diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
index e211c3d15a4ed2..9a435de1e0ef41 100644
--- a/Lib/test/test_bytes.py
+++ b/Lib/test/test_bytes.py
@@ -1799,6 +1799,26 @@ def test_setslice_trap(self):
         b[8:] = b
         self.assertEqual(b, bytearray(list(range(8)) + list(range(256))))
 
+    def test_setslice_reentrant_resize(self):
+        # gh-153578: a buffer argument whose __buffer__ resizes the bytearray
+        # while the buffer is being acquired must not leave the slice bounds
+        # with lo > hi, which drove a negative-size memmove (an out-of-bounds
+        # write) in the setslice path reached through extend().
+        class Evil:
+            def __init__(self, resize):
+                self.resize = resize
+            def __buffer__(self, flags):
+                self.resize()
+                return memoryview(b'ABCDEFGH')
+        # clear() during __buffer__: extend appends to the emptied bytearray.
+        b = bytearray(b'x' * 100)
+        b.extend(Evil(b.clear))
+        self.assertEqual(b, b'ABCDEFGH')
+        # partial shrink during __buffer__.
+        b = bytearray(b'x' * 100)
+        b.extend(Evil(lambda: b.__delitem__(slice(30, None))))
+        self.assertEqual(b, b'x' * 30 + b'ABCDEFGH')
+
     def test_iconcat(self):
         b = bytearray(b"abc")
         b1 = b
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-15-45-00.gh-issue-153578.Qm4Zt9.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-15-45-00.gh-issue-153578.Qm4Zt9.rst
new file mode 100644
index 00000000000000..cb98d02fd16973
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-15-45-00.gh-issue-153578.Qm4Zt9.rst
@@ -0,0 +1,2 @@
+Fix a crash in :meth:`bytearray.extend` when the argument's
+:meth:`~object.__buffer__` method resizes the bytearray. Patch by tonghuaroot.
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 696ddad8efaae5..9295a6d4ff08d2 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -670,8 +670,11 @@ bytearray_setslice(PyByteArrayObject *self, Py_ssize_t lo, Py_ssize_t hi,
         bytes = vbytes.buf;
     }
 
+    // gh-153578: __buffer__() may have resized self; re-clamp both bounds.
     if (lo < 0)
         lo = 0;
+    else if (lo > Py_SIZE(self))
+        lo = Py_SIZE(self);
     if (hi < lo)
         hi = lo;
     if (hi > Py_SIZE(self))