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SUNRPC: harden gss_krb5_unwrap_v2 against short tokens

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gss_krb5_unwrap_v2() reads the EC and RRC header fields at ptr+4 and
ptr+6 before validating that the token is at least GSS_KRB5_TOK_HDR_LEN
(16) bytes long, and its rotate_left() helper passes buf->len - base
to xdr_buf_subsegment() 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:

    gss_krb5_unwrap_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)
        xdr_buf_subsegment(buf, &subbuf,
                           base, buf->len - base)   /* u32 wrap when base > len */
        _rotate_left(&subbuf, shift)
          shift %= buf->len         /* divide-by-zero when base == len */

After decryption, the cleanup arithmetic has the same shape:

    movelen = min_t(unsigned int, buf->head[0].iov_len, len);
    movelen -= offset + GSS_KRB5_TOK_HDR_LEN + headskip;
    BUG_ON(offset + GSS_KRB5_TOK_HDR_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 < GSS_KRB5_TOK_HDR_LEN 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 _rotate_left() when buf->len is zero, so the
    shift %= buf->len modulo cannot fault;
  - replace the dead BUG_ON with a live check that returns
    GSS_S_DEFECTIVE_TOKEN before the movelen subtraction.

Fixes: de9c17eb4a ("gss_krb5: add support for new token formats in rfc4121")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260524010213.557424-5-cel@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chris Mason 2026-05-23 21:02:13 -04:00 committed by Chuck Lever
parent 3f491306dc
commit 6959297aaa
1 changed files with 9 additions and 2 deletions
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11
net/sunrpc/auth_gss/gss_krb5_wrap.c
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@ -73,6 +73,8 @@ static void _rotate_left(struct xdr_buf *buf, unsigned int shift)
int shifted = 0;
int this_shift;
if (!buf->len)
return;
shift %= buf->len;
while (shifted < shift) {
this_shift = min(shift - shifted, LOCAL_BUF_LEN);
@ -85,6 +87,8 @@ static void rotate_left(u32 base, struct xdr_buf *buf, unsigned int shift)
{
struct xdr_buf subbuf;
if (buf->len <= base)
return;
xdr_buf_subsegment(buf, &subbuf, base, buf->len - base);
_rotate_left(&subbuf, shift);
}
@ -154,6 +158,9 @@ gss_krb5_unwrap_v2(struct krb5_ctx *kctx, int offset, int len,
dprintk("RPC: %s\n", __func__);
if (len - offset <= GSS_KRB5_TOK_HDR_LEN)
return GSS_S_DEFECTIVE_TOKEN;
ptr = buf->head[0].iov_base + offset;
if (be16_to_cpu(*((__be16 *)ptr)) != KG2_TOK_WRAP)
@ -220,9 +227,9 @@ gss_krb5_unwrap_v2(struct krb5_ctx *kctx, int offset, int len,
* head buffer space rather than that actually occupied.
*/
movelen = min_t(unsigned int, buf->head[0].iov_len, len);
if (movelen < offset + GSS_KRB5_TOK_HDR_LEN + headskip)
return GSS_S_DEFECTIVE_TOKEN;
movelen -= offset + GSS_KRB5_TOK_HDR_LEN + headskip;
BUG_ON(offset + GSS_KRB5_TOK_HDR_LEN + headskip + movelen >
buf->head[0].iov_len);
memmove(ptr, ptr + GSS_KRB5_TOK_HDR_LEN + headskip, movelen);
buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;
buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);