Commit message: Fix buffer overflow when decrypting client SPICE ticket

reds_handle_ticket uses a fixed size 'password' buffer for the decrypted
password whose size is SPICE_MAX_PASSWORD_LENGTH. However,
RSA_private_decrypt which we call for the decryption expects the
destination buffer to be at least RSA_size(link->tiTicketing.rsa)
bytes long. On my spice-server build, SPICE_MAX_PASSWORD_LENGTH
is 60 while RSA_size() is 128, so we end up overflowing 'password'
when using long passwords (this was reproduced using the string:
'fullscreen=1proxy=#enter proxy here; e.g spice_proxy = http://[proxy]:[port]'
as a password).

When the overflow occurs, QEMU dies with:
*** stack smashing detected ***: qemu-system-x86_64 terminated

This commit ensures we use a corectly sized 'password' buffer,
and that it's correctly nul-terminated so that we can use strcmp
instead of strncmp. To keep using strncmp, we'd need to figure out
which one of 'password' and 'taTicket.password' is the smaller buffer,
and use that size.

This fixes rhbz#999839
CVE: CVE-2013-4282
CWE: CWE-119
Vulnerability description: Stack-based buffer overflow in the reds_handle_ticket function in server/reds.c in SPICE 0.12.0 allows remote attackers to cause a denial of service (crash) via a long password in a SPICE ticket.

Project: spice

 static void reds_handle_ticket(void *opaque)
 {
     RedLinkInfo *link = (RedLinkInfo *)opaque;
    char *password;
     time_t ltime;
    int password_size;
 
     time(&ltime);
    if (RSA_size(link->tiTicketing.rsa) < SPICE_MAX_PASSWORD_LENGTH) {
        spice_warning("RSA modulus size is smaller than SPICE_MAX_PASSWORD_LENGTH (%d < %d), "
                      "SPICE ticket sent from client may be truncated",
                      RSA_size(link->tiTicketing.rsa), SPICE_MAX_PASSWORD_LENGTH);
    }

    password = g_malloc0(RSA_size(link->tiTicketing.rsa) + 1);
    password_size = RSA_private_decrypt(link->tiTicketing.rsa_size,
                                        link->tiTicketing.encrypted_ticket.encrypted_data,
                                        (unsigned char *)password,
                                        link->tiTicketing.rsa,
                                        RSA_PKCS1_OAEP_PADDING);
    if (password_size == -1) {
        spice_warning("failed to decrypt RSA encrypted password: %s",
                      ERR_error_string(ERR_get_error(), NULL));
        goto error;
    }
    password[password_size] = '\0';
 
     if (ticketing_enabled && !link->skip_auth) {
         int expired =  taTicket.expiration_time < ltime;
 
         if (strlen(taTicket.password) == 0) {
             spice_warning("Ticketing is enabled, but no password is set. "
                          "please set a ticket first");
            goto error;
         }
 
        if (expired || strcmp(password, taTicket.password) != 0) {
             if (expired) {
                 spice_warning("Ticket has expired");
             } else {
                 spice_warning("Invalid password");
             }
            goto error;
         }
     }
 
     reds_handle_link(link);
    goto end;

error:
    reds_send_link_result(link, SPICE_LINK_ERR_PERMISSION_DENIED);
    reds_link_free(link);

end:
    g_free(password);
 }