Hi,
These patches fix two issues in the drm/drm_crtc driver. Initially I was hitting the BUG_ON() in a scenario as explained in the commit message of what is now the second patch in this series.
After posting, sashiko.dev noticed another issue, that was previously masked by the now-removed BUG_ON(). Since we can't have a loud BUG() be replaced with silent data corruption or worse, I've also added a patch to address this issue highlighted by sashiko.dev. I believe its observation and analysis to be correct.
Cheers, Andre'
Signed-off-by: André Draszik andre.draszik@linaro.org --- Changes in v3: - Philipp: - patch 1: update kerneldoc, add Fixes: - patch 2: shorten commit message - explicitly Cc: stable - collect tag - Link to v2: https://lore.kernel.org/r/20260708-linux-drm_crtc_fix2-v2-0-cf72be75d75a@lin...
Changes in v2: - add new patch 1 to address sashiko observation - original patch 1 becomes patch 2 - patch 2: - don't turn fence_to_crtc() into macro (Jani, Philipp) - update commit message to include reference to deprecated use of BUG - Link to v1: https://lore.kernel.org/r/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f34@lin...
--- André Draszik (2): drm/drm_crtc: ensure dma_fence_ops remain valid during device unbind drm/drm_crtc: fix race with dma_fence_signal() in ::get_driver_name()
drivers/gpu/drm/drm_crtc.c | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-) --- base-commit: 0718283ab28bc3907e10b61a6b4be6fefa1cbb2f change-id: 20260618-linux-drm_crtc_fix2-23a7c354a412
Best regards,
In [1], sashiko reported the following issue:
=== snip === Looking at how these fences are managed, drm_crtc_create_fence() creates a dma_fence without taking a reference to the drm_device or drm_crtc. Because the sync_file framework exposes this fence to userspace, the fence can outlive the CRTC.
The dma_fence contract requires that data accessed by dma_fence_ops (like get_driver_name) must remain valid for an RCU grace period after the fence is signaled. However, drm_crtc_cleanup() and the subsequent freeing of the device do not wait for an RCU grace period via synchronize_rcu().
If userspace calls ioctl(SYNC_IOC_FILE_INFO) concurrently with a device hot-unplug:
CPU1 (Userspace) sync_file_get_name() ops = rcu_dereference(fence->ops); if (!dma_fence_test_signaled_flag()) // Preempted or delayed here
CPU2 (Driver Teardown) Signals the fence (setting fence->ops = NULL) Destroys and frees the CRTC without waiting for an RCU grace period
CPU1 (Resumes) ops->get_driver_name(fence) -> drm_crtc_fence_get_driver_name() crtc = fence_to_crtc(fence); // Casts to the freed CRTC return crtc->dev->driver->name; // Use-after-free
...
Does the CRTC or DRM device need to be kept alive for the RCU grace period, or should the fence hold a proper reference to prevent the use-after-free when get_driver_name() and get_timeline_name() access the freed CRTC structure? === snap ===
I believe this to be a correct observation and this patch implements the suggestion of waiting for an RCU grace period before proceeding with destruction of the drm_crtc, so that get_driver_name() and get_timeline_name() can still work.
Link: https://sashiko.dev/#/patchset/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f3... Fixes: 6d6003c4b613 ("drm/fence: add fence timeline to drm_crtc") Cc: stable@vger.kernel.org Signed-off-by: André Draszik andre.draszik@linaro.org
--- v3: - Philipp: update kerneldoc, add Fixes:
v2: new patch --- drivers/gpu/drm/drm_crtc.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index 63ead8ba6756..e8e80c936852 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -493,14 +493,23 @@ EXPORT_SYMBOL(__drmm_crtc_alloc_with_planes); * drm_crtc_cleanup - Clean up the core crtc usage * @crtc: CRTC to cleanup * - * This function cleans up @crtc and removes it from the DRM mode setting - * core. Note that the function does *not* free the crtc structure itself, - * this is the responsibility of the caller. + * This function cleans up @crtc and removes it from the DRM mode setting core, + * after first waiting an RCU grace period to ensure @crtc->dev can safely be + * dereferenced by our dma_fence_ops. + * + * Note that the function does *not* free the crtc structure itself, this is the + * responsibility of the caller. */ void drm_crtc_cleanup(struct drm_crtc *crtc) { struct drm_device *dev = crtc->dev;
+ /* Ensure our dma_fence_ops remain valid for an RCU grace period after + * the fence is signaled. This is necessary because our dma_fence_ops + * dereference crtc->dev. + */ + synchronize_rcu(); + /* Note that the crtc_list is considered to be static; should we * remove the drm_crtc at runtime we would have to decrement all * the indices on the drm_crtc after us in the crtc_list.
On Tue, 2026-07-21 at 09:21 +0100, André Draszik wrote:
In [1], sashiko reported the following issue:
=== snip === Looking at how these fences are managed, drm_crtc_create_fence() creates a dma_fence without taking a reference to the drm_device or drm_crtc. Because the sync_file framework exposes this fence to userspace, the fence can outlive the CRTC.
The dma_fence contract requires that data accessed by dma_fence_ops (like get_driver_name) must remain valid for an RCU grace period after the fence is signaled. However, drm_crtc_cleanup() and the subsequent freeing of the device do not wait for an RCU grace period via synchronize_rcu().
If userspace calls ioctl(SYNC_IOC_FILE_INFO) concurrently with a device hot-unplug:
CPU1 (Userspace) sync_file_get_name() ops = rcu_dereference(fence->ops); if (!dma_fence_test_signaled_flag()) // Preempted or delayed here
nit: no one will be preempted here since the RCU read lock must be held. The Sashiko tool misses the point, which is simply that someone illegally frees up stuff that might be still in use, with or without delay or preemption, that's all irrelevant for the issue.
Anyways, thanks for fixing this:
[…]
Link: https://sashiko.dev/#/patchset/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f3... Fixes: 6d6003c4b613 ("drm/fence: add fence timeline to drm_crtc") Cc: stable@vger.kernel.org Signed-off-by: André Draszik andre.draszik@linaro.org
Reviewed-by: Philipp Stanner phasta@kernel.org
v3:
- Philipp: update kerneldoc, add Fixes:
v2: new patch
drivers/gpu/drm/drm_crtc.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index 63ead8ba6756..e8e80c936852 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -493,14 +493,23 @@ EXPORT_SYMBOL(__drmm_crtc_alloc_with_planes); * drm_crtc_cleanup - Clean up the core crtc usage * @crtc: CRTC to cleanup *
- This function cleans up @crtc and removes it from the DRM mode setting
- core. Note that the function does *not* free the crtc structure itself,
- this is the responsibility of the caller.
- This function cleans up @crtc and removes it from the DRM mode setting core,
- after first waiting an RCU grace period to ensure @crtc->dev can safely be
- dereferenced by our dma_fence_ops.
- Note that the function does *not* free the crtc structure itself, this is the
- responsibility of the caller.
*/ void drm_crtc_cleanup(struct drm_crtc *crtc) { struct drm_device *dev = crtc->dev;
- /* Ensure our dma_fence_ops remain valid for an RCU grace period after
* the fence is signaled. This is necessary because our dma_fence_ops* dereference crtc->dev.*/- synchronize_rcu();
/* Note that the crtc_list is considered to be static; should we * remove the drm_crtc at runtime we would have to decrement all * the indices on the drm_crtc after us in the crtc_list.
On 7/21/26 13:20, Philipp Stanner wrote:
On Tue, 2026-07-21 at 09:21 +0100, André Draszik wrote:
In [1], sashiko reported the following issue:
=== snip === Looking at how these fences are managed, drm_crtc_create_fence() creates a dma_fence without taking a reference to the drm_device or drm_crtc. Because the sync_file framework exposes this fence to userspace, the fence can outlive the CRTC.
The dma_fence contract requires that data accessed by dma_fence_ops (like get_driver_name) must remain valid for an RCU grace period after the fence is signaled. However, drm_crtc_cleanup() and the subsequent freeing of the device do not wait for an RCU grace period via synchronize_rcu().
If userspace calls ioctl(SYNC_IOC_FILE_INFO) concurrently with a device hot-unplug:
CPU1 (Userspace) sync_file_get_name() ops = rcu_dereference(fence->ops); if (!dma_fence_test_signaled_flag()) // Preempted or delayed here
nit: no one will be preempted here since the RCU read lock must be held. The Sashiko tool misses the point, which is simply that someone illegally frees up stuff that might be still in use, with or without delay or preemption, that's all irrelevant for the issue.
Anyways, thanks for fixing this:
Seconded.
