[PATCH 2/3] connection: Prevent integer overflow in DIV_ROUNDUP.

[Michal Srb]

The DIV_ROUNDUP macro would overflow when trying to round values higher
than MAX_UINT32 - (a - 1). The result is 0 after the division. This is
potential security issue when demarshalling an array because the length
check is performed with the overflowed value, but then the original huge
value is stored for later use.

The issue was present only on 32bit platforms. The use of size_t in the
DIV_ROUNDUP macro already promoted everything to 64 bit size on 64 bit
systems.
---
 src/connection.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/src/connection.c b/src/connection.c
index 294c521..31396bc 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -44,7 +44,12 @@
 #include "wayland-private.h"
 #include "wayland-os.h"
 
-#define DIV_ROUNDUP(n, a) ( ((n) + ((a) - 1)) / (a) )
+static inline uint32_t div_roundup(uint32_t n, size_t a) {
+    // The cast to uint64_t is necessary to prevent overflow when rounding
+    // values close to UINT32_MAX. After the division it is again safe to
+    // cast back to uint32_t.
+    return (uint32_t) (((uint64_t) n + (a - 1)) / a);
+}
 
 struct wl_buffer {
 	char data[4096];
@@ -734,7 +739,7 @@ wl_connection_demarshal(struct wl_connection *connection,
 				break;
 			}
 
-			next = p + DIV_ROUNDUP(length, sizeof *p);
+			next = p + div_roundup(length, sizeof *p);
 			if (next > end) {
 				wl_log("message too short, "
 				       "object (%d), message %s(%s)\n",
@@ -793,7 +798,7 @@ wl_connection_demarshal(struct wl_connection *connection,
 		case 'a':
 			length = *p++;
 
-			next = p + DIV_ROUNDUP(length, sizeof *p);
+			next = p + div_roundup(length, sizeof *p);
 			if (next > end) {
 				wl_log("message too short, "
 				       "object (%d), message %s(%s)\n",
@@ -1068,7 +1073,7 @@ buffer_size_for_closure(struct wl_closure *closure)
 			}
 
 			size = strlen(closure->args[i].s) + 1;
-			buffer_size += 1 + DIV_ROUNDUP(size, sizeof(uint32_t));
+			buffer_size += 1 + div_roundup(size, sizeof(uint32_t));
 			break;
 		case 'a':
 			if (closure->args[i].a == NULL) {
@@ -1077,7 +1082,7 @@ buffer_size_for_closure(struct wl_closure *closure)
 			}
 
 			size = closure->args[i].a->size;
-			buffer_size += (1 + DIV_ROUNDUP(size, sizeof(uint32_t)));
+			buffer_size += (1 + div_roundup(size, sizeof(uint32_t)));
 			break;
 		default:
 			break;
@@ -1139,11 +1144,11 @@ serialize_closure(struct wl_closure *closure, uint32_t *buffer,
 			size = strlen(closure->args[i].s) + 1;
 			*p++ = size;
 
-			if (p + DIV_ROUNDUP(size, sizeof *p) > end)
+			if (p + div_roundup(size, sizeof *p) > end)
 				goto overflow;
 
 			memcpy(p, closure->args[i].s, size);
-			p += DIV_ROUNDUP(size, sizeof *p);
+			p += div_roundup(size, sizeof *p);
 			break;
 		case 'a':
 			if (closure->args[i].a == NULL) {
@@ -1154,11 +1159,11 @@ serialize_closure(struct wl_closure *closure, uint32_t *buffer,
 			size = closure->args[i].a->size;
 			*p++ = size;
 
-			if (p + DIV_ROUNDUP(size, sizeof *p) > end)
+			if (p + div_roundup(size, sizeof *p) > end)
 				goto overflow;
 
 			memcpy(p, closure->args[i].a->data, size);
-			p += DIV_ROUNDUP(size, sizeof *p);
+			p += div_roundup(size, sizeof *p);
 			break;
 		default:
 			break;
-- 
2.16.4