[PATCH] drm/panfrost: Don't dereference bogus MMU pointers

[Robin Murphy]

It seems that killing an application while faults are occurring
(particularly with a GPU in FPGA at a whopping 40MHz) can lead to
handling a lingering page fault after all the address space contexts
have already been freed. In this situation, the LRU list is empty so
addr_to_drm_mm_node() ends up dereferencing the list head as if it were
a struct panfrost_mmu entry; this leaves "mmu->as" actually pointing at
the pfdev->alloc_mask bitmap, which is also empty, and given that the
fault has a high likelihood of being in AS0, hilarity ensues.

Sadly, the cleanest solution seems to involve another goto. Oh well, at
least it's robust...

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
 drivers/gpu/drm/panfrost/panfrost_mmu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
index e61984e26e0a..508765f80cfe 100644
--- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
+++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
@@ -413,11 +413,11 @@ addr_to_drm_mm_node(struct panfrost_device *pfdev, int as, u64 addr)
 	spin_lock(&pfdev->as_lock);
 	list_for_each_entry(mmu, &pfdev->as_lru_list, list) {
 		if (as == mmu->as)
-			break;
+			goto found_mmu;
 	}
-	if (as != mmu->as)
-		goto out;
+	goto out;
 
+found_mmu:
 	priv = container_of(mmu, struct panfrost_file_priv, mmu);
 
 	spin_lock(&priv->mm_lock);
-- 
2.21.0.dirty