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SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

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gss_krb5_unwrap_v2() sets buf->len to a logical
length, which can be much smaller than head[0].iov_len
(the allocated receive-page capacity).  It then calls
xdr_buf_trim() with a trim length derived from the 16-bit
"extra count" (ec) field in the Kerberos v2 token header.

The ec field is authenticated by the post-decrypt memcmp()
against the encrypted header copy, so a randomly-mutated
value is rejected.  However, any peer holding a valid GSS
context can legitimately encrypt a token whose ec exceeds
the plaintext length.  Per RFC 4121, such a token is
structurally malformed.

Although xdr_buf_trim() now clamps the buf->len subtraction
to avoid unsigned underflow, the buffer is still left in a
semantically invalid state (zero length, inconsistent iov
lengths) when ec is oversized.

Reject these tokens before calling xdr_buf_trim(), giving
callers a well-defined GSS_S_DEFECTIVE_TOKEN error and
keeping the xdr_buf internally consistent.  The wrapped blob
begins at a nonzero offset -- both callers pass len as
offset + opaque_len -- so buf->len still counts the offset
bytes that precede the blob.  Compare the trim length
against the remaining wrapped segment, buf->len - offset,
rather than the whole buffer; comparing against buf->len
alone leaves an offset-wide window in which an oversized ec
passes the test and xdr_buf_trim() cuts into the bytes ahead
of the blob.

Fixes: cf4c024b90 ("sunrpc: trim off EC bytes in GSSAPI v2 unwrap")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-tier2-v1-1-d026a1415e0b@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chuck Lever 2026-05-28 15:32:08 -04:00 committed by Chuck Lever
parent f9868174af
commit ad484748ee
1 changed files with 2 additions and 0 deletions
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2
net/sunrpc/auth_gss/gss_krb5_wrap.c
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@ -235,6 +235,8 @@ gss_krb5_unwrap_v2(struct krb5_ctx *kctx, int offset, int len,
buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);
/* Trim off the trailing "extra count" and checksum blob */
if (ec + GSS_KRB5_TOK_HDR_LEN + tailskip > buf->len - offset)
return GSS_S_DEFECTIVE_TOKEN;
xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);
*align = XDR_QUADLEN(GSS_KRB5_TOK_HDR_LEN + headskip);