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

[Subrata Modak <subrata@linux.vnet.ibm.com>]

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], ...))
> 
> 

I tried out and the compiler does not complain in this case.
Updated Patch below. Please review.

Signed-Off-By: Subrata Modak <subrata@linux.vnet.ibm.com>
To: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>,
Cc: H. Peter Anvin <hpa@zytor.com>,
Cc: Ingo Molnar <mingo@elte.hu>,
Cc: x86@kernel.org,
Cc: Sachin P Sant <sachinp@linux.vnet.ibm.com>,
Cc: Andi Kleen <andi@firstfloor.org>,
Cc: Andi Kleen <ak@linux.intel.com>,
Cc: Linux Kernel <linux-kernel@vger.kernel.org>,
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>,
Cc: Thomas Gleixner <tglx@linutronix.de>,
Cc: Ingo Molnar <mingo@redhat.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 11:50:52.000000000 +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