Re: [PATCH] powerpc/mm: Only read faulting instruction when necessary in do_page_fault()

[Christophe LEROY]

Le 24/04/2017 à 11:15, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
>
> > diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> > index 400f2d0d42f8..3d506589236c 100644
> > --- a/arch/powerpc/mm/fault.c
> > +++ b/arch/powerpc/mm/fault.c
> > @@ -356,8 +348,22 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
> >  		 * between the last mapped region and the stack will
> >  		 * expand the stack rather than segfaulting.
> >  		 */
> > -		if (address + 2048 < uregs->gpr[1] && !store_updates_sp(inst))
> > -			goto bad_area;
> > +		if (address + 2048 < uregs->gpr[1]) {
> > +			if (!inst) {
>
> That looks like it could lead to an infinite loop, if the instruction
> that caused the fault has been overwritten with NULL by another thread.

If the inst is NULL, store_updates_sp(inst) returns false so it jumps to 
bad_area_nosemaphore

Christophe

> Or did I miss something? (I only read the diff)
>
> > +				/*
> > +				 * We want to do this outside mmap_sem, because
> > +				 * reading code around nip can result in fault,
> > +				 * which will cause a deadlock when called with
> > +				 * mmap_sem held
> > +				 */
> > +				up_read(&mm->mmap_sem);
> > +				__get_user(inst,
> > +					   (unsigned int __user *)regs->nip);
> > +				if (!store_updates_sp(inst))
> > +					goto bad_area_nosemaphore;
> > +				goto retry;
> > +			}
>
>
> cheers