drm/amdgpu/userq: fix dma_fence refcount underflow in userq path

An extra dma_fence_put() can drop the last reference to a fence while it is
still attached to a dma_resv object. This frees the fence prematurely via
dma_fence_release() while other users still hold the pointer.

Later accesses through dma_resv iteration may then operate on the freed
fence object, leading to refcount underflow warnings and potential hangs
when walking reservation fences.

Fix this by correcting the fence lifetime so the dma_resv object retains a
valid reference until it is done with the fence.i

[   31.133803] refcount_t: underflow; use-after-free.
[   31.133805] WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x58/0x90, CPU#18: kworker/u96:1/188

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
index 8afd70667b4905b87093c8a77499cc86ed57066b..c8b0c4600a88da49121db51f14a6ce2f6ce65fcc 100644 (file)
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
@@ -881,12 +881,9 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
                                 * be good for now
                                 */
                                r = dma_fence_wait(fences[i], true);
-                               if (r) {
-                                       dma_fence_put(fences[i]);
+                               if (r)
                                        goto free_fences;
-                               }
 
-                               dma_fence_put(fences[i]);
                                continue;
                        }
 
@@ -908,7 +905,6 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
                        fence_info[cnt].va = fence_drv->va;
                        fence_info[cnt].value = fences[i]->seqno;
 
-                       dma_fence_put(fences[i]);
                        /* Increment the actual userq fence count */
                        cnt++;
                }