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Subject[PATCH 3.2 154/199] Fix missing sanity check in /dev/sg
3.2.87-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@ZenIV.linux.org.uk>

commit 137d01df511b3afe1f05499aea05f3bafc0fb221 upstream.

What happens is that a 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.

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

Found by Dmitry Vyukov and syzkaller.

[ The BUG_ON() got changed to a WARN_ON_ONCE(), but this fixes the
underlying issue. - Linus ]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: we're not using iov_iter, but can check the
byte length after truncation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/scsi/sg.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1710,6 +1710,10 @@ static int sg_start_req(Sg_request *srp,
iov_count = iov_shorten(iov, iov_count, hp->dxfer_len);
len = hp->dxfer_len;
}
+ if (len == 0) {
+ kfree(iov);
+ return -EINVAL;
+ }

res = blk_rq_map_user_iov(q, rq, md, (struct sg_iovec *)iov,
iov_count,
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