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

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Message-Id: <1242382591.7224.40.camel@subratamodak.linux.ibm.com>
Date:	Fri, 15 May 2009 15:46:31 +0530
From:	Subrata Modak <subrata@...ux.vnet.ibm.com>
To:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.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

On Fri, 2009-05-15 at 12:32 +0900, Hiroshi Shimamoto wrote:
> 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 ;-)
> 
> Hi Subrata, I have a question.
> Have you tried to compile on x86_64 whether the compiler claims the
> similar code in sys32_sigreturn() in arch/x86/ia32/ia32_signal.c?

Oops. No, the compiler does not complain here. It simply compiles fine. 

So, do you want to take a different view for the patch against
arch/x86/kernel/signal.c, or, i would resend it with the following
things fixed:


> looks good to me.
> BTW who writes the description?
> 
> Acked-by: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
> 

Regards--
Subrata

> 
> Thanks,
> Hiroshi

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