high_memory address in /proc/dri/*/vma

[Ben Hutchings]

On Mon, 2011-12-19 at 16:14 -0800, Kees Cook wrote:
> On Mon, Dec 19, 2011 at 4:09 PM, Ben Hutchings <ben at decadent.org.uk> wrote:
> > Kees, in commit 01e2f533a234dc62d16c0d3d4fb9d71cf1ce50c3 ("drm: do not
> > leak kernel addresses via /proc/dri/*/vma") you changed the logging of
> > high_memory:
> >
> > -       seq_printf(m, "vma use count: %d, high_memory = %p, 0x%08llx\n",
> > +       seq_printf(m, "vma use count: %d, high_memory = %pK, 0x%pK\n",
> >                   atomic_read(&dev->vma_count),
> > -                  high_memory, (u64)virt_to_phys(high_memory));
> > +                  high_memory, (void *)virt_to_phys(high_memory));
> >
> > This doesn't make sense because the physical address may be truncated
> > (in theory at least).
> 
> Leaking even a truncated address is still a problem, IMO. Or do you
> mean there is some side-effect causing a problem?

I mean that this the conversion to void * can be a narrowing conversion,
so when printing of kernel pointers is enabled the full physical address
may not be displayed.

> > I think it would make more sense to make this entire file readable by
> > root only, but I don't know whether anything depends on being able to
> > read it.  Its existence is conditional on DRM_DEBUG_CODE != 0 but that
> > is always true at the moment.
> 
> The kptr_restrict syscall (that controls %pK behavior) has 3 modes,
> including one that hides these values even from the root user, so I
> would prefer this stays as-is.
> 
> Sorry I'm being dense, but what problem is %pK causing here? I'd be
> happy to help get it fixed.

The problem is that it is not suitable for printing physical addresses,
because they are not pointers.
 
Ben.

-- 
Ben Hutchings
Humans are not rational beings; they are rationalising beings.
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