drm/i915: Prevent writing into a read-only object via a GGTT mmap

commit 3e977ac6 upstream.

If the user has created a read-only object, they should not be allowed
to circumvent the write protection by using a GGTT mmapping. Deny it.

Also most machines do not support read-only GGTT PTEs, so again we have
to reject attempted writes. Fortunately, this is known a priori, so we
can at least reject in the call to create the mmap (with a sanity check
in the fault handler).

v2: Check the vma->vm_flags during mmap() to allow readonly access.
v3: Remove VM_MAYWRITE to curtail mprotect()

Testcase: igt/gem_userptr_blits/readonly_mmap*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com> #v1
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712185315.3288-4-chris@chris-wilson.co.ukSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/drm_gem.c

@@ -996,6 +996,15 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 		return -EACCES;
 	}
 
+	if (node->readonly) {
+		if (vma->vm_flags & VM_WRITE) {
+			drm_gem_object_unreference_unlocked(obj);
+			return -EINVAL;
+		}
+
+		vma->vm_flags &= ~VM_MAYWRITE;
+	}
+
 	ret = drm_gem_mmap_obj(obj, drm_vma_node_size(node) << PAGE_SHIFT,
 			       vma);
 

drivers/gpu/drm/i915/i915_drv.h

@@ -2339,6 +2339,18 @@ i915_gem_object_put_unlocked(struct drm_i915_gem_object *obj)
 __deprecated
 extern void drm_gem_object_unreference_unlocked(struct drm_gem_object *);
 
+static inline void
+i915_gem_object_set_readonly(struct drm_i915_gem_object *obj)
+{
+	obj->base.vma_node.readonly = true;
+}
+
+static inline bool
+i915_gem_object_is_readonly(const struct drm_i915_gem_object *obj)
+{
+	return obj->base.vma_node.readonly;
+}
+
 static inline bool
 i915_gem_object_has_struct_page(const struct drm_i915_gem_object *obj)
 {

drivers/gpu/drm/i915/i915_gem.c

@@ -1773,6 +1773,10 @@ int i915_gem_fault(struct vm_area_struct *area, struct vm_fault *vmf)
 	unsigned int flags;
 	int ret;
 
+	/* Sanity check that we allow writing into this object */
+	if (i915_gem_object_is_readonly(obj) && write)
+		return VM_FAULT_SIGBUS;
+
 	/* We don't use vmf->pgoff since that has the fake offset */
 	page_offset = ((unsigned long)vmf->virtual_address - area->vm_start) >>
 		PAGE_SHIFT;

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -178,7 +178,7 @@ static int ppgtt_bind_vma(struct i915_vma *vma,
 	vma->pages = vma->obj->pages;
 
 	/* Applicable to VLV, and gen8+ */
-	if (vma->obj->gt_ro)
+	if (i915_gem_object_is_readonly(vma->obj))
 		pte_flags |= PTE_READ_ONLY;
 
 	vma->vm->insert_entries(vma->vm, vma->pages, vma->node.start,
@@ -2360,6 +2360,11 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 
 	rpm_atomic_seq = assert_rpm_atomic_begin(dev_priv);
 
+	/*
+	 * Note that we ignore PTE_READ_ONLY here. The caller must be careful
+	 * not to allow the user to override access to a read only page.
+	 */
+
 	gtt_entries = (gen8_pte_t __iomem *)ggtt->gsm + (start >> PAGE_SHIFT);
 
 	for_each_sgt_dma(addr, sgt_iter, st) {
@@ -2620,7 +2625,7 @@ static int ggtt_bind_vma(struct i915_vma *vma,
 		return ret;
 
 	/* Applicable to VLV (gen8+ do not support RO in the GGTT) */
-	if (obj->gt_ro)
+	if (i915_gem_object_is_readonly(obj))
 		pte_flags |= PTE_READ_ONLY;
 
 	vma->vm->insert_entries(vma->vm, vma->pages, vma->node.start,
@@ -2649,7 +2654,7 @@ static int aliasing_gtt_bind_vma(struct i915_vma *vma,
 
 	/* Currently applicable only to VLV */
 	pte_flags = 0;
-	if (vma->obj->gt_ro)
+	if (i915_gem_object_is_readonly(vma->obj))
 		pte_flags |= PTE_READ_ONLY;
 
 

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1966,7 +1966,7 @@ intel_ring_create_vma(struct drm_i915_private *dev_priv, int size)
 	 * if supported by the platform's GGTT.
 	 */
 	if (vm->has_read_only)
-		obj->gt_ro = 1;
+		i915_gem_object_set_readonly(obj);
 
 	vma = i915_vma_create(obj, vm, NULL);
 	if (IS_ERR(vma))

include/drm/drm_vma_manager.h

@@ -42,6 +42,7 @@ struct drm_vma_offset_node {
 	rwlock_t vm_lock;
 	struct drm_mm_node vm_node;
 	struct rb_root vm_files;
+	bool readonly:1;
 };
 
 struct drm_vma_offset_manager {