[PATCH v2] dma-buf/sw_sync: Optimize the retrieval of fence reference counts

[Edward Adam Davis]

There is a race condition between closing a sync file and the SW_SYNC_IOC_INC
operation on a sync obj. When a fence is created via SW_SYNC_IOC_CREATE_FENCE,
its reference count is initialized to 1. When the sync file is closed, the
fence's reference count drops to 0. A race occurs if SW_SYNC_IOC_INC accesses
the fence whose ref count has already reached 0 before dma_fence_release()
calls timeline_fence_release() to erase the corresponding pt_tree node from
the sync object, potentially hitting:

refcount_t: addition on 0; use-after-free.
------------[ cut here ]------------
kernel BUG at lib/refcount.c:34!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
Call Trace:
 kref_get include/linux/kref.h:45 [inline]
 dma_fence_get include/linux/dma-fence.h:323 [inline]
 sync_timeline_signal+0x6d9/0x8a0 drivers/dma-buf/sw_sync.c:224
 sw_sync_ioctl_inc drivers/dma-buf/sw_sync.c:405 [inline]
 sw_sync_ioctl+0x7b4/0x13f0 drivers/dma-buf/sw_sync.c:470

Use dma_fence_get_rcu() to replace dma_get_fence(), if fence reference
count is 0 then do like timeline fence release things: remove pt link
and erase pt node.

Fixes: e531fdb5cd5e ("dma-buf/sw_sync: Avoid recursive lock during fence signal")
Reported-by: syzbot+cba1a1f459230395a076@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=cba1a1f459230395a076
Signed-off-by: Edward Adam Davis <eadavis@xxxxxxxx>
---
v1 -> v2: use dma_fence_get_rcu() and remove link & erase pt node

 drivers/dma-buf/sw_sync.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
index 8df20b0218a9..261bc98b1ed6 100644
--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -221,7 +221,11 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
 		if (!timeline_fence_signaled(&pt->base))
 			break;
 
-		dma_fence_get(&pt->base);
+		if (!dma_fence_get_rcu(&pt->base)) {
+			list_del_init(&pt->link);
+			rb_erase(&pt->node, &obj->pt_tree);
+			continue;
+		}
 
 		list_move_tail(&pt->link, &signalled);
 		rb_erase(&pt->node, &obj->pt_tree);
-- 
2.43.0