linux-kernel - [PATCH 4.9 096/109] dma-buf/sw_sync: clean up list before signaling the fence

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Message-Id: <20171207125645.662924122@linuxfoundation.org>
Date:   Thu,  7 Dec 2017 13:57:20 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Chris Wilson <chris@...is-wilson.co.uk>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Subject: [PATCH 4.9 096/109] dma-buf/sw_sync: clean up list before signaling the fence

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gustavo Padovan <gustavo.padovan@...labora.com>

commit 3792b7c1a70815fe4e954221c096f9278638fd21 upstream.

If userspace already dropped its own reference by closing the sw_sync
fence fd we might end up in a deadlock where
dma_fence_is_signaled_locked() will trigger the release of the fence and
thus try to hold the lock to remove the fence from the list.

dma_fence_is_signaled_locked() tries to release/free the fence and hold
the lock in the process.

We fix that by changing the order operation and clean up the list and
rb-tree first.

v2: Drop fence get/put dance and manipulate the list first (Chris Wilson)

Cc: Chris Wilson <chris@...is-wilson.co.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.com>
Reviewed-by: Chris Wilson <chris@...is-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170729152217.8362-2-gustavo@padovan.org
[s/dma_fence/fence/g - gregkh]
Cc: Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/dma-buf/sw_sync.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -213,11 +213,21 @@ static void sync_timeline_signal(struct
 	obj->value += inc;
 
 	list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
-		if (!fence_is_signaled_locked(&pt->base))
+		if (!timeline_fence_signaled(&pt->base))
 			break;
 
 		list_del_init(&pt->link);
 		rb_erase(&pt->node, &obj->pt_tree);
+
+		/*
+		 * A signal callback may release the last reference to this
+		 * fence, causing it to be freed. That operation has to be
+		 * last to avoid a use after free inside this loop, and must
+		 * be after we remove the fence from the timeline in order to
+		 * prevent deadlocking on timeline->lock inside
+		 * timeline_fence_release().
+		 */
+		fence_signal_locked(&pt->base);
 	}
 
 	spin_unlock_irq(&obj->lock);