FAILED: patch "[PATCH] xen/scsifront: check for a NULL shadow entry on a backend response" failed to apply to 6.18-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 992df081bc8878d11e1eb5ae4a609910fd35c62b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107050948-failed-6.18-992df081bc88@kernel.org' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 992df081bc8878d11e1eb5ae4a609910fd35c62b Mon Sep 17 00:00:00 2001
From: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
Date: Mon, 5 Oct 2026 21:35:54 +0900
Subject: [PATCH] xen/scsifront: check for a NULL shadow entry on a backend
 response

scsifront_do_response() validates the rqid in a backend response against
VSCSIIF_MAX_REQS before using it to index info->shadow[], but then
dereferences the entry to test ->inflight without checking that the slot
is populated.  Shadow slots are NULL before a command is submitted (the
host private area is zeroed at allocation) and are reset to NULL in
_scsifront_put_rqid() once a request completes, so an in-range rqid that
does not correspond to an outstanding request makes the frontend
dereference a NULL pointer.

A malicious or buggy backend can thus crash the guest by returning a
response whose rqid is in range but not in flight -- for example a
spurious response before any command has been issued, or a duplicate of
one already completed.  The ring has only VSCSIIF_MAX_REQS (16) slots, so
before the first command every in-range rqid selects a NULL slot.

Reject a response whose shadow slot is not populated.

Fixes: 6d1c2f48f3fc ("xen/scsifront: harden driver against malicious backend")
Cc: stable@vger.kernel.org
Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20261005123555.253995-1-yhlee@isslab.korea.ac.kr>
---
 drivers/scsi/xen-scsifront.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/scsi/xen-scsifront.c b/drivers/scsi/xen-scsifront.c
index f86a47056ea8c..9d7e12e400896 100644
--- a/drivers/scsi/xen-scsifront.c
+++ b/drivers/scsi/xen-scsifront.c
@@ -367,6 +367,7 @@ static void scsifront_do_response(struct vscsifrnt_info *info,
 	struct vscsifrnt_shadow *shadow;
 
 	if (ring_rsp->rqid >= VSCSIIF_MAX_REQS ||
+	    !info->shadow[ring_rsp->rqid] ||
 	    !info->shadow[ring_rsp->rqid]->inflight) {
 		scsifront_set_error(info, "illegal rqid returned by backend!");
 		return;
-- 
2.53.0