ceph: fix pre-auth out-of-bounds read on snaptrace in ceph_handle_caps()

commit 4dbc71b upstream.

ceph_handle_caps() reads snap_trace_len from the wire-format
ceph_mds_caps header and uses it unconditionally to build a fake
end pointer (snaptrace + snaptrace_len) that is later handed to
ceph_update_snap_trace() in the CEPH_CAP_OP_IMPORT case:

    snaptrace     = h + 1;
    snaptrace_len = le32_to_cpu(h->snap_trace_len);
    p             = snaptrace + snaptrace_len;
    ...
    case CEPH_CAP_OP_IMPORT:
        if (snaptrace_len) {
            ...
            if (ceph_update_snap_trace(mdsc, snaptrace,
                                       snaptrace + snaptrace_len,
                                       false, &realm)) { ... }

ceph_update_snap_trace() then decodes a struct ceph_mds_snap_realm
from snaptrace using ceph_decode_need(&p, e, sizeof(*ri), bad)
with the attacker-supplied fake end e == snaptrace + snaptrace_len.
With snaptrace_len == 0xFFFFFFFF the bound check is trivially
satisfied, ri = p reads sizeof(struct ceph_mds_snap_realm) past
the legitimate msg->front buffer, and ri->num_snaps /
ri->num_prior_parent_snaps then drive further out-of-bounds
reads of the encoded snap arrays.

The eleven msg_version >= 2 .. msg_version >= 12 decoder blocks
above the op switch each catch this OOB through their
ceph_decode_*_safe() / ceph_decode_need() helpers, but they sit
behind a hdr.version-gated if, so a malicious or compromised
MDS that sets msg->hdr.version = 1 reaches the IMPORT path with
no version-gated decoder having validated snap_trace_len. The
shape has been present since ceph_handle_caps() was introduced.

Validate snap_trace_len against the message front buffer before
consuming it, using the canonical ceph_decode_need() / ceph_has_room()
helper.  The helper bounds the length with subtraction (n <= end - p,
guarded by end >= p) rather than pointer addition, so it is wrap-safe
for the attacker-controlled u32 length on 32-bit builds where
p + snap_trace_len could overflow the address space.  This matches the
rest of the ceph decode path (e.g. the pool_ns_len check a few lines
below), and the existing goto bad cleanup already covers this exit
path.

Cc: stable@vger.kernel.org
Fixes: a8599bd ("ceph: capability management")
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: bryamzxz

fs/ceph/caps.c

@@ -4378,6 +4378,7 @@ void ceph_handle_caps(struct ceph_mds_session *session,
 
 	snaptrace = h + 1;
 	snaptrace_len = le32_to_cpu(h->snap_trace_len);
+	ceph_decode_need(&snaptrace, end, snaptrace_len, bad);
 	p = snaptrace + snaptrace_len;
 
 	if (msg_version >= 2) {