APPLIED: [SRU Mantic] netfilter: nftables: exthdr: fix 4-byte stack OOB write

[Stefan Bader]

On 14.09.23 21:06, Thadeu Lima de Souza Cascardo wrote:
> 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>
> (cherry picked from commit fd94d9dadee58e09b49075240fe83423eb1dcd36)
> CVE-2023-4881
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
> ---

Applied to mantic:linux/master-next. Thanks.

-Stefan

>   net/netfilter/nft_exthdr.c | 22 ++++++++++++++--------
>   1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
> index 7f856ceb3a66..da8c14f71ddd 100644
> --- 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);
>   		}
>   
> @@ -392,9 +399,8 @@ 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;
>   		}

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