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 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. 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. 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. Typically an application will call this function twice.
In order to decrypt SM2 encrypted data an application is expected to call the API function EVP_PKEY_decrypt(). See How to fix? for Debian:10 relevant fixed versions and status. Note: Versions mentioned in the description apply only to the upstream openssl package and not the openssl package as distributed by Debian:10.
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