linux-kernel - Re: possible ERESTARTNOHAND leak into userspace

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Message-ID: <20111223150227.GA27059@redhat.com>
Date:	Fri, 23 Dec 2011 16:02:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Anders Johansson <ajohansson@...ell.com>,
	David Miller <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: possible ERESTARTNOHAND leak into userspace

On 12/23, Michal Hocko wrote:
>
> Hi,
> this has already been discussed few years back with reports that select
> returned with ERESTARTNOHAND in multi-threaded applications
> (http://forum.soft32.com/linux/PATCH-select-fix-sys_select-leak-ERESTARTNOHAND-userspace-ftopict338572.html)
>
> Dave has come up with a possible explanation of the race but there was
> no further follow up with a conclusion.
>
> Just for reference:
>         Thread_A				Thread_B
> 	CPU0					CPU1
> 						syscall_XYZ
> core_sys_select
>   ret = -ERESTARTNOHAND;
>   if (signal_pending(current))
>      					do_notify_resume
>      					  do_signal (clear signal pending)

"clear signal pending" can't affect Thread_A. Even if it steals
the signal sent to Thread_A.

>   return ret;
> return from syscall
> no pending signal

please see above. Only the task itself can clear its TIF_DIGPENDING.

> return ERESTARTNOHAND

do_signal() should take care and restart the syscall.

> The race window is rather small and hard to trigger but we have seen
> reports where people really saw select returning ERESTARTNOHAND (on
> 2.6.16 based kernel - x86_64).
> I am not able to reproduce that myself neither with .16 kernel nor with
> the current vanilla so I am not sure whether the problem has been fixed
> already. But I do not see what prevents the race with vanilla.

I hope the problem was already fixed, at least I do not see anything
wrong in core_sys_select().

Oleg.

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