Re: [PATCH] dm: fix dax_dev NULL dereference

[Pankaj Gupta <pagupta@redhat.com>]

+CC [jencce.kernel@gmail.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
> 
>