[PATCH 5.15.y] nvmet-tcp: fix use-before-check of sg in bounds validation

[Cengiz Can]

The stable backport of commit 52a0a9854934 ("nvmet-tcp: add bounds
checks in nvmet_tcp_build_pdu_iovec") placed the bounds checks after
the iov_len calculation:

    while (length) {
        u32 iov_len = min_t(u32, length, sg->length - sg_offset);

        if (!sg_remaining) {    /* too late: sg already dereferenced */

In mainline, the checks come first because C99 allows mid-block variable
declarations. The stable backport moved the declaration to the top of the
loop to satisfy C89 declaration rules, but this ended up placing the
sg->length dereference before the sg_remaining and sg->length guards.

If sg_next() returns NULL at the end of the scatterlist, the next
iteration dereferences a NULL pointer in the iov_len calculation before
the sg_remaining check can prevent it.

Fix this by moving the iov_len declaration to function scope and
keeping the assignment after the bounds checks, matching the ordering
in mainline.

Fixes: 42afe8ed8ad2 ("nvmet-tcp: add bounds checks in nvmet_tcp_build_pdu_iovec")
Cc: stable at vger.kernel.org
Cc: YunJe Shin <ioerts at kookmin.ac.kr>
Cc: Sagi Grimberg <sagi at grimberg.me>
Cc: Keith Busch <kbusch at kernel.org>
Cc: linux-nvme at lists.infradead.org
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 drivers/nvme/target/tcp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c
index 8f7984c53f3f..c6cc1dfef92c 100644
--- a/drivers/nvme/target/tcp.c
+++ b/drivers/nvme/target/tcp.c
@@ -312,7 +312,7 @@ static void nvmet_tcp_build_pdu_iovec(struct nvmet_tcp_cmd *cmd)
 {
 	struct bio_vec *iov = cmd->iov;
 	struct scatterlist *sg;
-	u32 length, offset, sg_offset;
+	u32 length, offset, sg_offset, iov_len;
 	unsigned int sg_remaining;
 	int nr_pages;
 
@@ -329,8 +329,6 @@ static void nvmet_tcp_build_pdu_iovec(struct nvmet_tcp_cmd *cmd)
 	sg_remaining = cmd->req.sg_cnt - cmd->sg_idx;
 
 	while (length) {
-		u32 iov_len = min_t(u32, length, sg->length - sg_offset);
-
 		if (!sg_remaining) {
 			nvmet_tcp_fatal_error(cmd->queue);
 			return;
@@ -340,6 +338,8 @@ static void nvmet_tcp_build_pdu_iovec(struct nvmet_tcp_cmd *cmd)
 			return;
 		}
 
+		iov_len = min_t(u32, length, sg->length - sg_offset);
+
 		iov->bv_page = sg_page(sg);
 		iov->bv_len = iov_len;
 		iov->bv_offset = sg->offset + sg_offset;
-- 
2.43.0