[PATCH AUTOSEL 5.4 28/32] drm/scheduler: fix rare NULL ptr race

Sasha Levin
2020-04-07T00:05:38Z

From: Yintian Tao <yttao@amd.com>

[ Upstream commit 77bb2f204f1f0a53a602a8fd15816d6826212077 ]

There is one one corner case at dma_fence_signal_locked
which will raise the NULL pointer problem just like below.
->dma_fence_signal
->dma_fence_signal_locked
->test_and_set_bit
here trigger dma_fence_release happen due to the zero of fence refcount.

->dma_fence_put
->dma_fence_release
->drm_sched_fence_release_scheduled
->call_rcu
here make the union fled “cb_list” at finished fence
to NULL because struct rcu_head contains two pointer
which is same as struct list_head cb_list

Therefore, to hold the reference of finished fence at drm_sched_process_job
to prevent the null pointer during finished fence dma_fence_signal

[ 732.912867] BUG: kernel NULL pointer dereference, address: 0000000000000008
[ 732.914815] #PF: supervisor write access in kernel mode
[ 732.915731] #PF: error_code(0x0002) - not-present page
[ 732.916621] PGD 0 P4D 0
[ 732.917072] Oops: 0002 [#1] SMP PTI
[ 732.917682] CPU: 7 PID: 0 Comm: swapper/7 Tainted: G OE 5.4.0-rc7 #1
[ 732.918980] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
[ 732.920906] RIP: 0010:dma_fence_signal_locked+0x3e/0x100
[ 732.938569] Call Trace:
[ 732.939003] <IRQ>
[ 732.939364] dma_fence_signal+0x29/0x50
[ 732.940036] drm_sched_fence_finished+0x12/0x20 [gpu_sched]
[ 732.940996] drm_sched_process_job+0x34/0xa0 [gpu_sched]
[ 732.941910] dma_fence_signal_locked+0x85/0x100
[ 732.942692] dma_fence_signal+0x29/0x50
[ 732.943457] amdgpu_fence_process+0x99/0x120 [amdgpu]
[ 732.944393] sdma_v4_0_process_trap_irq+0x81/0xa0 [amdgpu]

v2: hold the finished fence at drm_sched_process_job instead of
amdgpu_fence_process
v3: resume the blank line

Signed-off-by: Yintian Tao <yttao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/scheduler/sched_main.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index 2af64459b3d77..dfb29e6eeff1e 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -627,7 +627,9 @@ static void drm_sched_process_job(struct dma_fence *f, struct dma_fence_cb *cb)

trace_drm_sched_process_job(s_fence);

+ dma_fence_get(&s_fence->finished);
drm_sched_fence_finished(s_fence);
+ dma_fence_put(&s_fence->finished);
wake_up_interruptible(&sched->wake_up_worker);
}

--
2.20.1