[PATCH AUTOSEL 6.19-5.10] drm/atmel-hlcdc: fix use-after-free of drm_crtc_commit after release

Sasha Levin
2026-02-14T01:03:39Z

From: Ludovic Desroches <ludovic.desroches@microchip.com>

[ Upstream commit bc847787233277a337788568e90a6ee1557595eb ]

The atmel_hlcdc_plane_atomic_duplicate_state() callback was copying
the atmel_hlcdc_plane state structure without properly duplicating the
drm_plane_state. In particular, state->commit remained set to the old
state commit, which can lead to a use-after-free in the next
drm_atomic_commit() call.

Fix this by calling
__drm_atomic_helper_duplicate_plane_state(), which correctly clones
the base drm_plane_state (including the ->commit pointer).

It has been seen when closing and re-opening the device node while
another DRM client (e.g. fbdev) is still attached:

=============================================================================
BUG kmalloc-64 (Not tainted): Poison overwritten
-----------------------------------------------------------------------------

0xc611b344-0xc611b344 @offset=836. First byte 0x6a instead of 0x6b
FIX kmalloc-64: Restoring Poison 0xc611b344-0xc611b344=0x6b
Allocated in drm_atomic_helper_setup_commit+0x1e8/0x7bc age=178 cpu=0
pid=29
drm_atomic_helper_setup_commit+0x1e8/0x7bc
drm_atomic_helper_commit+0x3c/0x15c
drm_atomic_commit+0xc0/0xf4
drm_framebuffer_remove+0x4cc/0x5a8
drm_mode_rmfb_work_fn+0x6c/0x80
process_one_work+0x12c/0x2cc
worker_thread+0x2a8/0x400
kthread+0xc0/0xdc
ret_from_fork+0x14/0x28
Freed in drm_atomic_helper_commit_hw_done+0x100/0x150 age=8 cpu=0
pid=169
drm_atomic_helper_commit_hw_done+0x100/0x150
drm_atomic_helper_commit_tail+0x64/0x8c
commit_tail+0x168/0x18c
drm_atomic_helper_commit+0x138/0x15c
drm_atomic_commit+0xc0/0xf4
drm_atomic_helper_set_config+0x84/0xb8
drm_mode_setcrtc+0x32c/0x810
drm_ioctl+0x20c/0x488
sys_ioctl+0x14c/0xc20
ret_fast_syscall+0x0/0x54
Slab 0xef8bc360 objects=21 used=16 fp=0xc611b7c0
flags=0x200(workingset|zone=0)
Object 0xc611b340 @offset=832 fp=0xc611b7c0

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Reviewed-by: Manikandan Muralidharan <manikandan.m@microchip.com>
Link: https://patch.msgid.link/20251024-lcd_fixes_mainlining-v1-2-79b615130dc3@microchip.com
Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

This confirms the fix. The key lines in the helper are:

```c
state->fence = NULL;
state->commit = NULL; // THIS is the critical fix
state->fb_damage_clips = NULL;
```

The old code was missing the `state->commit = NULL` step. By using the
helper, the commit pointer is properly set to NULL in the duplicated
state, preventing the use-after-free when the old state's commit is
freed.

Additionally, `state->fence` and `state->fb_damage_clips` were also not
being properly handled in the old code — the helper fixes multiple
potential issues at once.

### 3. CLASSIFICATION

- **Bug type**: Use-after-free (UAF) — one of the most critical bug
categories for stable
- **Severity**: HIGH — UAF can lead to kernel crashes, data corruption,
and potentially security vulnerabilities
- **Reproducibility**: PROVEN — the commit includes a full stack trace
from a real reproduction

### 4. SCOPE AND RISK ASSESSMENT

- **Lines changed**: Net -1 line (replaced 2 lines with 1 line)
- **Files touched**: 1 file (`atmel_hlcdc_plane.c`)
- **Complexity**: Minimal — uses the standard, well-tested DRM helper
function
- **Risk of regression**: Extremely low — this replaces incorrect manual
code with the standard pattern used by virtually all other DRM drivers
- **The destroy_state function already uses
`__drm_atomic_helper_plane_destroy_state`**, so using the duplicate
counterpart is the correct and symmetric approach

### 5. USER IMPACT

- Affects users of Atmel HLCDC display controllers (common in
Atmel/Microchip SoCs used in embedded systems)
- The bug triggers when closing and re-opening display device nodes
while another client (e.g., fbdev emulation) remains — a realistic
scenario
- Impact when triggered: kernel BUG (slab corruption), which can lead to
crashes or worse

### 6. STABILITY INDICATORS

- **Reviewed-by**: Manikandan Muralidharan (subsystem maintainer for
this driver)
- **Author**: Ludovic Desroches from Microchip — the hardware vendor's
own engineer
- The fix uses a well-established DRM helper function that has been
stable for years

### 7. DEPENDENCY CHECK

- `__drm_atomic_helper_plane_duplicate_state()` has existed in the
kernel since at least 4.x series — no dependency issues for any active
stable tree
- The atmel-hlcdc driver exists in all active stable trees
- The fix is completely self-contained — no other patches needed

### Summary

This is a textbook stable backport candidate:
- **Obviously correct**: Replaces broken manual code with the standard
DRM helper
- **Fixes a real bug**: UAF proven with stack trace
- **Small and contained**: 1 file, net -1 line change
- **No new features**: Pure bug fix
- **Low regression risk**: Uses established, well-tested helper function
- **Real-world impact**: Reproducible crash scenario on embedded
hardware

**YES**

drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c
index ec1fb5f9549a2..e55e88d44e829 100644
--- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c
+++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c
@@ -1160,8 +1160,7 @@ atmel_hlcdc_plane_atomic_duplicate_state(struct drm_plane *p)
return NULL;
}

- if (copy->base.fb)
- drm_framebuffer_get(copy->base.fb);
+ __drm_atomic_helper_plane_duplicate_state(p, &copy->base);

return &copy->base;
}
--
2.51.0