline scsi_debug disk;
   write again to allocated area;
   expect fsync: IO error.
   If one of the disks is pmem ramdisk, it reproduces every time on my setup,
   on v5.3-rc2+.
   The mount -o dax option is not required to reproduce this panic.
   ...

  Fix this by returning false from 'device_synchronous' function when dax_dev
  is NULL.

 [ 1984.878208] BUG: kernel NULL pointer dereference, address: 00000000000002d0
 [ 1984.882546] #PF: supervisor read access in kernel mode
 [ 1984.885664] #PF: error_code(0x0000) - not-present page
 [ 1984.888626] PGD 0 P4D 0
 [ 1984.890140] Oops: 0000 [#1] SMP PTI
 ...
 ...
 [ 1984.943682] Call Trace:
 [ 1984.945007]  device_synchronous+0xe/0x20 [dm_mod]
 [ 1984.947328]  stripe_iterate_devices+0x48/0x60 [dm_mod]
 [ 1984.949947]  ? dm_set_device_limits+0x130/0x130 [dm_mod]
 [ 1984.952516]  dm_table_supports_dax+0x39/0x90 [dm_mod]
 [ 1984.954989]  dm_table_set_restrictions+0x248/0x5d0 [dm_mod]
 [ 1984.957685]  dm_setup_md_queue+0x66/0x110 [dm_mod]
 [ 1984.960280]  table_load+0x1e3/0x390 [dm_mod]
 [ 1984.962491]  ? retrieve_status+0x1c0/0x1c0 [dm_mod]
 [ 1984.964910]  ctl_ioctl+0x1d3/0x550 [dm_mod]
 [ 1984.967006]  ? path_lookupat+0xf4/0x200
 [ 1984.968890]  dm_ctl_ioctl+0xa/0x10 [dm_mod]
 [ 1984.970920]  do_vfs_ioctl+0xa9/0x630
 [ 1984.972701]  ksys_ioctl+0x60/0x90
 [ 1984.974335]  __x64_sys_ioctl+0x16/0x20
 [ 1984.976221]  do_syscall_64+0x5b/0x1d0
 [ 1984.978091]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

 [1] https://lore.kernel.org/linux-fsdevel/2011806368.5335560.1564469373050.JavaMail.zimbra@redhat.com/T/#mac662eb50b9d7bd282b23e6e8625a3f7a4687506

Fixes: 2e9ee0955d3c ("dm: enable synchronous dax")
Reported-by: jencce.kernel@gmail.com
Tested-by: jencce.kernel@gmail.com
Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
---
 drivers/md/dm-table.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index caaee8032afe..b065845c1bdd 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -894,6 +894,9 @@ int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
 static int device_synchronous(struct dm_target *ti, struct dm_dev *dev,
 				       sector_t start, sector_t len, void *data)
 {
+	if (!dev->dax_dev)
+		return false;
+
 	return dax_synchronous(dev->dax_dev);
 }
 
-- 
2.20.1