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

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Message-ID: <4A0CDA37.9040603@ct.jp.nec.com>
Date:	Fri, 15 May 2009 11:57:59 +0900
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	Subrata Modak <subrata@...ux.vnet.ibm.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sachin P Sant <sachinp@...ux.vnet.ibm.com>,
	Andi Kleen <ak@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] Fix Warnining in arch/x86/kernel/signal.c

Subrata Modak wrote:
> Hello Hiroshi-san,
> 
> On Thu, 2009-05-14 at 09:24 +0900, Hiroshi Shimamoto wrote:
> 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], ...))
>>
>>
> 
> How about now ? Thanks for pointing that out. My mistake ;-)
> 
> Signed-off-by: Subrata Modak <subrata@...ux.vnet.ibm.com>
> To: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,

looks good to me.
BTW who writes the description?

Acked-by: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>

> Cc: H. Peter Anvin <hpa@...or.com>,
> Cc: Ingo Molnar <mingo@...e.hu>,
> Cc: x86@...nel.org,
> Cc: Sachin P Sant <sachinp@...ux.vnet.ibm.com>,
> Cc: Andi Kleen <andi@...stfloor.org>,
> Cc: Andi Kleen <ak@...ux.intel.com>,
> Cc: Linux Kernel <linux-kernel@...r.kernel.org>,
> Cc: Balbir Singh <balbir@...ux.vnet.ibm.com>,
> Cc: Thomas Gleixner <tglx@...utronix.de>,
> Cc: Ingo Molnar <mingo@...hat.com>
> Subject: Re:[PATCH] Fix Warnining in arch/x86/kernel/signal.c
> ---
> 
> --- a/arch/x86/kernel/signal.c	2009-05-14 11:27:15.000000000 +0530
> +++ b/arch/x86/kernel/signal.c	2009-05-14 14:36:24.000000000 +0530
> @@ -576,9 +576,9 @@ unsigned long sys_sigreturn(struct pt_re
>  
>  	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 ((_NSIG_WORDS > 1 && __copy_from_user(&set.sig[1],
> +		&frame->extramask, sizeof(frame->extramask))) ||
> +		__get_user(set.sig[0], &frame->sc.oldmask))
>  		goto badframe;
>  
>  	sigdelsetmask(&set, ~_BLOCKABLE);
> 
> ---
> Regards--
> Subrata
> 
>> 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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