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CVE-2021-3782: Reference count overflow in shm leads to use-after-free (#224) · Issues · wayland / wayland · GitLab
An internal reference count is held on the buffer pool, incremented every time a new buffer is created from the pool. The reference count is maintained as an int; on LP64 systems this can cause the reference count to overflow if the client creates a large number of wl_shm buffer objects, or if it can coerce the server to create a large number of external references to the buffer storage. With the reference count overflowing, a use-after-free can be constructed on the wl_shm_pool tracking structure, where values may be incremented or decremented; it may also be possible to construct a limited oracle to leak 4 bytes of server-side memory to the attacking client at a time.
The shared memory code uses an int for the reference count. On 64-bit systems, a malicous client can create so many references that the int overflows. This is undefined behavior, but it will most likely cause the int to overflow, causing an exploitable use-after-free. A successful exploit will result in the execution of arbitrary code in the context of the Wayland compositor.
To fix this bug, uintptr_t (or intptr_t) should be used for all reference counts. There are only UINT_MAX possible distinct pointers, so a uintptr_t reference count can never overflow, and an intptr_t reference count cannot overflow if the referencing and referenced objects are at least 2 bytes. This assumes that INTPTR_MAX == (UINTPTR_MAX >> 1) && UINTPTR_MAX == ((uintptr_t)INTPTR_MAX << 1) + 1, which can be checked with a C11 _Static_assert.
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