On 01/12/2021 21:15, Liviu Dudau wrote:

On Wed, Dec 01, 2021 at 03:44:03PM +0000, Steven Price wrote:

...

On 30/11/2021 14:25, Zhou Qingyang wrote:

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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@umn.edu

---

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.

Sorry, for some reason Zhou's patch appeared twice on the list and I hadn't spotted your reply to the other version. My mistake.

...

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.

Great, thanks for taking a look - I'm afraid I couldn't see an obvious fix.

Regards,

Steven

On 03/12/2021 10:09, Liviu Dudau wrote:

If drm_universal_plane_init() fails early we jump to the common cleanup code that calls komeda_plane_destroy() which in turn could access the uninitalised drm_plane and crash. Return early if an error is detected without going through the common code.

Reported-by: Steven Price steven.price@arm.com Signed-off-by: Liviu Dudau liviu.dudau@arm.com

Reviewed-by: Steven Price steven.price@arm.com

Looks correct, although I note there is a path in __drm_universal_plane_init() which doesn't clean up properly. I'll send a patch for that too.

Thanks,

Steve

---

drivers/gpu/drm/arm/display/komeda/komeda_plane.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/arm/display/komeda/komeda_plane.c b/drivers/gpu/drm/arm/display/komeda/komeda_plane.c index aa193c58f4bf6d9..517b94c3bcaf966 100644 --- a/drivers/gpu/drm/arm/display/komeda/komeda_plane.c +++ b/drivers/gpu/drm/arm/display/komeda/komeda_plane.c @@ -279,8 +279,10 @@ static int komeda_plane_add(struct komeda_kms_dev *kms,

komeda_put_fourcc_list(formats);

• if (err)

• goto cleanup;
  

• if (err) {

• kfree(kplane);
  

• return err;
  

• }

  drm_plane_helper_add(plane, &komeda_plane_helper_funcs);

On 12/3/21 14:15, Carsten Haitzler wrote:

On 12/3/21 10:09, Liviu Dudau wrote:

...

If drm_universal_plane_init() fails early we jump to the common cleanup code that calls komeda_plane_destroy() which in turn could access the uninitalised drm_plane and crash. Return early if an error is detected without going through the common code.

Reported-by: Steven Price steven.price@arm.com Signed-off-by: Liviu Dudau liviu.dudau@arm.com

---

drivers/gpu/drm/arm/display/komeda/komeda_plane.c | 6 ++++--   1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/arm/display/komeda/komeda_plane.c b/drivers/gpu/drm/arm/display/komeda/komeda_plane.c index aa193c58f4bf6d9..517b94c3bcaf966 100644 --- a/drivers/gpu/drm/arm/display/komeda/komeda_plane.c +++ b/drivers/gpu/drm/arm/display/komeda/komeda_plane.c @@ -279,8 +279,10 @@ static int komeda_plane_add(struct komeda_kms_dev *kms,       komeda_put_fourcc_list(formats); -    if (err) -        goto cleanup; +    if (err) { +        kfree(kplane); +        return err; +    }       drm_plane_helper_add(plane, &komeda_plane_helper_funcs);

Ummm... can I disagree here? this goto cleanup I think is just fine because plane has been set before drm_universal_plane_init() is called which is before the if (err) here. komeda_plane_destroy() in cleanup: does all the right things, so this patch isn't needed. I think it's less clean as it adds a new "handle error" path special-case instance where a special case is not needed. The fix to Zhou's original patch was needed for exactly the reason Liviu said - but not this one... ?

Let me take that back - it seems an init fail shouldn't call cleanup but the init fail doesn't quite cleanup properly. Steven found this and already sent a patch.