[PATCH v2] ceph: fix bare ceph_decode_8 OOB in decode_lockers()

[Pavitra Jha <jhapavitra98@gmail.com>]

decode_lockers() in cls_lock_client.c contains a bare ceph_decode_8(p)
call after the decode_locker() loop that has no preceding bounds check.

If a malicious or compromised OSD sends a cls_lock_get_info_reply where
num_lockers is crafted such that the decode_locker() loop advances p
exactly to end (or if num_lockers=0 and p is already at end after
ceph_start_decoding() accepts struct_len=0), the subsequent bare
ceph_decode_8(p) reads one byte past the validated buffer boundary.

The result is passed directly into *type, which is subsequently used as
a lock type discriminator by callers. An OSD-controlled one-byte OOB
read at this position gives an attacker influence over the lock type
field with no further preconditions.

The safe variant ceph_decode_8_safe() already exists and is used
consistently throughout the codebase. This site is the only remaining
bare ceph_decode_8() in the decode_lockers() post-loop path.

The goto target is err_free_lockers (not err_inval) because *lockers is
already allocated at this point and must be freed on any decode failure.

v1 of this series fixed the bare ceph_decode_32() before kzalloc_objs()
and added the err_inval label. This v2 addresses the second bare decode
identified by Viacheslav Dubeyko's review.

Regarding the -EINVAL choice (raised in review): -EINVAL is correct for
the err_inval path. The failure is structural malformation of OSD-supplied
data, not a memory shortage. -ENOMEM would misrepresent the failure class
to callers and to stable@ backporters triaging error paths.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment can trigger this against any kernel client that issues the
lock.get_info class method (e.g. during RBD exclusive lock acquisition)
without any further privileges beyond OSD session establishment.

Fixes: d4ed4a530562 ("libceph: support for lock.lock_info")
Cc: stable@vger.kernel.org
Signed-off-by: Pavitra Jha <jhapavitra98@gmail.com>
---
v2: Replace bare *type = ceph_decode_8(p) with ceph_decode_8_safe(),
    goto err_free_lockers to correctly free *lockers on failure.
    Address Viacheslav Dubeyko's review question about this site and
    clarify -EINVAL rationale.
---
 net/ceph/cls_lock_client.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ceph/cls_lock_client.c b/net/ceph/cls_lock_client.c
index 4f27b3d15..c9183a348 100644
--- a/net/ceph/cls_lock_client.c
+++ b/net/ceph/cls_lock_client.c
@@ -314,7 +314,7 @@ static int decode_lockers(void **p, void *end, u8 *type, char **tag,
 			goto err_free_lockers;
 	}
 
-	*type = ceph_decode_8(p);
+	ceph_decode_8_safe(p, end, *type, err_free_lockers);
 	s = ceph_extract_encoded_string(p, end, NULL, GFP_NOIO);
 	if (IS_ERR(s)) {
 		ret = PTR_ERR(s);
-- 
2.53.0