netfilter: nftables: exthdr: fix 4-byte stack OOB write

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@strlen.de>

diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
index a9844eefedebc63b7b6e854534e82261fb00e966..3fbaa7bf41f9c74956c826ee63797406bff9f297 100644 (file)
--- a/net/netfilter/nft_exthdr.c
+++ b/net/netfilter/nft_exthdr.c
@@ -35,6 +35,14 @@ 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)
@@ -56,8 +64,7 @@ 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:
@@ -153,8 +160,7 @@ 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:
@@ -210,7 +216,8 @@ 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);
                }
 
@@ -388,9 +395,8 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
                            offset + ntohs(sch->length) > pkt->skb->len)
                                break;
 
-                       dest[priv->len / NFT_REG32_SIZE] = 0;
-                       if (skb_copy_bits(pkt->skb, offset + priv->offset,
-                                         dest, priv->len) < 0)
+                       if (nft_skb_copy_to_reg(pkt->skb, offset + priv->offset,
+                                               dest, priv->len) < 0)
                                break;
                        return;
                }