Re: [PATCH] drm: check for NULL parameter in exported drm_get_format_name() function.

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Tue, Nov 22, 2016 at 01:15:08PM -0500, Sean Paul wrote:
> On Tue, Nov 22, 2016 at 1:06 PM, Ville Syrjälä
> <ville.syrjala@linux.intel.com> wrote:
> > On Tue, Nov 22, 2016 at 12:35:53PM -0500, Rob Clark wrote:
> >> On Tue, Nov 22, 2016 at 12:31 PM, Ville Syrjälä
> >> <ville.syrjala@linux.intel.com> wrote:
> >> > On Tue, Nov 22, 2016 at 12:23:59PM -0500, Rob Clark wrote:
> >> >> On Tue, Nov 22, 2016 at 11:50 AM, Ville Syrjälä
> >> >> <ville.syrjala@linux.intel.com> wrote:
> >> >> > On Tue, Nov 22, 2016 at 04:41:06PM +0000, Liviu Dudau wrote:
> >> >> >> drm_get_format_name() de-references the buf parameter without checking
> >> >> >> if the pointer was not NULL. Given that the function is EXPORT-ed, lets
> >> >> >> sanitise the parameters before proceeding.
> >> >> >>
> >> >> >> Fixes: b3c11ac267d461d3d5 ("drm: move allocation out of drm_get_format_name())
> >> >> >> Cc: Eric Engestrom <eric@engestrom.ch>
> >> >> >> Cc: Rob Clark <robdclark@gmail.com>
> >> >> >> Cc: Jani Nikula <jani.nikula@intel.com>
> >> >> >> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >> >> >>
> >> >> >> Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
> >> >> >> ---
> >> >> >>  drivers/gpu/drm/drm_fourcc.c | 3 +++
> >> >> >>  1 file changed, 3 insertions(+)
> >> >> >>
> >> >> >> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> >> >> >> index 90d2cc8..0a3ff0b 100644
> >> >> >> --- a/drivers/gpu/drm/drm_fourcc.c
> >> >> >> +++ b/drivers/gpu/drm/drm_fourcc.c
> >> >> >> @@ -85,6 +85,9 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
> >> >> >>   */
> >> >> >>  const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
> >> >> >>  {
> >> >> >> +     if (!buf)
> >> >> >> +             return NULL;
> >> >> >> +
> >> >> >
> >> >> > Seems rather pointless to me. Why would you ever pass NULL to this guy?

I don't know, maybe because I'm a malicious driver writer that wants to crash the kernel?
The function is exported, so it can be called by any module, even out-of-tree. Looks
like an easy DoS to me.

> >> >>
> >> >> perhaps BUG_ON(!buf)...
> >> >
> >> > And how does that differ from just buf->foo?
> >>
> >> it gets you a file and line # in the error splat.. not that
> >> drm_get_format_name() is such a big function that it would be
> >> difficult to decipher the null deref crash, but if we added anything
> >> it should be BUG_ON() to make it clear that passing null isn't a
> >> caller error.
> >
> > Yeah, BUG_ON() at least documents the intent, so it's better than
> > the null check. But for something like this even BUG_ON() is
> > just wasted bytes IMO.
> 
> +1
> 
> The patch has a Fixes line, but AFAICT, the referenced patch didn't
> introduce any NULL opportunities that weren't previously there.

Really? Before the referenced patch the function only had one parameter,
an uint32_t. True that kmalloc could return a NULL pointer and we would've
crashed there as well, but at least we were running out of memory. Now
I can crash the kernel any time I want with a badly written driver/module.

Best regards,
Liviu

> 
> Sean
> 
> 
> >
> > BUG_ON() can be useful for those weird bugs where somewhere deep
> > down you hit a null pointer and you can't figure out where the
> > bad pointer came from. So you might sprinkle a few BUG_ONs()
> > further up to catch it sooner. Esp. if you can't reproduce the
> > bug yourself and have to rely on user(s) to find it for you.
> >
> > Even WARN_ON() w/ or w/o an early bailout might be a decent idea
> > sometimes since it might have a slightly higher chance of keeping
> > the kernel in working condition, but IMO just blindly throwing
> > it around everywhere is not a good approach.
> >
> > --
> > Ville Syrjälä
> > Intel OTC
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
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