Re: [PATCH net] nfnetlink_osf: validate individual option lengths in fingerprints

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Mar 19, 2026 at 03:32:44PM +0800, bestswngs@gmail.com wrote:
> From: Weiming Shi <bestswngs@gmail.com>
> 
> nfnl_osf_add_callback() validates opt_num bounds and string
> NUL-termination but does not check individual option length fields.
> A zero-length option causes nf_osf_match_one() to enter the option
> matching loop even when foptsize sums to zero, which matches packets
> with no TCP options where ctx->optp is NULL:
> 
>  Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
>  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
>  RIP: 0010:nf_osf_match_one (net/netfilter/nfnetlink_osf.c:98)
>  Call Trace:
>   <IRQ>
>   nf_osf_match (net/netfilter/nfnetlink_osf.c:227)
>   xt_osf_match_packet (net/netfilter/xt_osf.c:32)
>   ipt_do_table (net/ipv4/netfilter/ip_tables.c:293)
>   nf_hook_slow (net/netfilter/core.c:623)
>   ip_local_deliver (net/ipv4/ip_input.c:262)
>   ip_rcv (net/ipv4/ip_input.c:573)
>   </IRQ>
>  Kernel panic - not syncing: Fatal exception in interrupt
> 
> Additionally, an MSS option (kind=2) with length < 4 causes
> out-of-bounds reads when nf_osf_match_one() unconditionally accesses
> optp[2] and optp[3] for MSS value extraction.  While RFC 9293
> section 3.2 specifies that the MSS option is always exactly 4
> bytes (Kind=2, Length=4), the check uses "< 4" rather than
> "!= 4" because lengths greater than 4 do not cause memory
> safety issues -- the buffer is guaranteed to be at least
> foptsize bytes by the ctx->optsize == foptsize check.
> 
> Reject fingerprints where any option has zero length, or where an MSS
> option has length less than 4, at add time rather than trusting these
> values in the packet matching hot path.
> 
> Fixes: 11eeef41d5f6 ("netfilter: passive OS fingerprint xtables match")
> Reported-by: Xiang Mei <xmei5@asu.edu>
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>
> ---
>  net/netfilter/nfnetlink_osf.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/netfilter/nfnetlink_osf.c b/net/netfilter/nfnetlink_osf.c
> index 94e3eac5743a..c35b831dac5a 100644
> --- a/net/netfilter/nfnetlink_osf.c
> +++ b/net/netfilter/nfnetlink_osf.c
> @@ -303,6 +303,7 @@ static int nfnl_osf_add_callback(struct sk_buff *skb,
>  	struct nf_osf_user_finger *f;
>  	struct nf_osf_finger *kf = NULL, *sf;
>  	int err = 0;
> +	int i;
> 
>  	if (!capable(CAP_NET_ADMIN))
>  		return -EPERM;
> @@ -318,6 +319,13 @@ static int nfnl_osf_add_callback(struct sk_buff *skb,
>  	if (f->opt_num > ARRAY_SIZE(f->opt))
>  		return -EINVAL;
> 
> +	for (i = 0; i < f->opt_num; i++) {
> +		if (!f->opt[i].length)
> +			return -EINVAL;
> +		if (f->opt[i].kind == OSFOPT_MSS && f->opt[i].length < 4)
> +			return -EINVAL;
> +	}
> +
>  	if (!memchr(f->genre, 0, MAXGENRELEN) ||
>  	    !memchr(f->subtype, 0, MAXGENRELEN) ||
>  	    !memchr(f->version, 0, MAXGENRELEN))
> --
> 2.43.0
> 


Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
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