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Date:   Sun, 19 Feb 2017 07:15:27 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     James Bottomley <jejb@...ux.vnet.ibm.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Hannes Reinecke <hare@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: scsi: BUG in scsi_init_io

On Tue, Jan 31, 2017 at 07:41:51AM -0800, James Bottomley wrote:

> > Please-please-please, let's not use WARN for something that is not a
> > kernel bug and is user-triggerable.
> 
> It is a kernel bug and it should not be user triggerable, so it should
> have a warn_on or bug_on.  It means something called a data setup
> function with no data.  There's actually a root cause that patches like
> this won't fix, can we find it?

The root cause is unfixable without access to TARDIS and dose of
antipsychotics sufficient to prevent /dev/sg API creation.

What happens is that write to /dev/sg is given a request with non-zero
->iovec_count combined with zero ->dxfer_len.  Or with ->dxferp pointing
to an array full of empty iovecs.

AFAICS, the minimal fix would be something like this:

YAMissingSanityCheck in /dev/sg

write permission to /dev/sg shouldn't be equivalent to the ability to trigger
BUG_ON() while holding spinlocks...

Cc: stable@...r.kernel.org
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index dbe5b4b95df0..121de0aaa6ad 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1753,6 +1753,10 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 			return res;
 
 		iov_iter_truncate(&i, hp->dxfer_len);
+		if (!iov_iter_count(&i)) {
+			kfree(iov);
+			return -EINVAL;
+		}
 
 		res = blk_rq_map_user_iov(q, rq, md, &i, GFP_ATOMIC);
 		kfree(iov);

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