netdev - Re: [PATCH v2] tcp: restrict net.ipv4.tcp_adv_win

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Date:	Sun, 28 Nov 2010 10:39:20 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	adobriyan@...il.com
Cc:	eric.dumazet@...il.com, shemminger@...ux-foundation.org,
	netdev@...r.kernel.org, bhutchings@...arflare.com
Subject: Re: [PATCH v2] tcp: restrict net.ipv4.tcp_adv_win_scale (#20312)

From: Alexey Dobriyan <adobriyan@...il.com>
Date: Tue, 23 Nov 2010 00:54:21 +0200

> tcp_win_from_space() does the following:
> 
>       if (sysctl_tcp_adv_win_scale <= 0)
>               return space >> (-sysctl_tcp_adv_win_scale);
>       else
>               return space - (space >> sysctl_tcp_adv_win_scale);
> 
> "space" is int.
> 
> As per C99 6.5.7 (3) shifting int for 32 or more bits is
> undefined behaviour.
> 
> Indeed, if sysctl_tcp_adv_win_scale is exactly 32,
> space >> 32 equals space and function returns 0.
> 
> Which means we busyloop in tcp_fixup_rcvbuf().
> 
> Restrict net.ipv4.tcp_adv_win_scale to [-31, 31].
> 
> Fix https://bugzilla.kernel.org/show_bug.cgi?id=20312
> 
> Steps to reproduce:
> 
>       echo 32 >/proc/sys/net/ipv4/tcp_adv_win_scale
>       wget www.kernel.org
>       [softlockup]
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>

I'll aply this, thanks Alexey.
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