[added to the 3.18 stable tree] sg_start_req(): make sure that there's not too many elements in iovec

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

This patch has been added to the 3.18 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 451a2886b6bf90e2fb378f7c46c655450fb96e81 ]

unfortunately, allowing an arbitrary 16bit value means a possibility of
overflow in the calculation of total number of pages in bio_map_user_iov() -
we rely on there being no more than PAGE_SIZE members of sum in the
first loop there.  If that sum wraps around, we end up allocating
too small array of pointers to pages and it's easy to overflow it in
the second loop.

X-Coverup: TINC (and there's no lumber cartel either)
Cc: stable@vger.kernel.org # way, way back
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <redacted>
---
 drivers/scsi/sg.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 07b2ea1..b3c8aab 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c

@@ -1785,6 +1785,9 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 			md->from_user = 0;
 	}
 
+	if (unlikely(iov_count > UIO_MAXIOV))
+		return -EINVAL;
+
 	if (iov_count) {
 		int len, size = sizeof(struct sg_iovec) * iov_count;
 		struct iovec *iov;

-- 
2.1.4