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

[Hiroshi Shimamoto]

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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