linux-kernel - [PATCH] sg: recheck MMAP_IO request length with lock held

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Message-Id: <20170816044843.96697-1-toddpoynor@google.com>
Date:   Tue, 15 Aug 2017 21:48:43 -0700
From:   Todd Poynor <toddpoynor@...gle.com>
To:     Doug Gilbert <dgilbert@...erlog.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hannes Reinecke <hare@...e.de>,
        Todd Poynor <toddpoynor@...gle.com>
Subject: [PATCH] sg: recheck MMAP_IO request length with lock held

Commit 1bc0eb044615 ("scsi: sg: protect accesses to 'reserved' page
array") adds needed concurrency protection for the "reserve" buffer.
Some checks that are initially made outside the lock are replicated once
the lock is taken to ensure the checks and resulting decisions are made
using consistent state.

The check that a request with flag SG_FLAG_MMAP_IO set fits in the
reserve buffer also needs to be performed again under the lock to
ensure the reserve buffer length compared against matches the value in
effect when the request is linked to the reserve buffer.  An -ENOMEM
should be returned in this case, instead of switching over to an
indirect buffer as for non-MMAP_IO requests.

Signed-off-by: Todd Poynor <toddpoynor@...gle.com>
---
 drivers/scsi/sg.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index d7ff71e0c85c..3a44b4bc872b 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1735,9 +1735,12 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 		    !sfp->res_in_use) {
 			sfp->res_in_use = 1;
 			sg_link_reserve(sfp, srp, dxfer_len);
-		} else if ((hp->flags & SG_FLAG_MMAP_IO) && sfp->res_in_use) {
+		} else if (hp->flags & SG_FLAG_MMAP_IO) {
+			res = -EBUSY; /* sfp->res_in_use == 1 */
+			if (dxfer_len > rsv_schp->bufflen)
+				res = -ENOMEM;
 			mutex_unlock(&sfp->f_mutex);
-			return -EBUSY;
+			return res;
 		} else {
 			res = sg_build_indirect(req_schp, sfp, dxfer_len);
 			if (res) {
-- 
2.14.1.480.gb18f417b89-goog