bus-message: fix calculation of offsets table

The offsets specify the ends of variable length data. We would trust the
incoming data, putting the offsets specified in our message
into the offsets tables after doing some superficial verification.
But when actually reading the data we apply alignment, so we would take
the previous offset, align it, making it bigger then current offset, and
then we'd try to read data of negative length.

In the attached example, the message specifies the following offsets:
[1, 4]
but the alignment of those items is
[1, 8]
so we'd calculate the second item as starting at 8 and ending at 4.

src/libsystemd/sd-bus/bus-message.c

@@ -3115,6 +3115,7 @@ static int container_next_item(sd_bus_message *m, struct bus_container *c, size_
                         assert(alignment > 0);
 
                         *rindex = ALIGN_TO(c->offsets[c->offset_index], alignment);
+                        assert(c->offsets[c->offset_index+1] >= *rindex);
                         c->item_size = c->offsets[c->offset_index+1] - *rindex;
                 } else {
 
@@ -3154,6 +3155,7 @@ static int container_next_item(sd_bus_message *m, struct bus_container *c, size_
                 assert(alignment > 0);
 
                 *rindex = ALIGN_TO(c->offsets[c->offset_index], alignment);
+                assert(c->offsets[c->offset_index+1] >= *rindex);
                 c->item_size = c->offsets[c->offset_index+1] - *rindex;
 
                 c->offset_index++;
@@ -3700,7 +3702,7 @@ static int build_struct_offsets(
                 size_t *n_offsets) {
 
         unsigned n_variable = 0, n_total = 0, v;
-        size_t previous = 0, where;
+        size_t previous, where;
         const char *p;
         size_t sz;
         void *q;
@@ -3779,6 +3781,7 @@ static int build_struct_offsets(
 
         /* Second, loop again and build an offset table */
         p = signature;
+        previous = m->rindex;
         while (*p != 0) {
                 size_t n, offset;
                 int k;
@@ -3792,37 +3795,34 @@ static int build_struct_offsets(
                         memcpy(t, p, n);
                         t[n] = 0;
 
+                        size_t align = bus_gvariant_get_alignment(t);
+                        assert(align > 0);
+
+                        /* The possible start of this member after including alignment */
+                        size_t start = ALIGN_TO(previous, align);
+
                         k = bus_gvariant_get_size(t);
                         if (k < 0) {
                                 size_t x;
 
-                                /* variable size */
+                                /* Variable size */
                                 if (v > 0) {
                                         v--;
 
                                         x = bus_gvariant_read_word_le((uint8_t*) q + v*sz, sz);
                                         if (x >= size)
                                                 return -EBADMSG;
-                                        if (m->rindex + x < previous)
-                                                return -EBADMSG;
                                 } else
-                                        /* The last item's end
-                                         * is determined from
-                                         * the start of the
-                                         * offset array */
+                                        /* The last item's end is determined
+                                         * from the start of the offset array */
                                         x = size - (n_variable * sz);
 
                                 offset = m->rindex + x;
-
-                        } else {
-                                size_t align;
-
-                                /* fixed size */
-                                align = bus_gvariant_get_alignment(t);
-                                assert(align > 0);
-
-                                offset = (*n_offsets == 0 ? m->rindex  : ALIGN_TO((*offsets)[*n_offsets-1], align)) + k;
-                        }
+                                if (offset < start)
+                                        return -EBADMSG;
+                        } else
+                                /* Fixed size */
+                                offset = start + k;
                 }
 
                 previous = (*offsets)[(*n_offsets)++] = offset;