Re: [PATCH] drm/vc4: fix vc4_atomic_commit_tail() logic

[Marek Szyprowski]

On 08.06.2021 10:55, Mark Rutland wrote:
> In vc4_atomic_commit_tail() we iterate of the set of old CRTCs, and
> attempt to wait on any channels which are still in use. When we iterate
> over the CRTCs, we have:
>
> * `i` - the index of the CRTC
> * `channel` - the channel a CRTC is using
>
> When we check the channel state, we consult:
>
>    old_hvs_state->fifo_state[channel].in_use
>
> ... but when we wait for the channel, we erroneously wait on:
>
>    old_hvs_state->fifo_state[i].pending_commit
>
> ... rather than:
>
>     old_hvs_state->fifo_state[channel].pending_commit
>
> ... and this bogus access has been observed to result in boot-time hangs
> on some arm64 configurations, and can be detected using KASAN. FIx this
> by using the correct index.
>
> I've tested this on a Raspberry Pi 3 model B v1.2 with KASAN.
>
> Trimmed KASAN splat:
>
> | ==================================================================
> | BUG: KASAN: slab-out-of-bounds in vc4_atomic_commit_tail+0x1cc/0x910
> | Read of size 8 at addr ffff000007360440 by task kworker/u8:0/7
> | CPU: 2 PID: 7 Comm: kworker/u8:0 Not tainted 5.13.0-rc3-00009-g694c523e7267 #3
> |
> | Hardware name: Raspberry Pi 3 Model B (DT)
> | Workqueue: events_unbound deferred_probe_work_func
> | Call trace:
> |  dump_backtrace+0x0/0x2b4
> |  show_stack+0x1c/0x30
> |  dump_stack+0xfc/0x168
> |  print_address_description.constprop.0+0x2c/0x2c0
> |  kasan_report+0x1dc/0x240
> |  __asan_load8+0x98/0xd4
> |  vc4_atomic_commit_tail+0x1cc/0x910
> |  commit_tail+0x100/0x210
> | ...
> |
> | Allocated by task 7:
> |  kasan_save_stack+0x2c/0x60
> |  __kasan_kmalloc+0x90/0xb4
> |  vc4_hvs_channels_duplicate_state+0x60/0x1a0
> |  drm_atomic_get_private_obj_state+0x144/0x230
> |  vc4_atomic_check+0x40/0x73c
> |  drm_atomic_check_only+0x998/0xe60
> |  drm_atomic_commit+0x34/0x94
> |  drm_client_modeset_commit_atomic+0x2f4/0x3a0
> |  drm_client_modeset_commit_locked+0x8c/0x230
> |  drm_client_modeset_commit+0x38/0x60
> |  drm_fb_helper_set_par+0x104/0x17c
> |  fbcon_init+0x43c/0x970
> |  visual_init+0x14c/0x1e4
> | ...
> |
> | The buggy address belongs to the object at ffff000007360400
> |  which belongs to the cache kmalloc-128 of size 128
> | The buggy address is located 64 bytes inside of
> |  128-byte region [ffff000007360400, ffff000007360480)
> | The buggy address belongs to the page:
> | page:(____ptrval____) refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7360
> | flags: 0x3fffc0000000200(slab|node=0|zone=0|lastcpupid=0xffff)
> | raw: 03fffc0000000200 dead000000000100 dead000000000122 ffff000004c02300
> | raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
> | page dumped because: kasan: bad access detected
> |
> | Memory state around the buggy address:
> |  ffff000007360300: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> |  ffff000007360380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> | >ffff000007360400: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
> |                                            ^
> |  ffff000007360480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> |  ffff000007360500: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> | ==================================================================
>
> Link: https://lore.kernel.org/r/4d0c8318-bad8-2be7-e292-fc8f70c198de@xxxxxxxxxxx
> Link: https://lore.kernel.org/linux-arm-kernel/20210607151740.moncryl5zv3ahq4s@gilmour
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxx>
> Cc: Emma Anholt <emma@xxxxxxxxxx>
> Cc: Maxime Ripard <maxime@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/vc4/vc4_kms.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_kms.c b/drivers/gpu/drm/vc4/vc4_kms.c
> index bb5529a7a9c2..948b3a58aad1 100644
> --- a/drivers/gpu/drm/vc4/vc4_kms.c
> +++ b/drivers/gpu/drm/vc4/vc4_kms.c
> @@ -372,7 +372,7 @@ static void vc4_atomic_commit_tail(struct drm_atomic_state *state)
>   		if (!old_hvs_state->fifo_state[channel].in_use)
>   			continue;
>   
> -		ret = drm_crtc_commit_wait(old_hvs_state->fifo_state[i].pending_commit);
> +		ret = drm_crtc_commit_wait(old_hvs_state->fifo_state[channel].pending_commit);
>   		if (ret)
>   			drm_err(dev, "Timed out waiting for commit\n");
>   	}

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland