[PATCH net 13/13] netfilter: nft_payload: fix mask build for partial field offload

From: "Xiang Mei (Microsoft)" <[email protected]>

nft_payload_offload_mask() builds the offload match mask for a payload
expression that covers only part of a header field.  For a partial IPv6
address match (field_len = 16, priv_len = 1) that shift is 1 << 120, which
is undefined on the 32-bit int operand.  It also trims only one word, so
the remaining words stay 0xffffffff (and when priv_len is a multiple of 4
the trim is skipped entirely), leaving the mask covering more bytes than
the rule matches.

  UBSAN: shift-out-of-bounds in net/netfilter/nft_payload.c:278:20
  shift exponent 120 is too large for 32-bit type 'int'
  ...

The match is byte-granular and struct nft_data is zero-initialised, so the
correct mask is simply the first priv_len bytes set to 0xff. Set those
bytes directly and drop the word/shift trimming; this removes the undefined
shift and no longer over-masks the trailing bytes.

Fixes: a5d45bc0dc50 ("netfilter: nftables_offload: build mask based from the matching bytes")
Reported-by: [email protected]
Signed-off-by: Xiang Mei (Microsoft) <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>
---
 net/netfilter/nft_payload.c | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/net/netfilter/nft_payload.c b/net/netfilter/nft_payload.c
index 391539a1ceaa..8a4472fd77d9 100644
--- a/net/netfilter/nft_payload.c
+++ b/net/netfilter/nft_payload.c
@@ -259,9 +259,7 @@ static int nft_payload_dump(struct sk_buff *skb,
 static bool nft_payload_offload_mask(struct nft_offload_reg *reg,
 				     u32 priv_len, u32 field_len)
 {
-	unsigned int remainder, delta, k;
 	struct nft_data mask = {};
-	__be32 remainder_mask;
 
 	if (priv_len == field_len) {
 		memset(&reg->mask, 0xff, priv_len);
@@ -270,15 +268,7 @@ static bool nft_payload_offload_mask(struct nft_offload_reg *reg,
 		return false;
 	}
 
-	memset(&mask, 0xff, field_len);
-	remainder = priv_len % sizeof(u32);
-	if (remainder) {
-		k = priv_len / sizeof(u32);
-		delta = field_len - priv_len;
-		remainder_mask = htonl(~((1 << (delta * BITS_PER_BYTE)) - 1));
-		mask.data[k] = (__force u32)remainder_mask;
-	}
-
+	memset(&mask, 0xff, priv_len);
 	memcpy(&reg->mask, &mask, field_len);
 
 	return true;
-- 
2.47.3