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Date: Tue, 3 Sep 2019 09:41:48 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Cyrus Sh <sirus.shahini@...il.com>, davem@...emloft.net
Cc: shiraz.saleem@...el.com, jgg@...pe.ca, arnd@...db.de,
netdev@...r.kernel.org, sirus@...utah.edu
Subject: Re: [PATCH] Clock-independent TCP ISN generation
On 9/3/19 7:06 AM, Cyrus Sh wrote:
> This patch addresses the privacy issue of TCP ISN generation in Linux
> kernel. Currently an adversary can deanonymize a user behind an anonymity
> network by inducing a load pattern on the target machine and correlating
> its clock skew with the pattern. Since the kernel adds a clock-based
> counter to generated ISNs, the adversary can observe SYN packets with
> similar IP and port numbers to find out the clock skew of the target
> machine and this can help them identify the user. To resolve this problem
> I have changed the related function to generate the initial sequence
> numbers randomly and independent from the cpu clock. This feature is
> controlled by a new sysctl option called "tcp_random_isn" which I've added
> to the kernel. Once enabled the initial sequence numbers are guaranteed to
> be generated independently from each other and from the hardware clock of
> the machine. If the option is off, ISNs are generated as before. To get
> more information about this patch and its effectiveness you can refer to my
> post here:
> https://bitguard.wordpress.com/?p=982
<quote>
Firstly it’s unlikely that this happens at all,
</quote>
Sorry this happens all the time.
Some people use very disturbing setups really, and they are not trying to be malicious.
Clock skew seems quite secondary. Some firewall rules should prevent this kind of attacks ?
> and to see a discussion about the issue you can read this:
> https://trac.torproject.org/projects/tor/ticket/16659
Four years old discussion. Does not seem urgent matter :/
>
> Signed-off-by: Sirus Shahini <sirus.shahini@...il.com>
> ---
> include/net/tcp.h | 1 +
> include/uapi/linux/sysctl.h | 1 +
> kernel/sysctl_binary.c | 1 +
> net/core/secure_seq.c | 24 +++++++++++++++++++++++-
> net/ipv4/sysctl_net_ipv4.c | 7 +++++++
> net/ipv4/tcp_input.c | 1 +
> 6 files changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 81e8ade..4ad1bbf 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -241,6 +241,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
>
> /* sysctl variables for tcp */
> extern int sysctl_tcp_max_orphans;
> +extern int sysctl_tcp_random_isn;
All TCP sysctls are per netns these days. (Look in include/net/netns/ipv4.h )
> extern long sysctl_tcp_mem[3];
>
> #define TCP_RACK_LOSS_DETECTION 0x1 /* Use RACK to detect losses */
> diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
> index 87aa2a6..ba8927e 100644
> --- a/include/uapi/linux/sysctl.h
> +++ b/include/uapi/linux/sysctl.h
> @@ -426,6 +426,7 @@ enum
> NET_TCP_ALLOWED_CONG_CONTROL=123,
> NET_TCP_MAX_SSTHRESH=124,
> NET_TCP_FRTO_RESPONSE=125,
> + NET_IPV4_TCP_RANDOM_ISN=126,
Nope, we do not add new sysctls there anymore.
Everybody should now have /proc files to tune the values.
> };
>
> enum {
> diff --git a/kernel/sysctl_binary.c b/kernel/sysctl_binary.c
> index 73c1320..0faf7d4 100644
> --- a/kernel/sysctl_binary.c
> +++ b/kernel/sysctl_binary.c
> @@ -332,6 +332,7 @@ static const struct bin_table bin_net_ipv4_netfilter_table[] = {
> };
>
> static const struct bin_table bin_net_ipv4_table[] = {
> + {CTL_INT, NET_IPV4_TCP_RANDOM_ISN "tcp_random_isn"}
Same remark. We no longer add stuff there.
> {CTL_INT, NET_IPV4_FORWARD, "ip_forward" },
>
> { CTL_DIR, NET_IPV4_CONF, "conf", bin_net_ipv4_conf_table },
> diff --git a/net/core/secure_seq.c b/net/core/secure_seq.c
> index 7b6b1d2..b644bbe 100644
> --- a/net/core/secure_seq.c
> +++ b/net/core/secure_seq.c
> @@ -22,6 +22,7 @@
>
> static siphash_key_t net_secret __read_mostly;
> static siphash_key_t ts_secret __read_mostly;
> +static siphash_key_t last_secret = {{0,0}} ;
>
> static __always_inline void net_secret_init(void)
> {
> @@ -134,8 +135,29 @@ u32 secure_tcp_seq(__be32 saddr, __be32 daddr,
> __be16 sport, __be16 dport)
> {
> u32 hash;
> -
> + u32 temp;
> +
> net_secret_init();
> +
> + if (sysctl_tcp_random_isn){
> + if (!last_secret.key[0] && !last_secret.key[1]){
> + memcpy(&last_secret,&net_secret,sizeof(last_secret));
> +
> + }else{
Check your patch using scripts/checkpatch.pl
All these missing spaces should be spotted.
> + temp = *((u32*)&(net_secret.key[0]));
> + temp >>= 8;
> + last_secret.key[0]+=temp;
> + temp = *((u32*)&(net_secret.key[1]));
> + temp >>= 8;
> + last_secret.key[1]+=temp;
Why not simply use an official random generator, instead of these convoluted
and not SMP safe attempts ?
Check drivers/char/random.c for officially maintained generators.
> + }
> + hash = siphash_3u32((__force u32)saddr, (__force u32)daddr,
> + (__force u32)sport << 16 | (__force u32)dport,
> + &last_secret);
> + return hash;
> + }
> +
> +
> hash = siphash_3u32((__force u32)saddr, (__force u32)daddr,
> (__force u32)sport << 16 | (__force u32)dport,
> &net_secret);
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