Re: [PATCH] drm/komeda: Fix an undefined behavior bug in komeda_plane_add()

[Liviu Dudau]

On Wed, Dec 01, 2021 at 03:44:03PM +0000, Steven Price wrote:
> On 30/11/2021 14:25, Zhou Qingyang wrote:
> > In komeda_plane_add(), komeda_get_layer_fourcc_list() is assigned to
> > formats and used in drm_universal_plane_init().
> > drm_universal_plane_init() passes formats to
> > __drm_universal_plane_init(). __drm_universal_plane_init() further
> > passes formats to memcpy() as src parameter, which could lead to an
> > undefined behavior bug on failure of komeda_get_layer_fourcc_list().
> > 
> > Fix this bug by adding a check of formats.
> > 
> > This bug was found by a static analyzer. The analysis employs
> > differential checking to identify inconsistent security operations
> > (e.g., checks or kfrees) between two code paths and confirms that the
> > inconsistent operations are not recovered in the current function or
> > the callers, so they constitute bugs.
> > 
> > Note that, as a bug found by static analysis, it can be a false
> > positive or hard to trigger. Multiple researchers have cross-reviewed
> > the bug.
> > 
> > Builds with CONFIG_DRM_KOMEDA=m show no new warnings,
> > and our static analyzer no longer warns about this code.
> > 
> > Fixes: 61f1c4a8ab75 ("drm/komeda: Attach komeda_dev to DRM-KMS")
> > Signed-off-by: Zhou Qingyang <zhou1615@xxxxxxx>
> > ---
> >  drivers/gpu/drm/arm/display/komeda/komeda_plane.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/arm/display/komeda/komeda_plane.c b/drivers/gpu/drm/arm/display/komeda/komeda_plane.c
> > index d63d83800a8a..dd3f17e970dd 100644
> > --- a/drivers/gpu/drm/arm/display/komeda/komeda_plane.c
> > +++ b/drivers/gpu/drm/arm/display/komeda/komeda_plane.c
> > @@ -265,6 +265,10 @@ static int komeda_plane_add(struct komeda_kms_dev *kms,
> >  
> >  	formats = komeda_get_layer_fourcc_list(&mdev->fmt_tbl,
> >  					       layer->layer_type, &n_formats);
> > +	if (!formats) {
> > +		err = -ENOMEM;
> > +		goto cleanup;
> > +	}
> 
> If this executes it will cause undefined behaviour...
> 
> The cleanup code calls komeda_plane_destroy() which calls
> drm_plane_cleanup() which does (amongst other things) a
> list_del(&plane->head). But the plane hasn't been put on a list yet as
> that's done in drm_universal_plane_init().
> 
> So in this case we simple want to do:
> 
> if (!formats) {
> 	kfree(kplane);
> 	return -ENOMEM;
> }

Zhou has already posted v2 that contains this fix.

> 
> Note that without this 'fix' a NULL return from
> komeda_get_layer_fourcc_list() would leave n_formats==0, so while the
> NULL pointer is passed into memcpy() it is also passed a length of 0.
> Which I believe is safe.
> 
> However while looking at this function...
> 
> >  
> >  	err = drm_universal_plane_init(&kms->base, plane,
> >  			get_possible_crtcs(kms, c->pipeline),
> > 
> 
> This call to drm_universal_plane_init() can fail early before
> plane->head has been initialised. In which case the following:
> 
> > 	komeda_put_fourcc_list(formats);
> > 
> > 	if (err)
> > 		goto cleanup;
> 
> commits the exact same sin and would cause a similar NULL dereference in
> drm_plane_cleanup().

I will come up with a patch for this case and post it to the list tomorrow.

Best regards,
Liviu



> 
> Steve

-- 
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