CVE-2021-3711

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. Fixed in OpenSSL 1.1.1l (Affected 1.1.1-1.1.1k).