binder: fix race that allows malicious free of live buffer
commit 7bada55ab50697861eee6bb7d60b41e68a961a9c upstream. Malicious code can attempt to free buffers using the BC_FREE_BUFFER ioctl to binder. There are protections against a user freeing a buffer while in use by the kernel, however there was a window where BC_FREE_BUFFER could be used to free a recently allocated buffer that was not completely initialized. This resulted in a use-after-free detected by KASAN with a malicious test program. This window is closed by setting the buffer's allow_user_free attribute to 0 when the buffer is allocated or when the user has previously freed it instead of waiting for the caller to set it. The problem was that when the struct buffer was recycled, allow_user_free was stale and set to 1 allowing a free to go through. Signed-off-by: NTodd Kjos <tkjos@google.com> Acked-by: NArve Hjønnevåg <arve@android.com> Cc: stable <stable@vger.kernel.org> # 4.14 Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
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