[PATCH] drm/i915: Possible security hole in command parsing

["Rebecca N. Palmer" <rebecca_palmer@zoho.com>]

i915_parse_cmds returns -EACCES on chained batches, which "tells the
caller to abort and dispatch the workload as a non-secure batch",
but the mechanism implementing that was broken when
flags |= I915_DISPATCH_SECURE was moved from i915_gem_execbuffer_parse
to i915_gem_do_execbuffer (17cabf571e50677d980e9ab2a43c5f11213003ae):
i915_gem_execbuffer_parse returns the original batch_obj in this case,
and i915_gem_do_execbuffer doesn't check for that.

Is this being made secure some other way (in which case the obsolete
comments should probably be removed), or is this a security hole?

Warning: this is my first kernel patch, and has not been tested yet.
Signed-off-by: Rebecca Palmer <rebecca_palmer@zoho.com> 

--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1398,7 +1398,7 @@ i915_gem_do_execbuffer(struct drm_device
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct eb_vmas *eb;
-	struct drm_i915_gem_object *batch_obj;
+	struct drm_i915_gem_object *batch_obj, *orig_batch_obj;
 	struct drm_i915_gem_exec_object2 shadow_exec_entry;
 	struct intel_engine_cs *ring;
 	struct intel_context *ctx;
@@ -1511,7 +1511,7 @@ i915_gem_do_execbuffer(struct drm_device
 		goto err;
 
 	/* take note of the batch buffer before we might reorder the lists */
-	batch_obj = eb_get_batch(eb);
+	orig_batch_obj = eb_get_batch(eb);
 
 	/* Move the objects en-masse into the GTT, evicting if necessary. */
 	need_relocs = (args->flags & I915_EXEC_NO_RELOC) == 0;
@@ -1533,7 +1533,7 @@ i915_gem_do_execbuffer(struct drm_device
 	}
 
 	/* Set the pending read domains for the batch buffer to COMMAND */
-	if (batch_obj->base.pending_write_domain) {
+	if (orig_batch_obj->base.pending_write_domain) {
 		DRM_DEBUG("Attempting to use self-modifying batch buffer\n");
 		ret = -EINVAL;
 		goto err;
@@ -1543,7 +1543,7 @@ i915_gem_do_execbuffer(struct drm_device
 		batch_obj = i915_gem_execbuffer_parse(ring,
 						      &shadow_exec_entry,
 						      eb,
-						      batch_obj,
+						      orig_batch_obj,
 						      args->batch_start_offset,
 						      args->batch_len,
 						      file->is_master);
@@ -1559,7 +1559,7 @@ i915_gem_do_execbuffer(struct drm_device
 		 * don't want that set when the command parser is
 		 * enabled.
 		 */
-		if (USES_PPGTT(dev))
+		if (USES_PPGTT(dev) && batch_obj!=orig_batch_obj)
 			dispatch_flags |= I915_DISPATCH_SECURE;
 
 		exec_start = 0;

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