CVE-2026-31641rxrpc: Fix RxGK token loading to check bounds

Official description

In the Linux kernel, the following vulnerability has been resolved: rxrpc: Fix RxGK token loading to check bounds rxrpc_preparse_xdr_yfs_rxgk() reads the raw key length and ticket length from the XDR token as u32 values and passes each through round_up(x, 4) before using the rounded value for validation and allocation. When the raw length is >= 0xfffffffd, round_up() wraps to 0, so the bounds check and kzalloc both use 0 while the subsequent memcpy still copies the original ~4 GiB value, producing a heap buffer overflow reachable from an unprivileged add_key() call. Fix this by: (1) Rejecting raw key lengths above AFSTOKEN_GK_KEY_MAX and raw ticket lengths above AFSTOKEN_GK_TOKEN_MAX before rounding, consistent with the caps that the RxKAD path already enforces via AFSTOKEN_RK_TIX_MAX. (2) Sizing the flexible-array allocation from the validated raw key length via struct_size_t() instead of the rounded value. (3) Caching the raw lengths so that the later field assignments and memcpy calls do not re-read from the token, eliminating a class of TOCTOU re-parse. The control path (valid token with lengths within bounds) is unaffected.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/3e04596cba8a86cbff9c3f4bf0a524a3a488773c
• https://git.kernel.org/stable/c/49875b360c2b83a3c226e189c502e501d83e6445
• https://git.kernel.org/stable/c/d179a868dd755b0cfcf7582e00943d702b9943b8
• https://access.redhat.com/security/cve/CVE-2026-31641 vdb-entry, x_refsource_REDHAT
• RHBZ#2461548 issue-tracking, x_refsource_REDHAT
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-31641.json x_sadp-csaf-vex
• https://access.redhat.com/errata/RHSA-2026:27288 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:55618 vendor-advisory, x_refsource_REDHAT