Re: [PATCH] drm/sti: Fix return type of sti_{dvo,hda,hdmi}_connector_mode_valid()

[Kees Cook <keescook@chromium.org>]

On Mon, Nov 14, 2022 at 03:03:47PM -0700, Nathan Chancellor wrote:
> Hi all,
> 
> On Wed, Nov 02, 2022 at 08:56:23AM -0700, Nathan Chancellor wrote:
> > With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
> > indirect call targets are validated against the expected function
> > pointer prototype to make sure the call target is valid to help mitigate
> > ROP attacks. If they are not identical, there is a failure at run time,
> > which manifests as either a kernel panic or thread getting killed. A
> > proposed warning in clang aims to catch these at compile time, which
> > reveals:
> > 
> >   drivers/gpu/drm/sti/sti_hda.c:637:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_connector *, struct drm_display_mode *)' with an expression of type 'int (struct drm_connector *, struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
> >           .mode_valid = sti_hda_connector_mode_valid,
> >                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >   drivers/gpu/drm/sti/sti_dvo.c:376:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_connector *, struct drm_display_mode *)' with an expression of type 'int (struct drm_connector *, struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
> >           .mode_valid = sti_dvo_connector_mode_valid,
> >                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >   drivers/gpu/drm/sti/sti_hdmi.c:1035:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_connector *, struct drm_display_mode *)' with an expression of type 'int (struct drm_connector *, struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
> >           .mode_valid = sti_hdmi_connector_mode_valid,
> >                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > ->mode_valid() in 'struct drm_connector_helper_funcs' expects a return
> > type of 'enum drm_mode_status', not 'int'. Adjust the return type of
> > sti_{dvo,hda,hdmi}_connector_mode_valid() to match the prototype's to
> > resolve the warning and CFI failure.
> > 
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1750
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> >  drivers/gpu/drm/sti/sti_dvo.c  | 5 +++--
> >  drivers/gpu/drm/sti/sti_hda.c  | 5 +++--
> >  drivers/gpu/drm/sti/sti_hdmi.c | 5 +++--
> >  3 files changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/sti/sti_dvo.c b/drivers/gpu/drm/sti/sti_dvo.c
> > index b6ee8a82e656..076d5f30a09c 100644
> > --- a/drivers/gpu/drm/sti/sti_dvo.c
> > +++ b/drivers/gpu/drm/sti/sti_dvo.c
> > @@ -346,8 +346,9 @@ static int sti_dvo_connector_get_modes(struct drm_connector *connector)
> >  
> >  #define CLK_TOLERANCE_HZ 50
> >  
> > -static int sti_dvo_connector_mode_valid(struct drm_connector *connector,
> > -					struct drm_display_mode *mode)
> > +static enum drm_mode_status
> > +sti_dvo_connector_mode_valid(struct drm_connector *connector,
> > +			     struct drm_display_mode *mode)
> >  {
> >  	int target = mode->clock * 1000;
> >  	int target_min = target - CLK_TOLERANCE_HZ;
> > diff --git a/drivers/gpu/drm/sti/sti_hda.c b/drivers/gpu/drm/sti/sti_hda.c
> > index 03cc401ed593..a53b5a15c2a9 100644
> > --- a/drivers/gpu/drm/sti/sti_hda.c
> > +++ b/drivers/gpu/drm/sti/sti_hda.c
> > @@ -601,8 +601,9 @@ static int sti_hda_connector_get_modes(struct drm_connector *connector)
> >  
> >  #define CLK_TOLERANCE_HZ 50
> >  
> > -static int sti_hda_connector_mode_valid(struct drm_connector *connector,
> > -					struct drm_display_mode *mode)
> > +static enum drm_mode_status
> > +sti_hda_connector_mode_valid(struct drm_connector *connector,
> > +			     struct drm_display_mode *mode)
> >  {
> >  	int target = mode->clock * 1000;
> >  	int target_min = target - CLK_TOLERANCE_HZ;
> > diff --git a/drivers/gpu/drm/sti/sti_hdmi.c b/drivers/gpu/drm/sti/sti_hdmi.c
> > index cb82622877d2..09e0cadb6368 100644
> > --- a/drivers/gpu/drm/sti/sti_hdmi.c
> > +++ b/drivers/gpu/drm/sti/sti_hdmi.c
> > @@ -1004,8 +1004,9 @@ static int sti_hdmi_connector_get_modes(struct drm_connector *connector)
> >  
> >  #define CLK_TOLERANCE_HZ 50
> >  
> > -static int sti_hdmi_connector_mode_valid(struct drm_connector *connector,
> > -					struct drm_display_mode *mode)
> > +static enum drm_mode_status
> > +sti_hdmi_connector_mode_valid(struct drm_connector *connector,
> > +			      struct drm_display_mode *mode)
> >  {
> >  	int target = mode->clock * 1000;
> >  	int target_min = target - CLK_TOLERANCE_HZ;
> > 
> > base-commit: 9abf2313adc1ca1b6180c508c25f22f9395cc780
> > -- 
> > 2.38.1
> > 
> > 
> 
> Could someone please pick this up so that it makes 6.2? We would like
> to try and get this warning turned on so that it can catch more
> potential run time issues at compile time but that can only happen when
> all the warnings are fixed.

Same here; I'll pick this up tomorrow if no one else snags it.

-- 
Kees Cook