FAILED: patch "[PATCH] libceph: fix OOB read in decode_watchers() via missing bounds" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 00ead17c7de137a692edee59f2772e6af687e8eb
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026082057-aviator-unglazed-0547@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

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

From 00ead17c7de137a692edee59f2772e6af687e8eb Mon Sep 17 00:00:00 2001
From: Pavitra Jha <jhapavitra98@gmail.com>
Date: Wed, 8 Jul 2026 01:39:41 -0400
Subject: [PATCH] libceph: fix OOB read in decode_watchers() via missing bounds
 check

ceph_start_decoding() validates that struct_len bytes remain in the
buffer after the encoding header, but accepts struct_len=0 as valid:
ceph_decode_need(p, end, 0, bad) always passes. When a malicious or
compromised OSD sends an obj_list_watch_response_t reply with
struct_len=0, ceph_start_decoding() returns success with p == end,
leaving zero bytes guaranteed for subsequent reads.

The immediately following ceph_decode_32(p) in decode_watchers() has
no preceding bounds check. With p == end this is a 4-byte read past
the validated buffer boundary. The garbage value is then passed
directly to kzalloc_objs() as the watcher count.

The sibling function decode_watcher() already uses the safe variants
(ceph_decode_copy_safe, ceph_decode_64_safe, ceph_decode_skip_32)
after its own ceph_start_decoding() call. decode_watchers() is the
only site that uses the bare variant, confirming an oversight.

Fix by replacing ceph_decode_32(p) with ceph_decode_32_safe(p, end,
*num_watchers, bad), consistent with the established pattern.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment (e.g. cloud) can trigger this against any kernel client
that calls CEPH_OSD_OP_LIST_WATCHERS, without any further privileges
beyond OSD session establishment.

[ idryomov: trim changelog ]

Cc: stable@vger.kernel.org
Fixes: a4ed38d7a180 ("libceph: support for CEPH_OSD_OP_LIST_WATCHERS")
Signed-off-by: Pavitra Jha <jhapavitra98@gmail.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
index 2ff00070c181..28d76c2f6b3e 100644
--- a/net/ceph/osd_client.c
+++ b/net/ceph/osd_client.c
@@ -5030,7 +5030,7 @@ static int decode_watchers(void **p, void *end,
 	if (ret)
 		return ret;
 
-	*num_watchers = ceph_decode_32(p);
+	ceph_decode_32_safe(p, end, *num_watchers, bad);
 	*watchers = kzalloc_objs(**watchers, *num_watchers, GFP_NOIO);
 	if (!*watchers)
 		return -ENOMEM;
@@ -5044,6 +5044,9 @@ static int decode_watchers(void **p, void *end,
 	}
 
 	return 0;
+
+bad:
+	return -EINVAL;
 }
 
 /*