FAILED: patch "[PATCH] SUNRPC: Reject short RFC 4121 MIC tokens in" failed to apply to 6.12-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x b94f6719dcd9f7a609bc5f459f85795900e77d25
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090330-cranberry-daredevil-a4d4@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b94f6719dcd9f7a609bc5f459f85795900e77d25 Mon Sep 17 00:00:00 2001
From: Chuck Lever <chuck.lever@oracle.com>
Date: Sat, 23 May 2026 12:52:37 -0400
Subject: [PATCH] SUNRPC: Reject short RFC 4121 MIC tokens in
 gss_krb5_verify_mic_v2

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")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260523165237.510204-1-cel@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

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;