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Subject: Re: [PATCH] scsi: core: initialize cmd->cmnd before it is used
To:     Bart Van Assche <bvanassche@acm.org>,
        Tadeusz Struk <tadeusz.struk@linaro.org>,
        linux-scsi@vger.kernel.org
Cc:     Christoph Hellwig <hch@lst.de>,
        "James E . J . Bottomley" <jejb@linux.ibm.com>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        syzbot+5516b30f5401d4dcbcae@syzkaller.appspotmail.com
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 <17a1b72e-2c2a-8492-cb92-4dec36a6531d@acm.org>
From:   Douglas Gilbert <dgilbert@interlog.com>
Message-ID: <cad499a9-7587-1fa9-9f7d-223e66a18efa@interlog.com>
Date:   Tue, 2 Nov 2021 17:33:51 -0400
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On 2021-11-01 11:06 p.m., Bart Van Assche wrote:
> On 11/1/21 18:56, Douglas Gilbert wrote:
>> On 2021-11-01 4:20 p.m., Bart Van Assche wrote:
>>> One of the functions in the call stack in the first message of this email
>>> thread is sg_io(). I am not aware of any documentation that specifies whether
>>> it is valid to set cmd_len in the sg_io header to zero. My opinion is that
>>> the SG_IO implementation should either reject cmd_len == 0 or set cmd_len
>>> to a valid value if it is zero.
>>
>> For the sg driver in production, the v3 interface users (including
>> ioctl(<sg_fd>, SG_IO,) ) have this check:
>>
>>         if ((!hp->cmdp) || (hp->cmd_len < 6) || (hp->cmd_len > sizeof (cmnd))) {
>>                  sg_remove_request(sfp, srp);
>>                  return -EMSGSIZE;
>>          }
> 
> Hi Doug,
> 
> Thanks for having taken a look. I found the above check in sg_new_write(). To me 
> that function seems to come from a code path that is unrelated to sg_io(), the 
> function shown in the call stack in the email at the start of this thread. Maybe 
> I overlooked something but I haven't found a minimum size check for hdr->cmd_len 
> in sg_io() before the blk_execute_rq() call. Should such a check perhaps be added?

I guess it came from ioctl(<non_sg_fd>, SG_IO, ) and I found no lower bound
check when I looked in lk 5.15.0 . No-one has complained to me about the
    hp->cmd_len < 6

check in the sg driver ***. So I think such a check may be useful in the 
scsi_fill_sghdr_rq() function in drivers/scsi/scsi_ioctl.c . And a return
of -EMSGSIZE seems to be tailor made for this situation.

Doug Gilbert


*** It is possible a vendor specific command could be between 1 and 5 bytes
long, but that would probably be an unwise choice.