From ad484748eec0a66eac0f13ab53b3fbedb7333c91 Mon Sep 17 00:00:00 2001 From: Chuck Lever Date: Thu, 28 May 2026 15:32:08 -0400 Subject: [PATCH] SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field 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: cf4c024b9083 ("sunrpc: trim off EC bytes in GSSAPI v2 unwrap") Cc: stable@vger.kernel.org Reviewed-by: Jeff Layton Link: https://patch.msgid.link/20260528-tier2-v1-1-d026a1415e0b@oracle.com Signed-off-by: Chuck Lever --- net/sunrpc/auth_gss/gss_krb5_wrap.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c b/net/sunrpc/auth_gss/gss_krb5_wrap.c index d84c35f779f5..d3f61c4b5a13 100644 --- a/net/sunrpc/auth_gss/gss_krb5_wrap.c +++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c @@ -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);