[PATCH 2/2] drm/msm: protect against faults from copy_from_user() in submit ioctl

[Rob Clark <robdclark@gmail.com>]

An evil userspace could try to cause deadlock by passing an unfaulted-in
GEM bo as submit->bos (or submit->cmds) table.  Which will trigger
msm_gem_fault() while we already hold struct_mutex.  See:

https://github.com/freedreno/msmtest/blob/master/evilsubmittest.c

Cc: stable@vger.kernel.org
Signed-off-by: Rob Clark <robdclark@gmail.com>
---
 drivers/gpu/drm/msm/msm_drv.h        | 6 ++++++
 drivers/gpu/drm/msm/msm_gem.c        | 9 +++++++++
 drivers/gpu/drm/msm/msm_gem_submit.c | 3 +++
 3 files changed, 18 insertions(+)

diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
index a35c1b6..957801e 100644
--- a/drivers/gpu/drm/msm/msm_drv.h
+++ b/drivers/gpu/drm/msm/msm_drv.h
@@ -157,6 +157,12 @@ struct msm_drm_private {
 	struct shrinker shrinker;
 
 	struct msm_vblank_ctrl vblank_ctrl;
+
+	/* task holding struct_mutex.. currently only used in submit path
+	 * to detect and reject faults from copy_from_user() for submit
+	 * ioctl.
+	 */
+	struct task_struct *struct_mutex_task;
 };
 
 struct msm_format {
diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index 8dfdeec..f6b8945 100644
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/drivers/gpu/drm/msm/msm_gem.c
@@ -196,11 +196,20 @@ int msm_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
 	struct drm_gem_object *obj = vma->vm_private_data;
 	struct drm_device *dev = obj->dev;
+	struct msm_drm_private *priv = dev->dev_private;
 	struct page **pages;
 	unsigned long pfn;
 	pgoff_t pgoff;
 	int ret;
 
+	/* This should only happen if userspace tries to pass a mmap'd
+	 * but unfaulted gem bo vaddr into submit ioctl, triggering
+	 * a page fault while struct_mutex is already held.  This is
+	 * not a valid use-case so just bail.
+	 */
+	if (priv->struct_mutex_task == current)
+		return VM_FAULT_SIGBUS;
+
 	/* Make sure we don't parallel update on a fault, nor move or remove
 	 * something from beneath our feet
 	 */
diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
index 03d4ce2..0be57a9 100644
--- a/drivers/gpu/drm/msm/msm_gem_submit.c
+++ b/drivers/gpu/drm/msm/msm_gem_submit.c
@@ -426,6 +426,8 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
 	if (ret)
 		return ret;
 
+	priv->struct_mutex_task = current;
+
 	if (args->flags & MSM_SUBMIT_FENCE_FD_OUT) {
 		out_fence_fd = get_unused_fd_flags(O_CLOEXEC);
 		if (out_fence_fd < 0) {
@@ -549,6 +551,7 @@ out:
 out_unlock:
 	if (ret && (out_fence_fd >= 0))
 		put_unused_fd(out_fence_fd);
+	priv->struct_mutex_task = NULL;
 	mutex_unlock(&dev->struct_mutex);
 	return ret;
 }
-- 
2.7.4