linux-kernel - [PATCH] block: allow WRITE_SAME commands with the SG

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Date:   Sat, 10 Dec 2016 12:56:06 -0600
From:   Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>
To:     axboe@...nel.dk, linux-block@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, pbonzini@...hat.com
Subject: [PATCH] block: allow WRITE_SAME commands with the SG_IO ioctl

The WRITE_SAME commands are not present in the blk_default_cmd_filter
write_ok list, and thus are failed with -EPERM when the SG_IO ioctl()
is executed without CAP_SYS_RAWIO capability (e.g., unprivileged users).
[ sg_io() -> blk_fill_sghdr_rq() > blk_verify_command() -> -EPERM ]

The problem can be reproduced with the sg_write_same command

  # sg_write_same --num 1 --xferlen 512 /dev/sda
  #

  # capsh --drop=cap_sys_rawio -- -c \
    'sg_write_same --num 1 --xferlen 512 /dev/sda'
    Write same: pass through os error: Operation not permitted
  #

For comparison, the WRITE_VERIFY command does not observe this problem,
since it is in that list:

  # capsh --drop=cap_sys_rawio -- -c \
    'sg_write_verify --num 1 --ilen 512 --lba 0 /dev/sda'
  #

So, this patch adds the WRITE_SAME commands to the list, in order
for the SG_IO ioctl to finish successfully:

  # capsh --drop=cap_sys_rawio -- -c \
    'sg_write_same --num 1 --xferlen 512 /dev/sda'
  #

That case happens to be exercised by QEMU KVM guests with 'scsi-block' devices
(qemu "-device scsi-block" [1], libvirt "<disk type='block' device='lun'>" [2]),
which employs the SG_IO ioctl() and runs as an unprivileged user (libvirt-qemu).

In that scenario, when a filesystem (e.g., ext4) performs its zero-out calls,
which are translated to write-same calls in the guest kernel, and then into
SG_IO ioctls to the host kernel, SCSI I/O errors may be observed in the guest:

  [...] sd 0:0:0:0: [sda] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
  [...] sd 0:0:0:0: [sda] tag#0 Sense Key : Aborted Command [current]
  [...] sd 0:0:0:0: [sda] tag#0 Add. Sense: I/O process terminated
  [...] sd 0:0:0:0: [sda] tag#0 CDB: Write Same(10) 41 00 01 04 e0 78 00 00 08 00
  [...] blk_update_request: I/O error, dev sda, sector 17096824

Links:
[1] http://git.qemu.org/?p=qemu.git;a=commit;h=336a6915bc7089fb20fea4ba99972ad9a97c5f52
[2] https://libvirt.org/formatdomain.html#elementsDisks (see 'disk' -> 'device')

Signed-off-by: Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>
Signed-off-by: Brahadambal Srinivasan <latha@...ux.vnet.ibm.com>
Reported-by: Manjunatha H R <manjuhr1@...ibm.com>
---
 block/scsi_ioctl.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/scsi_ioctl.c b/block/scsi_ioctl.c
index 0774799..c6fee74 100644
--- a/block/scsi_ioctl.c
+++ b/block/scsi_ioctl.c
@@ -182,6 +182,9 @@ static void blk_set_cmd_filter_defaults(struct blk_cmd_filter *filter)
 	__set_bit(WRITE_16, filter->write_ok);
 	__set_bit(WRITE_LONG, filter->write_ok);
 	__set_bit(WRITE_LONG_2, filter->write_ok);
+	__set_bit(WRITE_SAME, filter->write_ok);
+	__set_bit(WRITE_SAME_16, filter->write_ok);
+	__set_bit(WRITE_SAME_32, filter->write_ok);
 	__set_bit(ERASE, filter->write_ok);
 	__set_bit(GPCMD_MODE_SELECT_10, filter->write_ok);
 	__set_bit(MODE_SELECT, filter->write_ok);
-- 
2.7.4