[PATCH net] xfrm: Check for underflow in xfrm_state_mtu

[David Ahern <dsahern@kernel.org>]

From: David Ahern <dahern@nvidia.com>

Leo Lin reported OOB write issue in esp component:

  xfrm_state_mtu() returns u32 but performs its arithmetic in unsigned
  modulo-2^32 space using an attacker-influenced "header_len + authsize +
  net_adj" subtracted from a small "mtu" argument. A nobody user can
  install an IPv4 ESP tunnel SA with a large authentication key
  (XFRMA_ALG_AUTH_TRUNC, e.g. hmac(sha512), 64-byte key, 64-byte trunc),
  configure a small interface MTU (68 bytes), and set XFRMA_TFCPAD to a
  large value. When a single UDP datagram is then sent through the
  tunnel, xfrm_state_mtu() underflows to a near-2^32 value, and
  esp_output() consumes it as a signed int via:

        padto      = min(x->tfcpad, xfrm_state_mtu(x, mtu_cached))
        esp.tfclen = padto - skb->len   (assigned to int)

  esp.tfclen ends up negative (e.g. -207). It is sign-extended to size_t
  when passed to memset() inside esp_output_fill_trailer(), producing a
  ~16 EB write of zeroes at skb_tail_pointer(skb). KASAN logs it as
  "Write of size 18446744073709551537 at addr ffff888...".

Check for underflow and return 1. This causes the sendmsg attempt to
fail with ENETUNREACH.

Fixes: c5c252389374 ("[XFRM]: Optimize MTU calculation")
Reported-by: Leo Lin <leo@depthfirst.com>
Assisted-by: Codex:26.506.31004
Signed-off-by: David Ahern <dahern@nvidia.com>
---
 net/xfrm/xfrm_state.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 686014d39429..f597e4996bb2 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -3114,10 +3114,14 @@ u32 xfrm_state_mtu(struct xfrm_state *x, int mtu)
 	const struct xfrm_type *type = READ_ONCE(x->type);
 	struct crypto_aead *aead;
 	u32 blksize, net_adj = 0;
+	u32 overhead, payload_mtu;
 
 	if (x->km.state != XFRM_STATE_VALID ||
-	    !type || type->proto != IPPROTO_ESP)
+	    !type || type->proto != IPPROTO_ESP) {
+		if (mtu <= x->props.header_len)
+			return 1;
 		return mtu - x->props.header_len;
+	}
 
 	aead = x->data;
 	blksize = ALIGN(crypto_aead_blocksize(aead), 4);
@@ -3140,8 +3144,17 @@ u32 xfrm_state_mtu(struct xfrm_state *x, int mtu)
 		break;
 	}
 
-	return ((mtu - x->props.header_len - crypto_aead_authsize(aead) -
-		 net_adj) & ~(blksize - 1)) + net_adj - 2;
+	overhead = x->props.header_len + crypto_aead_authsize(aead) + net_adj;
+	if (mtu <= overhead)
+		return 1;
+
+	payload_mtu = mtu - overhead;
+	payload_mtu &= ~(blksize - 1);
+	if (payload_mtu <= 2)
+		return 1;
+
+	return payload_mtu + net_adj - 2;
+
 }
 EXPORT_SYMBOL_GPL(xfrm_state_mtu);
 
-- 
2.43.0