drm/i915: Prevent integer overflow when validating the execbuffer

Commit 2549d6c2 removed the vmalloc used for temporary storage of the
relocation lists used during execbuffer. However, our use of vmalloc was
being protected by an integer overflow check which we do want to
preserve!
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -3630,8 +3630,15 @@ validate_exec_list(struct drm_i915_gem_exec_object2 *exec,
 
 	for (i = 0; i < count; i++) {
 		char __user *ptr = (char __user *)(uintptr_t)exec[i].relocs_ptr;
-		size_t length = exec[i].relocation_count * sizeof(struct drm_i915_gem_relocation_entry);
+		int length; /* limited by fault_in_pages_readable() */
 
+		/* First check for malicious input causing overflow */
+		if (exec[i].relocation_count >
+		    INT_MAX / sizeof(struct drm_i915_gem_relocation_entry))
+			return -EINVAL;
+
+		length = exec[i].relocation_count *
+			sizeof(struct drm_i915_gem_relocation_entry);
 		if (!access_ok(VERIFY_READ, ptr, length))
 			return -EFAULT;