linux-kernel - Re: [PATCH] Fix Warnining in arch/x86/kernel/signal.c

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Date:	Thu, 14 May 2009 09:24:12 +0900
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Subrata Modak <subrata@...ux.vnet.ibm.com>, x86@...nel.org,
	Sachin P Sant <sachinp@...ux.vnet.ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] Fix Warnining in arch/x86/kernel/signal.c

H. Peter Anvin wrote:
> Ingo Molnar wrote:
>>>>>  
>>>>>  	if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
>>>>>  		goto badframe;
>>>>> -	if (__get_user(set.sig[0], &frame->sc.oldmask) || (_NSIG_WORDS > 1
>>>>> -		&& __copy_from_user(&set.sig[1], &frame->extramask,
>>>>> -				    sizeof(frame->extramask))))
>>>>> +
>>>>> +        if ( (__copy_from_user(&set.sig[1], &frame->extramask,
>>>>> +                sizeof(frame->extramask)) && _NSIG_WORDS > 1) || 
>>>>> +                __get_user(set.sig[0], &frame->sc.oldmask))
>>>>>  		goto badframe;
>>>> I'm not sure why this eliminates that warning.
>>>> set.sig[0] may not be initialized too, if __copy_from_user() failed.
>>> True, but only when either or both of __copy_from_user() and
>>> (_NSIG_WORDS > 1) fails. But in all instances set.sig[1] gets
>>> initialized.
>>>
>>>> I don't have enough time to look at this right now, sorry.
>>>>
>>>> Another question, __copy_from_user() will be called even if
>>>> _NSIG_WORDS is less than 2, perhaps it never occurs.
>>>> I think, to check _NSIG_WORDS > 1 before calling __copy_from_user()
>>>> is better.
>>> Fine. Let Ingo/Thomas/Peter decide whether they would like this fix or
>>> drop it.
>> If you get the Acked-by from Hiroshi-san it looks good to me. He 
>> modified this code last.
>>
> 
> This seriously looks wrong to me.  If _NSIG_WORDS == 1, then calling
> __copy_from_user here is a serious error.

Right. If _NSIG_WORDS is 1, sigset_t set has only sig[0], writing to
set.sig[1] means stack corruption.

Subrata, could you try like this?
if ((_NSIG_WORDS > 1 && __copy_from_user(&set.sig[1], ...) ||
	__get_user(set.sig[0], ...))

I wonder whether gcc really complains about the case of
__get_user(set.sig[0], ...) failure.
Why, the case which sig[0] initialized and sig[1] uninitialized is NG
and the case which sig[0] uninitialized and sig[1] initialized is OK.

Thanks,
Hiroshi
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