SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

PATCH In review Chuck Lever 3 participants 4 messages May 23 – May 25

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Chuck Lever

author maintainer

2026-05-23 16:52 UTC #0 lore raw

PATCH 1/1 SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2

From: Chuck Lever <[email protected]>

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 <[email protected]>

Fixes de9c17eb4a91 ("gss_krb5: add support for new token formats in rfc4121")

Signed-off-by Chuck Lever <[email protected]>

---
 net/sunrpc/auth_gss/gss_krb5_unseal.c | 3 +++
 1 file changed, 3 insertions(+)

net/sunrpc/auth_gss/gss_krb5_unseal.c +3 -0

index b5fb70419faa..4d12d49434c2 100644
@@ -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;

Signature · 1 line

2.54.0

Jeff Layton

maintainer Reviewed-by on v1 1/1

2026-05-24 10:47 UTC #1 lore raw

Quoted 47 lines · Chuck Lever

From: Chuck Lever <[email protected]>
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 <[email protected]>
Fixes de9c17eb4a91 ("gss_krb5: add support for new token formats in rfc4121")
Signed-off-by Chuck Lever <[email protected]>
---
 net/sunrpc/auth_gss/gss_krb5_unseal.c | 3 +++
 1 file changed, 3 insertions(+)
net/sunrpc/auth_gss/gss_krb5_unseal.c +0 -0
index b5fb70419faa..4d12d49434c2 100644

net/sunrpc/auth_gss/gss_krb5_unseal.c gss_krb5_verify_mic_v2(struct krb5_ctx *ctx, struct xdr_buf *message_buffer, · line 97

Quoted 9 lines

net/sunrpc/auth_gss/gss_krb5_unseal.c +3 -0

@@ -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;

Reviewed-by Jeff Layton <[email protected]>

kernel test robot

build bot

2026-05-25 16:12 UTC #2 lore raw

In reply to Chuck Lever

kernel test robot noticed the following build errors:

Hi Chuck,

[This is a private test report for your RFC patch.] kernel test robot noticed the following build errors:

[auto build test ERROR on trondmy-nfs/linux-next]
[also build test ERROR on linus/master v7.1-rc5 next-20260522] [If your patch is applied to the wrong git tree, kindly drop us a note. And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Chuck-Lever/SUNRPC-Reject-short-RFC-4121-MIC-tokens-in-gss_krb5_verify_mic_v2/20260524-005345
base:   git://git.linux-nfs.org/projects/trondmy/linux-nfs.git linux-next
patch link:    https://lore.kernel.org/r/20260523165237.510204-1-cel%40kernel.org
patch subject: [PATCH] SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2
config: sh-allmodconfig (https://download.01.org/0day-ci/archive/20260526/[email protected]/config)
compiler: sh4-linux-gcc (GCC) 15.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260526/[email protected]/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <[email protected]>
| Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/

All errors (new ones prefixed by >>):

net/sunrpc/auth_gss/gss_krb5_unseal.c: In function 'gss_krb5_verify_mic_v2':

>> net/sunrpc/auth_gss/gss_krb5_unseal.c:89:54: error: 'cksum_len' undeclared (first use in this function)
      89 |         if (read_token->len < GSS_KRB5_TOK_HDR_LEN + cksum_len)
         |                                                      ^~~~~~~~~

net/sunrpc/auth_gss/gss_krb5_unseal.c:89:54: note: each undeclared identifier is reported only once for each function it appears in

vim +/cksum_len +89 net/sunrpc/auth_gss/gss_krb5_unseal.c

    69	
    70	u32
    71	gss_krb5_verify_mic_v2(struct krb5_ctx *ctx, struct xdr_buf *message_buffer,
    72			       struct xdr_netobj *read_token)
    73	{
    74		struct crypto_ahash *tfm = ctx->initiate ?
    75					   ctx->acceptor_sign : ctx->initiator_sign;
    76		char cksumdata[GSS_KRB5_MAX_CKSUM_LEN];
    77		struct xdr_netobj cksumobj = {
    78			.len	= ctx->gk5e->cksumlength,
    79			.data	= cksumdata,
    80		};
    81		u8 *ptr = read_token->data;
    82		__be16 be16_ptr;
    83		time64_t now;
    84		u8 flags;
    85		int i;
    86	
    87		dprintk("RPC:       %s\n", __func__);
    88	
  > 89		if (read_token->len < GSS_KRB5_TOK_HDR_LEN + cksum_len)

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