CVE-2026-89542

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

SUNRPC: harden gsskrb5unwrap_v2 against short tokens

gsskrb5unwrap_v2() reads the EC and RRC header fields at ptr+4 and

ptr+6 before validating that the token is at least GSSKRB5TOKHDRLEN

(16) bytes long, and its rotate_left() helper passes buf->len - base

to xdrbufsubsegment() without verifying that base <= buf->len. When

a caller hands in a sub-16-byte token, or a token whose declared len

leaves base past the end of the buffer, three distinct failures follow:

    gsskrb5unwrap_v2(offset, len, buf)

      ptr = buf->head[0].iov_base + offset

      ec  = (ptr + 4)              / OOB read on short head */

      rrc = (ptr + 6)              / OOB read on short head */

      rotate_left(offset + 16, buf, rrc)

        xdrbufsubsegment(buf, &subbuf,

                           base, buf->len - base)   / u32 wrap when base > len /

        rotateleft(&subbuf, shift)

          shift %= buf->len         / divide-by-zero when base == len /

After decryption, the cleanup arithmetic has the same shape:

    movelen = mint(unsigned int, buf->head[0].iovlen, len);

    movelen -= offset + GSSKRB5TOKHDRLEN + headskip;

    BUGON(offset + GSSKRB5TOKHDR_LEN + headskip + movelen >

                                            buf->head[0].iov_len);

The BUG_ON re-adds the value just subtracted, so it reduces to

min(A, B) > A and is permanently false; it cannot catch the unsigned

underflow of movelen, which then drives a ~UINT_MAX-byte memmove().

Add four defense-in-depth guards inside the unwrap core so it is safe

regardless of what its callers validate:

• reject tokens with len - offset < GSSKRB5TOKHDRLEN before

    touching ptr+4/ptr+6;

• bail from rotate_left() when buf->len <= base, covering both the

    underflow and zero-length cases;

• return early from rotateleft() when buf->len is zero, so the

    shift %= buf->len modulo cannot fault;

• replace the dead BUG_ON with a live check that returns

    GSSSDEFECTIVE_TOKEN before the movelen subtraction.