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

Overview

In the Linux kernel, the following vulnerability has been resolved:

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

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.

Affected

• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < f913192fc782288e060dafc329b2346934be34cc
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < 0c011137194036424e974677e0f1592e22a33d8c
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < cc93f68a31c9b831abf2db8647b5f5b10329d793
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < 9081c71796724ffe96cba253f68fbe42363c5295
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < 03b417afce19ee6b6e61f1bbbbebac924c9f36d1
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < a4228b93706fb74a484e6ffb271c1cc2af3a2ddb
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < 71893c342a26bcff92eaab0b2b75d64aed19308a
• Linux >= a8599bd821d084d04a3290fffae1071624ec00ea < 4dbc71bcaf9a30abf3920a4e2cc4ed33bba78c02
• Linux 2.6.34

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.