SM2 Decryption Buffer Overflow

In order to decrypt SM2 encrypted data an application is expected to call the
API function EVP_PKEY_decrypt(). Typically an application will call this
function twice. The first time, on entry, the “out” parameter can be NULL and,
on exit, the “outlen” parameter is populated with the buffer size required to
hold the decrypted plaintext. The application can then allocate a sufficiently
sized buffer and call EVP_PKEY_decrypt() again, but this time passing a non-NULL
value for the “out” parameter.

A bug in the implementation of the SM2 decryption code means that the
calculation of the buffer size required to hold the plaintext returned by the
first call to EVP_PKEY_decrypt() can be smaller than the actual size required by
the second call. This can lead to a buffer overflow when EVP_PKEY_decrypt() is
called by the application a second time with a buffer that is too small.

A malicious attacker who is able present SM2 content for decryption to an
application could cause attacker chosen data to overflow the buffer by up to a
maximum of 62 bytes altering the contents of other data held after the
buffer, possibly changing application behaviour or causing the application to
crash. The location of the buffer is application dependent but is typically
heap allocated.