linux-kernel - Re: [RFC c/r 4/4] c/r: prctl: Extend PR_SET_MM to set up more mm

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Message-ID: <4F23099D.1090606@jp.fujitsu.com>
Date:	Fri, 27 Jan 2012 15:31:25 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	gorcunov@...nvz.org
CC:	keescook@...omium.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, ebiederm@...ssion.com, xemul@...nvz.org,
	mtk.manpages@...il.com, tj@...nel.org, avagin@...nvz.org,
	serge.hallyn@...onical.com, xemul@...allels.com,
	segoon@...nwall.com, kamezawa.hiroyu@...fujitsu.com
Subject: Re: [RFC c/r 4/4] c/r: prctl: Extend PR_SET_MM to set up more mm_struct
 entries

>>> +
>>> +       case PR_SET_MM_AUXV:
>>> +               if (arg4 > sizeof(mm->saved_auxv))
>>> +                       goto out;
>>> +               up_read(&mm->mmap_sem);
>>> +
>>> +               error = -EFAULT;
>>> +               if (!copy_from_user(mm->saved_auxv, (const void __user *)addr, arg4))
>>> +                       error = 0;
>>> +
>>> +               return error;
>>
>> Is the mmap_sem released here because of the copy_from_user()? Is it
>> still safe to write to saved_auxv?
>>
> 
> At moment I believe yes (if only I'm not missing something), we poke this
> vector at elf loading procedure, so it's up to user to sync access to "own"
> saved_auxv and write sane values inside.

Think prctl(PR_SET_MM_AUXV) vs prctl(PR_SET_MM_AUXV) race. That might make to write corrupted value.
syscall should be atomic from a point of userland view.
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