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

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

[Liviu Dudau]

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;
+
 	snprintf(buf->str, sizeof(buf->str),
 		 "%c%c%c%c %s-endian (0x%08x)",
 		 printable_char(format & 0xff),
-- 
2.10.2

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

[Ville Syrjälä]

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?

>  	snprintf(buf->str, sizeof(buf->str),
>  		 "%c%c%c%c %s-endian (0x%08x)",
>  		 printable_char(format & 0xff),
> -- 
> 2.10.2
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel OTC

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

[Rob Clark]

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?

perhaps BUG_ON(!buf)...

BR,
-R

>>       snprintf(buf->str, sizeof(buf->str),
>>                "%c%c%c%c %s-endian (0x%08x)",
>>                printable_char(format & 0xff),
>> --
>> 2.10.2
>>
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Ville Syrjälä
> Intel OTC
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

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

[Ville Syrjälä]

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?
> 
> perhaps BUG_ON(!buf)...

And how does that differ from just buf->foo?

> 
> BR,
> -R
> 
> >>       snprintf(buf->str, sizeof(buf->str),
> >>                "%c%c%c%c %s-endian (0x%08x)",
> >>                printable_char(format & 0xff),
> >> --
> >> 2.10.2
> >>
> >> _______________________________________________
> >> dri-devel mailing list
> >> dri-devel@lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> > --
> > Ville Syrjälä
> > Intel OTC
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel OTC

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

[Rob Clark]

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?
>>
>> 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.

BR,
-R

>>
>> BR,
>> -R
>>
>> >>       snprintf(buf->str, sizeof(buf->str),
>> >>                "%c%c%c%c %s-endian (0x%08x)",
>> >>                printable_char(format & 0xff),
>> >> --
>> >> 2.10.2
>> >>
>> >> _______________________________________________
>> >> dri-devel mailing list
>> >> dri-devel@lists.freedesktop.org
>> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>> >
>> > --
>> > Ville Syrjälä
>> > Intel OTC
>> > _______________________________________________
>> > dri-devel mailing list
>> > dri-devel@lists.freedesktop.org
>> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Ville Syrjälä
> Intel OTC

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

[Ville Syrjälä]

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?
> >>
> >> 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.

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

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

[Sean Paul]

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?
>> >>
>> >> 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.

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

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

[Liviu Dudau]

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

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯

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

[Rob Clark]

On Tue, Nov 22, 2016 at 1:47 PM, Liviu Dudau <Liviu.Dudau@arm.com> wrote:
> 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.

if you are a kernel module, in tree or not, all bets are off..  an out
of tree module could just as easily call sprintf(NULL, "foo")
directly..

BR,
-R

>> >> >>
>> >> >> 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
>
> --
> ====================
> | I would like to |
> | fix the world,  |
> | but they're not |
> | giving me the   |
>  \ source code!  /
>   ---------------
>     ¯\_(ツ)_/¯

[PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Liviu Dudau]

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.

v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.

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>
---
I still think sanity checking the parameters of an exported function is worth
doing, even if the way one triggers the NULL pointer crash is priviledged. Not
a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
pointer, but that is a matter of taste.


 drivers/gpu/drm/drm_fourcc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
index 90d2cc8..6d80239 100644
--- a/drivers/gpu/drm/drm_fourcc.c
+++ b/drivers/gpu/drm/drm_fourcc.c
@@ -85,6 +85,8 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
  */
 const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
 {
+	BUG_ON(!buf);
+
 	snprintf(buf->str, sizeof(buf->str),
 		 "%c%c%c%c %s-endian (0x%08x)",
 		 printable_char(format & 0xff),
-- 
2.10.2

Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Jani Nikula]

On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.com> 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.
>
> v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
>
> 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>
> ---
> I still think sanity checking the parameters of an exported function is worth
> doing, even if the way one triggers the NULL pointer crash is priviledged. Not
> a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
> pointer, but that is a matter of taste.

There really is no meaningful difference between doing BUG_ON(!bug)
vs. just letting buf->str oops. The kernel is full of functions that
expect sensible pointers, and I don't see why this one in particular
should be so special to warrant a BUG_ON().

BR,
Jani.

>
>
>  drivers/gpu/drm/drm_fourcc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index 90d2cc8..6d80239 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -85,6 +85,8 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
>   */
>  const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
>  {
> +	BUG_ON(!buf);
> +
>  	snprintf(buf->str, sizeof(buf->str),
>  		 "%c%c%c%c %s-endian (0x%08x)",
>  		 printable_char(format & 0xff),

-- 
Jani Nikula, Intel Open Source Technology Center

Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Liviu Dudau]

On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
> On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.com> 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.
> >
> > v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
> >
> > 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>
> > ---
> > I still think sanity checking the parameters of an exported function is worth
> > doing, even if the way one triggers the NULL pointer crash is priviledged. Not
> > a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
> > pointer, but that is a matter of taste.
> 
> There really is no meaningful difference between doing BUG_ON(!bug)
> vs. just letting buf->str oops. The kernel is full of functions that
> expect sensible pointers, and I don't see why this one in particular
> should be so special to warrant a BUG_ON().

Agree. That is why I prefer v1 where I return immediately on NULL pointers.

