drm/i915/userptr: Hold mmref whilst calling get-user-pages

Holding a reference to the containing task_struct is not sufficient to
prevent the mm_struct from being reaped under memory pressure. If this
happens whilst we are calling get_user_pages(), explosions erupt -
sometimes an immediate GPF, sometimes page flag corruption. To prevent
the target mm from being reaped as we are reading from it, acquire a
reference before we begin.

Testcase: igt/gem_shrink/*userptr
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459864801-28606-2-git-send-email-chris@chris-wilson.co.uk
(cherry picked from commit 40313f0cd0b711a7a5905e5182422799e157d8aa)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

1 files changed, 17 insertions, 12 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 18ba8139e922..4d30b60defda 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -501,19 +501,24 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
         if (pvec != NULL) {
                 struct mm_struct *mm = obj->userptr.mm->mm;
 
-                down_read(&mm->mmap_sem);
-                while (pinned < npages) {
-                        ret = get_user_pages_remote(work->task, mm,
-                                        obj->userptr.ptr + pinned * PAGE_SIZE,
-                                        npages - pinned,
-                                        !obj->userptr.read_only, 0,
-                                        pvec + pinned, NULL);
-                        if (ret < 0)
-                                break;
-
-                        pinned += ret;
+                ret = -EFAULT;
+                if (atomic_inc_not_zero(&mm->mm_users)) {
+                        down_read(&mm->mmap_sem);
+                        while (pinned < npages) {
+                                ret = get_user_pages_remote
+                                        (work->task, mm,
+                                         obj->userptr.ptr + pinned * PAGE_SIZE,
+                                         npages - pinned,
+                                         !obj->userptr.read_only, 0,
+                                         pvec + pinned, NULL);
+                                if (ret < 0)
+                                        break;
+
+                                pinned += ret;
+                        }
+                        up_read(&mm->mmap_sem);
+                        mmput(mm);
                 }
-                up_read(&mm->mmap_sem);
         }
 
         mutex_lock(&dev->struct_mutex);