SUNRPC: svcauth_gss: enforce krb5 token minimum length

Overview

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

SUNRPC: svcauth_gss: enforce krb5 token minimum length

svcauth_gss_unwrap_priv() validates only an upper bound on the wire-supplied opaque length before handing the buffer to gss_unwrap():

if (len > xdr_stream_remaining(xdr))
        goto unwrap_failed;
offset = xdr_stream_pos(xdr);
...
maj_stat = gss_unwrap(ctx, offset, offset + len, buf);

The wire value len flows unchanged as the upper bound into the krb5 unwrap path, so a len in [0, 16] passes this check and is handed to gss_unwrap(). For a krb5 v2 context that lands in gss_krb5_unwrap_v2(), which reads the 16-byte RFC 4121 token header fields at ptr+4 and ptr+6 and then calls rotate_left() before any integrity check. With a sub-header length the header reads run past the token, and _rotate_left()'s shift %= buf->len path can divide by zero when buf->len has been driven to zero by the truncated token. A header-only token (len == 16) is equally invalid: with a non-zero RRC field and the opaque blob ending at the XDR buffer boundary, rotate_left() builds a zero-length subbuffer, reaching the same division.

Reject the token at the server entry point before it reaches the krb5 unwrap core. A valid sealed RFC 4121 token must contain the 16-byte header plus at least some encrypted payload.

Fix by adding a minimum-length check immediately after the existing upper-bound check:

if (len <= GSS_KRB5_TOK_HDR_LEN)
        goto unwrap_failed;

Affected

• Linux >= 7c9fdcfb1b64c47ed618c103b617af3f86e1239c < 0ea5b0c7f212c2772d32c2b88287b89a9cdf6edd
• Linux >= 7c9fdcfb1b64c47ed618c103b617af3f86e1239c < dd6afc6cab8c5d387d1ed2f069562ef7bdadd651
• Linux >= 7c9fdcfb1b64c47ed618c103b617af3f86e1239c < de942dd8c2c8358bcad04ce44271954c48924423
• Linux >= 7c9fdcfb1b64c47ed618c103b617af3f86e1239c < 2eed1e6a976a44015c3ee78841fe336796e2b21c
• Linux >= 7c9fdcfb1b64c47ed618c103b617af3f86e1239c < a919c5c88769cf8fb3ec071e6078d830bf512489
• Linux 2.6.18

Remediation

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