[SRU][Focal][PATCH 1/1] netfilter: nftables: exthdr: fix 4-byte stack OOB write

[Yuxuan Luo]

From: Florian Westphal <fw at strlen.de>

If priv->len is a multiple of 4, then dst[len / 4] can write past
the destination array which leads to stack corruption.

This construct is necessary to clean the remainder of the register
in case ->len is NOT a multiple of the register size, so make it
conditional just like nft_payload.c does.

The bug was added in 4.1 cycle and then copied/inherited when
tcp/sctp and ip option support was added.

Bug reported by Zero Day Initiative project (ZDI-CAN-21950,
ZDI-CAN-21951, ZDI-CAN-21961).

Fixes: 49499c3e6e18 ("netfilter: nf_tables: switch registers to 32 bit addressing")
Fixes: 935b7f643018 ("netfilter: nft_exthdr: add TCP option matching")
Fixes: 133dc203d77d ("netfilter: nft_exthdr: Support SCTP chunks")
Fixes: dbb5281a1f84 ("netfilter: nf_tables: add support for matching IPv4 options")
Signed-off-by: Florian Westphal <fw at strlen.de>
(backported from commit fd94d9dadee58e09b49075240fe83423eb1dcd36)
[yuxuan.luo: ignore nft_exthdr_sctp_eval hunks as it is yet to be introduced
 in this tree.
]
Signed-off-by: Yuxuan Luo <yuxuan.luo at canonical.com>
---
 net/netfilter/nft_exthdr.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
index 670dd146fb2b1..ca268293cfa12 100644
--- a/net/netfilter/nft_exthdr.c
+++ b/net/netfilter/nft_exthdr.c
@@ -33,6 +33,14 @@ static unsigned int optlen(const u8 *opt, unsigned int offset)
 		return opt[offset + 1];
 }
 
+static int nft_skb_copy_to_reg(const struct sk_buff *skb, int offset, u32 *dest, unsigned int len)
+{
+	if (len % NFT_REG32_SIZE)
+		dest[len / NFT_REG32_SIZE] = 0;
+
+	return skb_copy_bits(skb, offset, dest, len);
+}
+
 static void nft_exthdr_ipv6_eval(const struct nft_expr *expr,
 				 struct nft_regs *regs,
 				 const struct nft_pktinfo *pkt)
@@ -54,8 +62,7 @@ static void nft_exthdr_ipv6_eval(const struct nft_expr *expr,
 	}
 	offset += priv->offset;
 
-	dest[priv->len / NFT_REG32_SIZE] = 0;
-	if (skb_copy_bits(pkt->skb, offset, dest, priv->len) < 0)
+	if (nft_skb_copy_to_reg(pkt->skb, offset, dest, priv->len) < 0)
 		goto err;
 	return;
 err:
@@ -151,8 +158,7 @@ static void nft_exthdr_ipv4_eval(const struct nft_expr *expr,
 	}
 	offset += priv->offset;
 
-	dest[priv->len / NFT_REG32_SIZE] = 0;
-	if (skb_copy_bits(pkt->skb, offset, dest, priv->len) < 0)
+	if (nft_skb_copy_to_reg(pkt->skb, offset, dest, priv->len) < 0)
 		goto err;
 	return;
 err:
@@ -208,7 +214,8 @@ static void nft_exthdr_tcp_eval(const struct nft_expr *expr,
 		if (priv->flags & NFT_EXTHDR_F_PRESENT) {
 			*dest = 1;
 		} else {
-			dest[priv->len / NFT_REG32_SIZE] = 0;
+			if (priv->len % NFT_REG32_SIZE)
+				dest[priv->len / NFT_REG32_SIZE] = 0;
 			memcpy(dest, opt + offset, priv->len);
 		}
 
-- 
2.34.1