Best regards,
Liviu

> 
> BR,
> Jani.
> 
> >
> >
> >  drivers/gpu/drm/drm_fourcc.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> > index 90d2cc8..6d80239 100644
> > --- a/drivers/gpu/drm/drm_fourcc.c
> > +++ b/drivers/gpu/drm/drm_fourcc.c
> > @@ -85,6 +85,8 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
> >   */
> >  const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
> >  {
> > +	BUG_ON(!buf);
> > +
> >  	snprintf(buf->str, sizeof(buf->str),
> >  		 "%c%c%c%c %s-endian (0x%08x)",
> >  		 printable_char(format & 0xff),
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯

Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Daniel Vetter]

On Wed, Nov 23, 2016 at 11:23:23AM +0000, Liviu Dudau wrote:
> On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
> > On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.com> 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.
> > >
> > > v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
> > >
> > > 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>
> > > ---
> > > I still think sanity checking the parameters of an exported function is worth
> > > doing, even if the way one triggers the NULL pointer crash is priviledged. Not
> > > a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
> > > pointer, but that is a matter of taste.
> > 
> > There really is no meaningful difference between doing BUG_ON(!bug)
> > vs. just letting buf->str oops. The kernel is full of functions that
> > expect sensible pointers, and I don't see why this one in particular
> > should be so special to warrant a BUG_ON().
> 
> Agree. That is why I prefer v1 where I return immediately on NULL pointers.

The question for v1 is why did you hit that? "broken driver code" isn't
really a good reason, au contraire it's a reason to not merge your patch:
We do not want to hide driver bugs silently.

There's definitely cases where handling NULL automatically is reasonable,
e.g. kfree(). But a NULL drm_format_name_buf sounds like, at least a quick
grep shows that all callers just put this struct onto the stack.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Jani Nikula]

On Wed, 23 Nov 2016, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Wed, Nov 23, 2016 at 11:23:23AM +0000, Liviu Dudau wrote:
>> On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
>> > On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.com> 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.
>> > >
>> > > v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
>> > >
>> > > 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>
>> > > ---
>> > > I still think sanity checking the parameters of an exported function is worth
>> > > doing, even if the way one triggers the NULL pointer crash is priviledged. Not
>> > > a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
>> > > pointer, but that is a matter of taste.
>> > 
>> > There really is no meaningful difference between doing BUG_ON(!bug)
>> > vs. just letting buf->str oops. The kernel is full of functions that
>> > expect sensible pointers, and I don't see why this one in particular
>> > should be so special to warrant a BUG_ON().
>> 
>> Agree. That is why I prefer v1 where I return immediately on NULL pointers.
>
> The question for v1 is why did you hit that? "broken driver code" isn't
> really a good reason, au contraire it's a reason to not merge your patch:
> We do not want to hide driver bugs silently.

Moreover, v1 puts the burden back on the *caller* of the function to
check for NULL return, while it previously could not even return NULL.

The function is fine. It isn't broken. Don't try to fix it.

BR,
Jani.



>
> There's definitely cases where handling NULL automatically is reasonable,
> e.g. kfree(). But a NULL drm_format_name_buf sounds like, at least a quick
> grep shows that all callers just put this struct onto the stack.
> -Daniel

-- 
Jani Nikula, Intel Open Source Technology Center

Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Liviu Dudau]

On Wed, Nov 23, 2016 at 02:47:53PM +0200, Jani Nikula wrote:
> On Wed, 23 Nov 2016, Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Wed, Nov 23, 2016 at 11:23:23AM +0000, Liviu Dudau wrote:
> >> On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
> >> > On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.com> 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.
> >> > >
> >> > > v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
> >> > >
> >> > > 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>
> >> > > ---
> >> > > I still think sanity checking the parameters of an exported function is worth
> >> > > doing, even if the way one triggers the NULL pointer crash is priviledged. Not
> >> > > a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
> >> > > pointer, but that is a matter of taste.
> >> > 
> >> > There really is no meaningful difference between doing BUG_ON(!bug)
> >> > vs. just letting buf->str oops. The kernel is full of functions that
> >> > expect sensible pointers, and I don't see why this one in particular
> >> > should be so special to warrant a BUG_ON().
> >> 
> >> Agree. That is why I prefer v1 where I return immediately on NULL pointers.
> >
> > The question for v1 is why did you hit that? "broken driver code" isn't
> > really a good reason, au contraire it's a reason to not merge your patch:
> > We do not want to hide driver bugs silently.

I was updating a stashed series and discovered that signature of the function has changed.
When I looked at how it changed and I got past the "you pass as a parameter a pointer 
to a struct that is used as a buffer and then that buffer is returned by function" weirdness,
I thought that at least checking for bad parameters should be done.

> 
> Moreover, v1 puts the burden back on the *caller* of the function to
> check for NULL return, while it previously could not even return NULL.
> 
> The function is fine. It isn't broken. Don't try to fix it.

OK. I just like defensive programming, that's all. :)

Best regards,
Liviu

> 
> BR,
> Jani.
> 
> 
> 
> >
> > There's definitely cases where handling NULL automatically is reasonable,
> > e.g. kfree(). But a NULL drm_format_name_buf sounds like, at least a quick
> > grep shows that all callers just put this struct onto the stack.
> > -Daniel
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯

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

[Liviu Dudau]

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?
> 
> perhaps BUG_ON(!buf)...

Nah, I don't want to start spamming the logs. You passed nothing, you get nothing,
no reason to throw a tantrum.

Best regards,
Liviu

> 
> BR,
> -R
> 
> >>       snprintf(buf->str, sizeof(buf->str),
> >>                "%c%c%c%c %s-endian (0x%08x)",
> >>                printable_char(format & 0xff),
> >> --
> >> 2.10.2
> >>
> >> _______________________________________________
> >> dri-devel mailing list
> >> dri-devel@lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> > --
> > Ville Syrjälä
> > Intel OTC
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