tpm2: Check size of buffer before accessing it (CVE-2023-1017 & -1018)

Check that there are sufficient bytes in the buffer before reading the
cipherSize from it. Also, reduce the bufferSize variable by the number
of bytes that make up the cipherSize to avoid reading and writing bytes
beyond the buffer in subsequent steps that do in-place decryption.

This fixes CVE-2023-1017 & CVE-2023-1018.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>

src/tpm2/CryptUtil.c

@@ -830,6 +830,10 @@ CryptParameterDecryption(
 			  + sizeof(session->sessionKey.t.buffer)));
     TPM2B_HMAC_KEY          key;            // decryption key
     UINT32                  cipherSize = 0; // size of cipher text
+
+    if (leadingSizeInByte > bufferSize)
+	return TPM_RC_INSUFFICIENT;
+
     // Retrieve encrypted data size.
     if(leadingSizeInByte == 2)
 	{
@@ -837,6 +841,7 @@ CryptParameterDecryption(
 	    // data to be decrypted
 	    cipherSize = (UINT32)BYTE_ARRAY_TO_UINT16(buffer);
 	    buffer = &buffer[2];   // advance the buffer
+	    bufferSize -= 2;
 	}
 #ifdef  TPM4B
     else if(leadingSizeInByte == 4)
@@ -844,6 +849,7 @@ CryptParameterDecryption(
 	    // the leading size is four bytes so get the four byte size field
 	    cipherSize = BYTE_ARRAY_TO_UINT32(buffer);
 	    buffer = &buffer[4];   //advance pointer
+	    bufferSize -= 4;
 	}
 #endif
     else