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Subject[PATCH] libata: fix length validation of ATAPI-relayed SCSI commands
From: Eric Biggers <>

syzkaller reported a crash in ata_bmdma_fill_sg() when writing to
/dev/sg1. The immediate cause was that the ATA command's scatterlist
was not DMA-mapped, which causes 'pi - 1' to underflow, resulting in a
write to 'qc->ap->bmdma_prd[0xffffffff]'.

Strangely though, the flag ATA_QCFLAG_DMAMAP was set in qc->flags. The
root cause is that when __ata_scsi_queuecmd() is preparing to relay a
SCSI command to an ATAPI device, it doesn't correctly validate the CDB
length before copying it into the 16-byte buffer 'cdb' in 'struct
ata_queued_cmd'. Namely, it validates the fixed CDB length expected
based on the SCSI opcode but not the actual CDB length, which can be
larger due to the use of the SG_NEXT_CMD_LEN ioctl. Since 'flags' is
the next member in ata_queued_cmd, a buffer overflow corrupts it.

Fix it by requiring that the actual CDB length be <= 16 (ATAPI_CDB_LEN).

[Really it seems the length should be required to be <= dev->cdb_len,
but the current behavior seems to have been intentionally introduced by
commit 607126c2a21c ("libata-scsi: be tolerant of 12-byte ATAPI commands
in 16-byte CDBs") to work around a userspace bug in mplayer. Probably
the workaround is no longer needed (mplayer was fixed in 2007), but
continuing to allow lengths to up 16 appears harmless for now.]

Here's a reproducer that works in QEMU when /dev/sg1 refers to the
CD-ROM drive that qemu-system-x86_64 creates by default:

#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>

#define SG_NEXT_CMD_LEN 0x2283

int main()
char buf[53] = { [36] = 0x7e, [52] = 0x02 };
int fd = open("/dev/sg1", O_RDWR);
ioctl(fd, SG_NEXT_CMD_LEN, &(int){ 17 });
write(fd, buf, sizeof(buf));

The crash was:

BUG: unable to handle kernel paging request at ffff8cb97db37ffc
IP: ata_bmdma_fill_sg drivers/ata/libata-sff.c:2623 [inline]
IP: ata_bmdma_qc_prep+0xa4/0xc0 drivers/ata/libata-sff.c:2727
PGD fb6c067 P4D fb6c067 PUD 0
Oops: 0002 [#1] SMP
CPU: 1 PID: 150 Comm: syz_ata_bmdma_q Not tainted 4.15.0-next-20180202 #99
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-20171110_100015-anatol 04/01/2014
Call Trace:
ata_qc_issue+0x100/0x1d0 drivers/ata/libata-core.c:5421
ata_scsi_translate+0xc9/0x1a0 drivers/ata/libata-scsi.c:2024
__ata_scsi_queuecmd drivers/ata/libata-scsi.c:4326 [inline]
ata_scsi_queuecmd+0x8c/0x210 drivers/ata/libata-scsi.c:4375
scsi_dispatch_cmd+0xa2/0xe0 drivers/scsi/scsi_lib.c:1727
scsi_request_fn+0x24c/0x530 drivers/scsi/scsi_lib.c:1865
__blk_run_queue_uncond block/blk-core.c:412 [inline]
__blk_run_queue+0x3a/0x60 block/blk-core.c:432
blk_execute_rq_nowait+0x93/0xc0 block/blk-exec.c:78
sg_common_write.isra.7+0x272/0x5a0 drivers/scsi/sg.c:806
sg_write+0x1ef/0x340 drivers/scsi/sg.c:677
__vfs_write+0x31/0x160 fs/read_write.c:480
vfs_write+0xa7/0x160 fs/read_write.c:544
SYSC_write fs/read_write.c:589 [inline]
SyS_write+0x4d/0xc0 fs/read_write.c:581
do_syscall_64+0x5e/0x110 arch/x86/entry/common.c:287

Fixes: 607126c2a21c ("libata-scsi: be tolerant of 12-byte ATAPI commands in 16-byte CDBs")
Cc: <> # v2.6.24+
Signed-off-by: Eric Biggers <>
drivers/ata/libata-scsi.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index 66be961c93a4e..47d421666451c 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -4309,7 +4309,9 @@ static inline int __ata_scsi_queuecmd(struct scsi_cmnd *scmd,
if (likely((scsi_op != ATA_16) || !atapi_passthru16)) {
/* relay SCSI command to ATAPI device */
int len = COMMAND_SIZE(scsi_op);
- if (unlikely(len > scmd->cmd_len || len > dev->cdb_len))
+ if (unlikely(len > scmd->cmd_len ||
+ len > dev->cdb_len ||
+ scmd->cmd_len > ATAPI_CDB_LEN))
goto bad_cdb_len;

xlat_func = atapi_xlat;
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