cpuinfo: Check for overflow in SIMD allocation

If the size to be allocated is very large and untrusted, then adding
the padding etc. might be enough to cause unsigned overflow, after
which a very small amount of memory will be allocated.

Signed-off-by: Simon McVittie <smcv@collabora.com>

src/cpuinfo/SDL_cpuinfo.c

@@ -1059,9 +1059,17 @@ SDL_SIMDAlloc(const size_t len)
 {
     const size_t alignment = SDL_SIMDGetAlignment();
     const size_t padding = (alignment - (len % alignment)) % alignment;
-    const size_t padded = len + padding;
     Uint8 *retval = NULL;
-    Uint8 *ptr = (Uint8 *) SDL_malloc(padded + alignment + sizeof (void *));
+    Uint8 *ptr;
+    size_t to_allocate;
+
+    /* alignment + padding + sizeof (void *) is bounded (a few hundred
+     * bytes max), so no need to check for overflow within that argument */
+    if (SDL_size_add_overflow(len, alignment + padding + sizeof (void *), &to_allocate)) {
+        return NULL;
+    }
+
+    ptr = (Uint8 *) SDL_malloc(to_allocate);
     if (ptr) {
         /* store the actual allocated pointer right before our aligned pointer. */
         retval = ptr + sizeof (void *);
@@ -1076,11 +1084,17 @@ SDL_SIMDRealloc(void *mem, const size_t len)
 {
     const size_t alignment = SDL_SIMDGetAlignment();
     const size_t padding = (alignment - (len % alignment)) % alignment;
-    const size_t padded = len + padding;
     Uint8 *retval = (Uint8*) mem;
     void *oldmem = mem;
     size_t memdiff = 0, ptrdiff;
     Uint8 *ptr;
+    size_t to_allocate;
+
+    /* alignment + padding + sizeof (void *) is bounded (a few hundred
+     * bytes max), so no need to check for overflow within that argument */
+    if (SDL_size_add_overflow(len, alignment + padding + sizeof (void *), &to_allocate)) {
+        return NULL;
+    }
 
     if (mem) {
         void **realptr = (void **) mem;
@@ -1091,7 +1105,7 @@ SDL_SIMDRealloc(void *mem, const size_t len)
         memdiff = ((size_t) oldmem) - ((size_t) mem);
     }
 
-    ptr = (Uint8 *) SDL_realloc(mem, padded + alignment + sizeof (void *));
+    ptr = (Uint8 *) SDL_realloc(mem, to_allocate);
 
     if (ptr == NULL) {
         return NULL; /* Out of memory, bail! */