[PATCH 4.18 061/350] drm: Get ref on CRTC commit object when waiting for flip_done

Greg Kroah-Hartman
2018-11-11T23:40:45Z

From: Leo Li <sunpeng.li@amd.com>

[ Upstream commit 4364bcb2cd21d042bde4776448417ddffbc54045 ]

This fixes a general protection fault, caused by accessing the contents
of a flip_done completion object that has already been freed. It occurs
due to the preemption of a non-blocking commit worker thread W by
another commit thread X. X continues to clear its atomic state at the
end, destroying the CRTC commit object that W still needs. Switching
back to W and accessing the commit objects then leads to bad results.

Worker W becomes preemptable when waiting for flip_done to complete. At
this point, a frequently occurring commit thread X can take over. Here's
an example where W is a worker thread that flips on both CRTCs, and X
does a legacy cursor update on both CRTCs:

...
1. W does flip work
2. W runs commit_hw_done()
3. W waits for flip_done on CRTC 1
4. > flip_done for CRTC 1 completes
5. W finishes waiting for CRTC 1
6. W waits for flip_done on CRTC 2

7. > Preempted by X
8. > flip_done for CRTC 2 completes
9. X atomic_check: hw_done and flip_done are complete on all CRTCs
10. X updates cursor on both CRTCs
11. X destroys atomic state
12. X done

13. > Switch back to W
14. W waits for flip_done on CRTC 2
15. W raises general protection fault

The error looks like so:

general protection fault: 0000 [#1] PREEMPT SMP PTI
**snip**
Call Trace:
lock_acquire+0xa2/0x1b0
_raw_spin_lock_irq+0x39/0x70
wait_for_completion_timeout+0x31/0x130
drm_atomic_helper_wait_for_flip_done+0x64/0x90 [drm_kms_helper]
amdgpu_dm_atomic_commit_tail+0xcae/0xdd0 [amdgpu]
commit_tail+0x3d/0x70 [drm_kms_helper]
process_one_work+0x212/0x650
worker_thread+0x49/0x420
kthread+0xfb/0x130
ret_from_fork+0x3a/0x50
Modules linked in: x86_pkg_temp_thermal amdgpu(O) chash(O)
gpu_sched(O) drm_kms_helper(O) syscopyarea sysfillrect sysimgblt
fb_sys_fops ttm(O) drm(O)

Note that i915 has this issue masked, since hw_done is signaled after
waiting for flip_done. Doing so will block the cursor update from
happening until hw_done is signaled, preventing the cursor commit from
destroying the state.

v2: The reference on the commit object needs to be obtained before
hw_done() is signaled, since that's the point where another commit
is allowed to modify the state. Assuming that the
new_crtc_state->commit object still exists within flip_done() is
incorrect.

Fix by getting a reference in setup_commit(), and releasing it
during default_clear().

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1539611200-6184-1-git-send-email-sunpeng.li@amd.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/gpu/drm/drm_atomic.c | 5 +++++
drivers/gpu/drm/drm_atomic_helper.c | 12 ++++++++----
include/drm/drm_atomic.h | 11 +++++++++++
3 files changed, 24 insertions(+), 4 deletions(-)

--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -173,6 +173,11 @@ void drm_atomic_state_default_clear(stru
state->crtcs[i].state = NULL;
state->crtcs[i].old_state = NULL;
state->crtcs[i].new_state = NULL;
+
+ if (state->crtcs[i].commit) {
+ drm_crtc_commit_put(state->crtcs[i].commit);
+ state->crtcs[i].commit = NULL;
+ }
}

for (i = 0; i < config->num_total_plane; i++) {
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1384,15 +1384,16 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for
void drm_atomic_helper_wait_for_flip_done(struct drm_device *dev,
struct drm_atomic_state *old_state)
{
- struct drm_crtc_state *new_crtc_state;
struct drm_crtc *crtc;
int i;

- for_each_new_crtc_in_state(old_state, crtc, new_crtc_state, i) {
- struct drm_crtc_commit *commit = new_crtc_state->commit;
+ for (i = 0; i < dev->mode_config.num_crtc; i++) {
+ struct drm_crtc_commit *commit = old_state->crtcs[i].commit;
int ret;

- if (!commit)
+ crtc = old_state->crtcs[i].ptr;
+
+ if (!crtc || !commit)
continue;

ret = wait_for_completion_timeout(&commit->flip_done, 10 * HZ);
@@ -1906,6 +1907,9 @@ int drm_atomic_helper_setup_commit(struc
drm_crtc_commit_get(commit);

commit->abort_completion = true;
+
+ state->crtcs[i].commit = commit;
+ drm_crtc_commit_get(commit);
}

for_each_oldnew_connector_in_state(state, conn, old_conn_state, new_conn_state, i) {
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -153,6 +153,17 @@ struct __drm_planes_state {
struct __drm_crtcs_state {
struct drm_crtc *ptr;
struct drm_crtc_state *state, *old_state, *new_state;
+
+ /**
+ * @commit:
+ *
+ * A reference to the CRTC commit object that is kept for use by
+ * drm_atomic_helper_wait_for_flip_done() after
+ * drm_atomic_helper_commit_hw_done() is called. This ensures that a
+ * concurrent commit won't free a commit object that is still in use.
+ */
+ struct drm_crtc_commit *commit;
+
s32 __user *out_fence_ptr;
u64 last_vblank_count;
};