Patch "drm/ttm: fix use-after-free races in vm fault handling" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

drm/ttm: fix use-after-free races in vm fault handling

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-ttm-fix-use-after-free-races-in-vm-fault-handling.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: =?UTF-8?q?Nicolai=20H=C3=A4hnle?= <[email protected]>
Date: Sat, 18 Feb 2017 22:59:56 +0100
Subject: drm/ttm: fix use-after-free races in vm fault handling
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Nicolai Hähnle <[email protected]>

commit 3089c1df10e2931b1d72d2ffa7d86431084c86b3 upstream.

The vm fault handler relies on the fact that the VMA owns a reference to the BO. However, once mmap_sem is released, other tasks are free to destroy the VMA, which can lead to the BO being freed. Fix two code paths where that can happen, both related to vm fault retries.

Found via a lock debugging warning which flagged &bo->wu_mutex as locked while being destroyed.

---
 drivers/gpu/drm/ttm/ttm_bo_vm.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

@@ -66,8 +66,11 @@ static int ttm_bo_vm_fault_idle(struct t
 		if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_unlock;
 
+		ttm_bo_reference(bo);
 		up_read(&vma->vm_mm->mmap_sem);
 		(void) fence_wait(bo->moving, true);
+		ttm_bo_unreserve(bo);
+		ttm_bo_unref(&bo);
 		goto out_unlock;
 	}
 

@@ -120,8 +123,10 @@ static int ttm_bo_vm_fault(struct vm_are
 
 		if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
 			if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
+				ttm_bo_reference(bo);
 				up_read(&vma->vm_mm->mmap_sem);
 				(void) ttm_bo_wait_unreserved(bo);
+				ttm_bo_unref(&bo);
 			}
 
 			return VM_FAULT_RETRY;

@@ -166,6 +171,13 @@ static int ttm_bo_vm_fault(struct vm_are
 	ret = ttm_bo_vm_fault_idle(bo, vma, vmf);
 	if (unlikely(ret != 0)) {
 		retval = ret;
+
+		if (retval == VM_FAULT_RETRY &&
+		    !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
+			/* The BO has already been unreserved. */
+			return retval;
+		}
+
 		goto out_unlock;
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/drm-ttm-fix-use-after-free-races-in-vm-fault-handling.patch