Re: [PATCH] x86/traps: fix an off-by-one error

[Hongyan Xia <hx242@xxxxxxx>]

On Tue, 2020-05-05 at 15:38 +0200, Jan Beulich wrote:
> On 05.05.2020 13:06, Hongyan Xia wrote:
> > --- a/xen/arch/x86/traps.c
> > +++ b/xen/arch/x86/traps.c
> > @@ -300,6 +300,7 @@ static void show_guest_stack(struct vcpu *v,
> > const struct cpu_user_regs *regs)
> >      int i;
> >      unsigned long *stack, addr;
> >      unsigned long mask = STACK_SIZE;
> > +    void *stack_page = NULL;
> >  
> >      /* Avoid HVM as we don't know what the stack looks like. */
> >      if ( is_hvm_vcpu(v) )
> > @@ -328,7 +329,7 @@ static void show_guest_stack(struct vcpu *v,
> > const struct cpu_user_regs *regs)
> >          vcpu = maddr_get_owner(read_cr3()) == v->domain ? v :
> > NULL;
> >          if ( !vcpu )
> >          {
> > -            stack = do_page_walk(v, (unsigned long)stack);
> > +            stack_page = stack = do_page_walk(v, (unsigned
> > long)stack);
> >              if ( (unsigned long)stack < PAGE_SIZE )
> >              {
> >                  printk("Inaccessible guest memory.\n");
> > @@ -358,7 +359,7 @@ static void show_guest_stack(struct vcpu *v,
> > const struct cpu_user_regs *regs)
> >      if ( mask == PAGE_SIZE )
> >      {
> >          BUILD_BUG_ON(PAGE_SIZE == STACK_SIZE);
> > -        unmap_domain_page(stack);
> > +        unmap_domain_page(stack_page);
> >      }
> 
> With this I think you want to change the whole construct here to
> 
>     if ( stack_page )
>         unmap_domain_page(stack_page);
> 
> i.e. with the then no longer relevant BUILD_BUG_ON() also dropped.

I wonder if such a construct is better with UNMAP_DOMAIN_PAGE(), since
it deals with NULL and will nullify it to prevent misuse.

> What's more important though - please also fix the same issue in
> compat_show_guest_stack(). Unless I'm mistaken of course, in which
> case it would be nice if the description could mention why the
> other similar function isn't affected.

Compat suffers from the same problem. Thanks for pointing that out.

Hongyan