[PATCH] dma-buf/sw_sync: Synchronize signal vs syncpt free

[Chris Wilson]

During release of the syncpt, we remove it from the list of syncpt and
the tree, but only if it is not already been removed. However, during
signaling, we first remove the syncpt from the list. So, if we
concurrently free and signal the syncpt, the free may decide that it is
not part of the tree and immediately free itself -- meanwhile the
signaler goes onto to use the now freed datastructure.

In particular, we get struct by commit 0e2f733addbf ("dma-buf: make
dma_fence structure a bit smaller v2") as the cb_list is immediately
clobbered by the kfree_rcu.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111381
Fixes: d3862e44daa7 ("dma-buf/sw-sync: Fix locking around sync_timeline lists")
References: 0e2f733addbf ("dma-buf: make dma_fence structure a bit smaller v2")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
Cc: Sean Paul <seanpaul at chromium.org>
Cc: Gustavo Padovan <gustavo at padovan.org>
Cc: Christian König <christian.koenig at amd.com>
Cc: <stable at vger.kernel.org> # v4.14+
---
 drivers/dma-buf/sw_sync.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
index 051f6c2873c7..27b1d549ed38 100644
--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -132,17 +132,14 @@ static void timeline_fence_release(struct dma_fence *fence)
 {
 	struct sync_pt *pt = dma_fence_to_sync_pt(fence);
 	struct sync_timeline *parent = dma_fence_parent(fence);
+	unsigned long flags;
 
+	spin_lock_irqsave(fence->lock, flags);
 	if (!list_empty(&pt->link)) {
-		unsigned long flags;
-
-		spin_lock_irqsave(fence->lock, flags);
-		if (!list_empty(&pt->link)) {
-			list_del(&pt->link);
-			rb_erase(&pt->node, &parent->pt_tree);
-		}
-		spin_unlock_irqrestore(fence->lock, flags);
+		list_del(&pt->link);
+		rb_erase(&pt->node, &parent->pt_tree);
 	}
+	spin_unlock_irqrestore(fence->lock, flags);
 
 	sync_timeline_put(parent);
 	dma_fence_free(fence);
-- 
2.23.0.rc1