py/modstruct: Check and prevent buffer-read overflow in struct unpacking

Prior to this patch, the size of the buffer given to unpack/unpack_from was
checked for being too small by using the count of the arguments, not their
actual size.  For example, a format spec of '4I' would only check that
there was 4 bytes available, not 16; and 'I' would check for 1 byte, not 4.

This bug is fixed in this patch by calculating the total size of the format
spec at the start of the unpacking function.  This function anyway needs to
calculate the number of items at the start, so calculating the total size
can be done at the same time.

py/modstruct.c

@@ -82,26 +82,10 @@ STATIC mp_uint_t get_fmt_num(const char **p) {
     return val;
 }
 
-STATIC uint calcsize_items(const char *fmt) {
-    uint cnt = 0;
-    while (*fmt) {
-        int num = 1;
-        if (unichar_isdigit(*fmt)) {
-            num = get_fmt_num(&fmt);
-            if (*fmt == 's') {
-                num = 1;
-            }
-        }
-        cnt += num;
-        fmt++;
-    }
-    return cnt;
-}
-
-STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
-    const char *fmt = mp_obj_str_get_str(fmt_in);
+STATIC size_t calc_size_items(const char *fmt, size_t *total_sz) {
     char fmt_type = get_fmt_type(&fmt);
-    mp_uint_t size;
+    size_t total_cnt = 0;
+    size_t size;
     for (size = 0; *fmt; fmt++) {
         mp_uint_t cnt = 1;
         if (unichar_isdigit(*fmt)) {
@@ -109,8 +93,10 @@ STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
         }
 
         if (*fmt == 's') {
+            total_cnt += 1;
             size += cnt;
         } else {
+            total_cnt += cnt;
             mp_uint_t align;
             size_t sz = mp_binary_get_size(fmt_type, *fmt, &align);
             while (cnt--) {
@@ -120,6 +106,14 @@ STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
             }
         }
     }
+    *total_sz = size;
+    return total_cnt;
+}
+
+STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
+    const char *fmt = mp_obj_str_get_str(fmt_in);
+    size_t size;
+    calc_size_items(fmt, &size);
     return MP_OBJ_NEW_SMALL_INT(size);
 }
 MP_DEFINE_CONST_FUN_OBJ_1(struct_calcsize_obj, struct_calcsize);
@@ -130,8 +124,9 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
     // Since we implement unpack and unpack_from using the same function
     // we relax the "exact" requirement, and only implement "big enough".
     const char *fmt = mp_obj_str_get_str(args[0]);
+    size_t total_sz;
+    size_t num_items = calc_size_items(fmt, &total_sz);
     char fmt_type = get_fmt_type(&fmt);
-    uint num_items = calcsize_items(fmt);
     mp_obj_tuple_t *res = MP_OBJ_TO_PTR(mp_obj_new_tuple(num_items, NULL));
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[1], &bufinfo, MP_BUFFER_READ);
@@ -152,21 +147,23 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
         p += offset;
     }
 
-    for (uint i = 0; i < num_items;) {
-        mp_uint_t sz = 1;
+    // Check that the input buffer is big enough to unpack all the values
+    if (p + total_sz > end_p) {
+        mp_raise_ValueError("buffer too small");
+    }
+
+    for (size_t i = 0; i < num_items;) {
+        mp_uint_t cnt = 1;
         if (unichar_isdigit(*fmt)) {
-            sz = get_fmt_num(&fmt);
-        }
-        if (p + sz > end_p) {
-            mp_raise_ValueError("buffer too small");
+            cnt = get_fmt_num(&fmt);
         }
         mp_obj_t item;
         if (*fmt == 's') {
-            item = mp_obj_new_bytes(p, sz);
-            p += sz;
+            item = mp_obj_new_bytes(p, cnt);
+            p += cnt;
             res->items[i++] = item;
         } else {
-            while (sz--) {
+            while (cnt--) {
                 item = mp_binary_get_val(fmt_type, *fmt, &p);
                 res->items[i++] = item;
             }

tests/basics/struct1.py

@@ -39,6 +39,12 @@ print(v == (10, 100, 200, 300))
 # network byte order
 print(struct.pack('!i', 123))
 
+# check that we get an error if the buffer is too small
+try:
+    struct.unpack('I', b'\x00\x00\x00')
+except:
+    print('struct.error')
+
 # first arg must be a string
 try:
     struct.pack(1, 2)

tests/basics/struct2.py

@@ -25,6 +25,12 @@ print(struct.calcsize('0s1s0H2H'))
 print(struct.unpack('<0s1s0H2H', b'01234'))
 print(struct.pack('<0s1s0H2H', b'abc', b'abc', 258, 515))
 
+# check that we get an error if the buffer is too small
+try:
+    struct.unpack('2H', b'\x00\x00')
+except:
+    print('Exception')
+
 # check that unknown types raise an exception
 try:
     struct.unpack('z', b'1')