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

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: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712185315.3288-4-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_gem.c

@@ -1036,6 +1036,15 @@ 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_put_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_gem.c

@@ -2012,6 +2012,10 @@ vm_fault_t i915_gem_fault(struct vm_fault *vmf)
 	pgoff_t page_offset;
 	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 = (vmf->address - area->vm_start) >> PAGE_SHIFT;
 

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -206,7 +206,7 @@ static int ppgtt_bind_vma(struct i915_vma *vma,
 
 	/* Applicable to VLV, and gen8+ */
 	pte_flags = 0;
-	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, cache_level, pte_flags);
@@ -2491,8 +2491,10 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 	const gen8_pte_t pte_encode = gen8_pte_encode(0, level, 0);
 	dma_addr_t addr;
 
-	/* The GTT does not support read-only mappings */
-	GEM_BUG_ON(flags & PTE_READ_ONLY);
+	/*
+	 * 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;
 	gtt_entries += vma->node.start >> PAGE_SHIFT;
@@ -2731,7 +2733,7 @@ static int ggtt_bind_vma(struct i915_vma *vma,
 
 	/* Applicable to VLV (gen8+ do not support RO in the GGTT) */
 	pte_flags = 0;
-	if (obj->gt_ro)
+	if (i915_gem_object_is_readonly(obj))
 		pte_flags |= PTE_READ_ONLY;
 
 	intel_runtime_pm_get(i915);
@@ -2769,7 +2771,7 @@ 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;
 
 	if (flags & I915_VMA_LOCAL_BIND) {

drivers/gpu/drm/i915/i915_gem_object.h

@@ -141,7 +141,6 @@ struct drm_i915_gem_object {
 	 * Is the object to be mapped as read-only to the GPU
 	 * Only honoured if hardware has relevant pte bit
 	 */
-	unsigned long gt_ro:1;
 	unsigned int cache_level:3;
 	unsigned int cache_coherent:2;
 #define I915_BO_CACHE_COHERENT_FOR_READ BIT(0)
@@ -358,6 +357,18 @@ static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj)
 	reservation_object_unlock(obj->resv);
 }
 
+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/intel_ringbuffer.c

@@ -1100,7 +1100,7 @@ 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_instance(obj, vm, NULL);
 	if (IS_ERR(vma))

drivers/gpu/drm/i915/selftests/i915_gem_context.c

@@ -487,7 +487,8 @@ static int igt_ctx_readonly(void *arg)
 					goto out_unlock;
 				}
 
-				obj->gt_ro = prandom_u32_state(&prng);
+				if (prandom_u32_state(&prng) & 1)
+					i915_gem_object_set_readonly(obj);
 			}
 
 			intel_runtime_pm_get(i915);
@@ -518,7 +519,7 @@ static int igt_ctx_readonly(void *arg)
 		unsigned int num_writes;
 
 		num_writes = rem;
-		if (obj->gt_ro)
+		if (i915_gem_object_is_readonly(obj))
 			num_writes = 0;
 
 		err = cpu_check(obj, num_writes);

include/drm/drm_vma_manager.h

@@ -41,6 +41,7 @@ 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 {