[PATCH] SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

gss_krb5_verify_mic_v2() reads the token ID at ptr[0..1], the flags
byte at ptr[2], and padding at ptr[3..7], then passes
ptr + GSS_KRB5_TOK_HDR_LEN and cksum_len to gss_krb5_mic_build_sg().
None of these accesses check read_token->len first.

The minimum safe token size is GSS_KRB5_TOK_HDR_LEN (16) plus
ctx->krb5e->cksum_len (12-24, depending on the enctype).  All callers
accept shorter tokens from the wire:

 - gss_unwrap_resp_integ() enforces only an upper bound
   (offset + len <= rcv_buf->len) before allocating
   mic.data = kmalloc(len) and passing it to gss_verify_mic().
   A malicious NFS server can therefore supply a short checksum
   opaque, producing a small slab allocation that the Kerberos MIC
   verifier reads past.

 - gss_validate() enforces only len <= RPC_MAX_AUTH_SIZE (400)
   before passing the wire-supplied length to
   gss_validate_seqno_mic(), which constructs a mic xdr_netobj
   and calls gss_verify_mic().

 - svcauth_gss_verify_header() enforces only
   checksum.len >= XDR_UNIT (4 bytes) before dispatching to
   gss_verify_mic().

 - svcauth_gss_unwrap_integ() checks only that the checksum fits
   in gsd->gsd_scratch.

Add a length guard at the top of gss_krb5_verify_mic_v2(), before any
ptr[] access or scatterlist construction.  Well-formed MIC tokens from
gss_krb5_get_mic_v2() already have exactly GSS_KRB5_TOK_HDR_LEN +
cksum_len bytes, so valid traffic is unaffected.

Reported-by: Chris Mason <clm@meta.com>
Fixes: de9c17eb4a91 ("gss_krb5: add support for new token formats in rfc4121")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/auth_gss/gss_krb5_unseal.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sunrpc/auth_gss/gss_krb5_unseal.c b/net/sunrpc/auth_gss/gss_krb5_unseal.c
index b5fb70419faa..4d12d49434c2 100644
--- a/net/sunrpc/auth_gss/gss_krb5_unseal.c
+++ b/net/sunrpc/auth_gss/gss_krb5_unseal.c
@@ -89,6 +89,9 @@ gss_krb5_verify_mic_v2(struct krb5_ctx *ctx, struct xdr_buf *message_buffer,
 
 	dprintk("RPC:       %s\n", __func__);
 
+	if (read_token->len < GSS_KRB5_TOK_HDR_LEN + cksum_len)
+		return GSS_S_DEFECTIVE_TOKEN;
+
 	memcpy(&be16_ptr, (char *) ptr, 2);
 	if (be16_to_cpu(be16_ptr) != KG2_TOK_MIC)
 		return GSS_S_DEFECTIVE_TOKEN;
-- 
2.54.0