linux-kernel - Re: [PATCH] libata: fix length validation of ATAPI-relayed SCSI commands

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Message-ID: <20180212172010.GB695913@devbig577.frc2.facebook.com>
Date:   Mon, 12 Feb 2018 09:20:10 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Mark Lord <mlord@...ox.com>, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] libata: fix length validation of ATAPI-relayed SCSI
 commands

On Sat, Feb 03, 2018 at 08:30:56PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> 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).

Applied to libata/for-4.16-fixes.

Thanks.

-- 
tejun