[…]
Link: https://sashiko.dev/#/patchset/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f3... Fixes: 6d6003c4b613 ("drm/fence: add fence timeline to drm_crtc") Cc: stable@vger.kernel.org Signed-off-by: André Draszik andre.draszik@linaro.org
Reviewed-by: Philipp Stanner phasta@kernel.org
Reviewed-by: Christian König christian.koenig@amd.com for both patches.
v3:
- Philipp: update kerneldoc, add Fixes:
v2: new patch
drivers/gpu/drm/drm_crtc.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index 63ead8ba6756..e8e80c936852 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -493,14 +493,23 @@ EXPORT_SYMBOL(__drmm_crtc_alloc_with_planes); * drm_crtc_cleanup - Clean up the core crtc usage * @crtc: CRTC to cleanup *
- This function cleans up @crtc and removes it from the DRM mode setting
- core. Note that the function does *not* free the crtc structure itself,
- this is the responsibility of the caller.
- This function cleans up @crtc and removes it from the DRM mode setting core,
- after first waiting an RCU grace period to ensure @crtc->dev can safely be
- dereferenced by our dma_fence_ops.
- Note that the function does *not* free the crtc structure itself, this is the
- responsibility of the caller.
*/ void drm_crtc_cleanup(struct drm_crtc *crtc) { struct drm_device *dev = crtc->dev;
- /* Ensure our dma_fence_ops remain valid for an RCU grace period after
* the fence is signaled. This is necessary because our dma_fence_ops* dereference crtc->dev.*/- synchronize_rcu();
/* Note that the crtc_list is considered to be static; should we * remove the drm_crtc at runtime we would have to decrement all * the indices on the drm_crtc after us in the crtc_list.
Since commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"), I'm seeing the BUG_ON() triggering in drm_crtc's fence_to_crtc() via drm_crtc_fence_get_driver_name() regularly:
Call trace: panic+0x58/0x5c die+0x160/0x178 bug_brk_handler+0x70/0xa4 call_el1_break_hook+0x3c/0x1a0 do_el1_brk64+0x24/0x74 el1_brk64+0x34/0x54 el1h_64_sync_handler+0x80/0xfc el1h_64_sync+0x84/0x88 drm_crtc_fence_get_driver_name+0x60/0x68 (P) sync_file_get_name+0x184/0x45c sync_file_ioctl+0x404/0xf70 __arm64_sys_ioctl+0x124/0x1dc
This looks to be caused by a code flow similar to the following:
+++ snip +++ thread A thread B
ioctl(SYNC_IOC_FILE_INFO) sync_file_ioctl() sync_file_get_name() dma_fence_signal_timestamp_locked() dma_fence_driver_name() ops = rcu_dereference(fence->ops) if (!dma_fence_test_signaled_flag()) ops->get_driver_name(fence) i.e. drm_crtc_fence_get_driver_name() test_and_set_bit(SIGNALED) RCU_INIT_POINTER(fence->ops, NULL) drm_crtc_fence_get_driver_name() BUG_ON(rcu_access_pointer(fence->ops) != &drm_crtc_fence_ops) +++ snap +++
In other words, a fence's ops pointer can now change to NULL on signal, which wasn't the case before the culprit commit and hence the BUG_ON() triggers.
Simply drop the BUG_ON() as it's usage of fence ops is invalid now, and because using BUG_ON() and friends to take down the system is an unacceptable way to handle a failure.
Note that the adjacent drm_crtc_fence_get_timeline_name() has the same problem and is fixed by this patch as well.
Fixes: 541c8f2468b9 ("dma-buf: detach fence ops on signal v3") Cc: stable@vger.kernel.org Reviewed-by: Philipp Stanner phasta@kernel.org Signed-off-by: André Draszik andre.draszik@linaro.org --- v3: - Philipp: - shorten commit message - explicitly Cc: stable - collect tag
v2: - don't turn fence_to_crtc() into macro - update commit message to include reference to unacceptable use of BUG --- drivers/gpu/drm/drm_crtc.c | 3 --- 1 file changed, 3 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index e8e80c936852..5295ec122e0d 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -154,11 +154,8 @@ static void drm_crtc_crc_fini(struct drm_crtc *crtc) #endif }
-static const struct dma_fence_ops drm_crtc_fence_ops; - static struct drm_crtc *fence_to_crtc(struct dma_fence *fence) { - BUG_ON(rcu_access_pointer(fence->ops) != &drm_crtc_fence_ops); return container_of(fence->extern_lock, struct drm_crtc, fence_lock); }
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