CVE-2026-89537

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: Reject short RFC 4121 MIC tokens in gsskrb5verifymicv2

gsskrb5verifymicv2() reads the token ID at ptr[0..1], the flags

byte at ptr[2], and padding at ptr[3..7], then passes

ptr + GSSKRB5TOKHDRLEN and cksumlen to gsskrb5micbuild_sg().

None of these accesses check read_token->len first.

The minimum safe token size is GSSKRB5TOKHDRLEN (16) plus

ctx->krb5e->cksum_len (12-24, depending on the enctype).  All callers

accept shorter tokens from the wire:

• gssunwrapresp_integ() enforces only an upper bound

   (offset + len <= rcv_buf->len) before allocating

   mic.data = kmalloc(len) and passing it to gssverifymic().

   A malicious NFS server can therefore supply a short checksum

   opaque, producing a small slab allocation that the Kerberos MIC

   verifier reads past.

• gssvalidate() enforces only len <= RPCMAXAUTHSIZE (400)

   before passing the wire-supplied length to

   gssvalidateseqnomic(), which constructs a mic xdrnetobj

   and calls gssverifymic().

• svcauthgssverify_header() enforces only

   checksum.len >= XDR_UNIT (4 bytes) before dispatching to

   gssverifymic().

• svcauthgssunwrap_integ() checks only that the checksum fits

   in gsd->gsd_scratch.

Add a length guard at the top of gsskrb5verifymicv2(), before any

ptr[] access or scatterlist construction.  Well-formed MIC tokens from

gsskrb5getmicv2() already have exactly GSSKRB5TOKHDRLEN +

cksum_len bytes, so valid traffic is unaffected.