py/objarray: Fix use-after-free if extending a bytearray from itself.

Two cases, one assigning to a slice.
Closes https://github.com/micropython/micropython/issues/13283

Second is extending a slice from itself, similar logic.

In both cases the problem occurs when m_renew causes realloc to move the
buffer, leaving a dangling pointer behind.

There are more complex and hard to fix cases when either argument is a
memoryview into the buffer, currently resizing to a new address breaks
memoryviews into that object.

Reproducing this bug and confirming the fix was done by running the unix
port under valgrind with GC-aware extensions.

Note in default configurations with GIL this bug exists but has no impact
(the free buffer won't be reused while the function is still executing, and
is no longer referenced after it returns).

Signed-off-by: Angus Gratton <angus@redyak.com.au>

py/objarray.c

@@ -424,6 +424,13 @@ static mp_obj_t array_extend(mp_obj_t self_in, mp_obj_t arg_in) {
     if (self->free < len) {
         self->items = m_renew(byte, self->items, (self->len + self->free) * sz, (self->len + len) * sz);
         self->free = 0;
+
+        if (self_in == arg_in) {
+            // Get arg_bufinfo again in case self->items has moved
+            //
+            // (Note not possible to handle case that arg_in is a memoryview into self)
+            mp_get_buffer_raise(arg_in, &arg_bufinfo, MP_BUFFER_READ);
+        }
     } else {
         self->free -= len;
     }
@@ -456,7 +463,8 @@ static mp_obj_t array_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value
                 #if MICROPY_PY_ARRAY_SLICE_ASSIGN
                 // Assign
                 size_t src_len;
-                void *src_items;
+                uint8_t *src_items;
+                size_t src_offs = 0;
                 size_t item_sz = mp_binary_get_size('@', o->typecode & TYPECODE_MASK, NULL);
                 if (mp_obj_is_obj(value) && MP_OBJ_TYPE_GET_SLOT_OR_NULL(((mp_obj_base_t *)MP_OBJ_TO_PTR(value))->type, subscr) == array_subscr) {
                     // value is array, bytearray or memoryview
@@ -469,7 +477,7 @@ static mp_obj_t array_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value
                     src_items = src_slice->items;
                     #if MICROPY_PY_BUILTINS_MEMORYVIEW
                     if (mp_obj_is_type(value, &mp_type_memoryview)) {
-                        src_items = (uint8_t *)src_items + (src_slice->memview_offset * item_sz);
+                        src_offs = src_slice->memview_offset * item_sz;
                     }
                     #endif
                 } else if (mp_obj_is_type(value, &mp_type_bytes)) {
@@ -504,13 +512,17 @@ static mp_obj_t array_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value
                         // TODO: alloc policy; at the moment we go conservative
                         o->items = m_renew(byte, o->items, (o->len + o->free) * item_sz, (o->len + len_adj) * item_sz);
                         o->free = len_adj;
+                        // m_renew may have moved o->items
+                        if (src_items == dest_items) {
+                            src_items = o->items;
+                        }
                         dest_items = o->items;
                     }
                     mp_seq_replace_slice_grow_inplace(dest_items, o->len,
-                        slice.start, slice.stop, src_items, src_len, len_adj, item_sz);
+                        slice.start, slice.stop, src_items + src_offs, src_len, len_adj, item_sz);
                 } else {
                     mp_seq_replace_slice_no_grow(dest_items, o->len,
-                        slice.start, slice.stop, src_items, src_len, item_sz);
+                        slice.start, slice.stop, src_items + src_offs, src_len, item_sz);
                     // Clear "freed" elements at the end of list
                     // TODO: This is actually only needed for typecode=='O'
                     mp_seq_clear(dest_items, o->len + len_adj, o->len, item_sz);

tests/basics/bytearray_add.py

@@ -15,4 +15,11 @@ print(b)
 
 # this inplace add tests the code when the buffer doesn't need to be increased
 b = bytearray()
-b += b''
+b += b""
+
+# extend a bytearray from itself
+b = bytearray(b"abcdefgh")
+for _ in range(4):
+    c = bytearray(b)  # extra allocation, as above
+    b.extend(b)
+print(b)

tests/basics/bytearray_add_self.py

@@ -0,0 +1,8 @@
+# add a bytearray to itself
+# This is not supported by CPython as of 3.11.18.
+
+b = bytearray(b"123456789")
+for _ in range(4):
+    c = bytearray(b)  # extra allocation increases chance 'b' has to relocate
+    b += b
+print(b)

tests/basics/bytearray_add_self.py.exp

@@ -0,0 +1 @@
+bytearray(b'123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789123456789')

tests/basics/bytearray_slice_assign.py

@@ -18,7 +18,7 @@ l = bytearray(x)
 l[1:3] = bytearray()
 print(l)
 l = bytearray(x)
-#del l[1:3]
+# del l[1:3]
 print(l)
 
 l = bytearray(x)
@@ -28,7 +28,7 @@ l = bytearray(x)
 l[:3] = bytearray()
 print(l)
 l = bytearray(x)
-#del l[:3]
+# del l[:3]
 print(l)
 
 l = bytearray(x)
@@ -38,7 +38,7 @@ l = bytearray(x)
 l[:-3] = bytearray()
 print(l)
 l = bytearray(x)
-#del l[:-3]
+# del l[:-3]
 print(l)
 
 # slice assignment that extends the array
@@ -61,8 +61,14 @@ b[1:1] = b"12345"
 print(b)
 
 # Growth of bytearray via slice extension
-b = bytearray(b'12345678')
-b.append(57) # expand and add a bit of unused space at end of the bytearray
+b = bytearray(b"12345678")
+b.append(57)  # expand and add a bit of unused space at end of the bytearray
 for i in range(400):
-    b[-1:] = b'ab' # grow slowly into the unused space
+    b[-1:] = b"ab"  # grow slowly into the unused space
+print(len(b), b)
+
+# Growth of bytearray via slice extension from itself
+b = bytearray(b"1234567")
+for i in range(3):
+    b[-1:] = b
 print(len(b), b)