libceph: fix OOB read in decode_watchers() via missing bounds check

commit 00ead17 upstream.

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: a4ed38d ("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>
[ kept the tree's `kcalloc()` context line instead of upstream's `kzalloc_objs()` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: pavitrajha

net/ceph/osd_client.c

@@ -5001,7 +5001,7 @@ 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 = kcalloc(*num_watchers, sizeof(**watchers), GFP_NOIO);
 	if (!*watchers)
 		return -ENOMEM;
@@ -5015,6 +5015,9 @@ static int decode_watchers(void **p, void *end,
 	}
 
 	return 0;
+
+bad:
+	return -EINVAL;
 }
 
 /*