netdev - Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie

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Date:	Mon, 02 Nov 2009 11:56:06 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	William Allen Simpson <william.allen.simpson@...il.com>
CC:	Linux Kernel Developers <linux-kernel@...r.kernel.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie

William Allen Simpson a écrit :
>
> The (buggy?) syncookie code that I'm avoiding/replacing is:
> 
> static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
>               ipv4_cookie_scratch);
> 
> static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16
> dport,
>                u32 count, int c)
> {
>     __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
> 
>     memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
>     tmp[0] = (__force u32)saddr;
>     tmp[1] = (__force u32)daddr;
>     tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
>     tmp[3] = count;
>     sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
> 
>     return tmp[17];
> }
> 
> It appears to me that the operations could be interrupted at any time, and
> the fairly expensive sha transform (or probably any of the assignments)
> could be corrupted?
> 
> That is, cpu0 grabs a scratch area, copies some data into it, interrupts,
> cpu0 comes along again with another packet, points into the same workspace,
> mashes the data, while cpu1 completes the previous operation with the
> old tmp pointer on the stack.
> 
> Worse, this is called twice, each time getting tmp, and mashing the data,
> and at the same time others are calling it twice more for verification.
> 
> Since syncookies only occur under stress, the code isn't well tested, and
> the only symptom would be the returned packet would be dropped after
> failing to verify.  Since this only happens when lots of packets are
> arriving, dropped packets probably aren't noticed.
> 
> However, that would be unacceptable for my code.
>


cookie_hash() runs in a non preemptable context. CPU cannot change under us.

(or else, we would not use __get_cpu_var(ipv4_cookie_scratch); )

And of course, each cpu gets its own scratch area, thanks to __get_cpu_var()

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