[PATCH] drm/vc4: Fix atomicity violation in vc4_crtc_send_vblank()

[PATCH] drm/vc4: Fix atomicity violation in vc4_crtc_send_vblank()

[Qiu-ji Chen]

Atomicity violation occurs when the vc4_crtc_send_vblank function is
executed simultaneously with modifications to crtc->state or
crtc->state->event. Consider a scenario where both crtc->state and
crtc->state->event are non-null. They can pass the validity check, but at
the same time, crtc->state or crtc->state->event could be set to null. In
this case, the validity check in vc4_crtc_send_vblank might act on the old
crtc->state and crtc->state->event (before locking), allowing invalid
values to pass the validity check, leading to null pointer dereference.

To address this issue, it is recommended to include the validity check of
crtc->state and crtc->state->event within the locking section of the
function. This modification ensures that the values of crtc->state->event
and crtc->state do not change during the validation process, maintaining
their valid conditions.

This possible bug is found by an experimental static analysis tool
developed by our team. This tool analyzes the locking APIs
to extract function pairs that can be concurrently executed, and then
analyzes the instructions in the paired functions to identify possible
concurrency bugs including data races and atomicity violations.

Fixes: 68e4a69aec4d ("drm/vc4: crtc: Create vblank reporting function")
Cc: stable@vger.kernel.org
Signed-off-by: Qiu-ji Chen <chenqiuji666@gmail.com>
---
 drivers/gpu/drm/vc4/vc4_crtc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
index 8b5a7e5eb146..98885f519827 100644
--- a/drivers/gpu/drm/vc4/vc4_crtc.c
+++ b/drivers/gpu/drm/vc4/vc4_crtc.c
@@ -575,10 +575,12 @@ void vc4_crtc_send_vblank(struct drm_crtc *crtc)
 	struct drm_device *dev = crtc->dev;
 	unsigned long flags;
 
-	if (!crtc->state || !crtc->state->event)
+	spin_lock_irqsave(&dev->event_lock, flags);
+	if (!crtc->state || !crtc->state->event) {
+		spin_unlock_irqrestore(&dev->event_lock, flags);
 		return;
+	}
 
-	spin_lock_irqsave(&dev->event_lock, flags);
 	drm_crtc_send_vblank_event(crtc, crtc->state->event);
 	crtc->state->event = NULL;
 	spin_unlock_irqrestore(&dev->event_lock, flags);
-- 
2.34.1

Re: [PATCH] drm/vc4: Fix atomicity violation in vc4_crtc_send_vblank()

[Simona Vetter]

On Fri, Sep 13, 2024 at 05:10:53PM +0800, Qiu-ji Chen wrote:
> Atomicity violation occurs when the vc4_crtc_send_vblank function is
> executed simultaneously with modifications to crtc->state or
> crtc->state->event. Consider a scenario where both crtc->state and
> crtc->state->event are non-null. They can pass the validity check, but at
> the same time, crtc->state or crtc->state->event could be set to null. In
> this case, the validity check in vc4_crtc_send_vblank might act on the old
> crtc->state and crtc->state->event (before locking), allowing invalid
> values to pass the validity check, leading to null pointer dereference.
> 
> To address this issue, it is recommended to include the validity check of
> crtc->state and crtc->state->event within the locking section of the
> function. This modification ensures that the values of crtc->state->event
> and crtc->state do not change during the validation process, maintaining
> their valid conditions.
> 
> This possible bug is found by an experimental static analysis tool
> developed by our team. This tool analyzes the locking APIs
> to extract function pairs that can be concurrently executed, and then
> analyzes the instructions in the paired functions to identify possible
> concurrency bugs including data races and atomicity violations.
> 
> Fixes: 68e4a69aec4d ("drm/vc4: crtc: Create vblank reporting function")
> Cc: stable@vger.kernel.org
> Signed-off-by: Qiu-ji Chen <chenqiuji666@gmail.com>
> ---
>  drivers/gpu/drm/vc4/vc4_crtc.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
> index 8b5a7e5eb146..98885f519827 100644
> --- a/drivers/gpu/drm/vc4/vc4_crtc.c
> +++ b/drivers/gpu/drm/vc4/vc4_crtc.c
> @@ -575,10 +575,12 @@ void vc4_crtc_send_vblank(struct drm_crtc *crtc)
>  	struct drm_device *dev = crtc->dev;
>  	unsigned long flags;
>  
> -	if (!crtc->state || !crtc->state->event)
> +	spin_lock_irqsave(&dev->event_lock, flags);

crtc->state isn't protected by this spinlock, which also points at the
more fundamental bug here: We need to pass the crtc_state from the caller,
because those have it (or well, can look it up with
drm_atomic_get_new_crtc_state). Then we also do not need a spinlock to
protect access to state->event, because in both callers we are the owners
of this struct field.
-Sima

> +	if (!crtc->state || !crtc->state->event) {
> +		spin_unlock_irqrestore(&dev->event_lock, flags);
>  		return;
> +	}
>  
> -	spin_lock_irqsave(&dev->event_lock, flags);
>  	drm_crtc_send_vblank_event(crtc, crtc->state->event);
>  	crtc->state->event = NULL;
>  	spin_unlock_irqrestore(&dev->event_lock, flags);
> -- 
> 2.34.1
> 

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm/vc4: Fix atomicity violation in vc4_crtc_send_vblank()

[Qiu-ji Chen]

Hi,

In the drm_device structure, it is mentioned: "@event_lock: Protects
@vblank_event_list and event delivery in general." I believe that the
validity check and the subsequent null assignment operation are part
of the event delivery process, and all of these should be protected by
the event_lock. If there is no lock protection before the validity
check, it is possible for a null crtc->state->event to be passed into
the drm_crtc_send_vblank_event() function, leading to a null pointer
dereference error.

We have observed its callers and found that they are from the
drm_crtc_helper_funcs driver interface. We believe that functions
within driver interfaces can be concurrent, potentially causing a data
race on crtc->state->event.

Qiu-ji Chen