auth-sasl.c

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/*-------------------------------------------------------------------------
 *
 * auth-sasl.c
 *    Routines to handle authentication via SASL
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/libpq/auth-sasl.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "libpq/auth.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "libpq/sasl.h"
 
/*
 * Perform a SASL exchange with a libpq client, using a specific mechanism
 * implementation.
 *
 * shadow_pass is an optional pointer to the stored secret of the role
 * authenticated, from pg_authid.rolpassword.  For mechanisms that use
 * shadowed passwords, a NULL pointer here means that an entry could not
 * be found for the role (or the user does not exist), and the mechanism
 * should fail the authentication exchange.
 *
 * Some SASL mechanisms (e.g. OAUTHBEARER) define special exchanges for
 * parameter discovery. These exchanges will always result in STATUS_ERROR,
 * since we can't let the connection continue, but we shouldn't consider them to
 * be failed authentication attempts. *abandoned will be set to true in this
 * case.
 *
 * Mechanisms must take care not to reveal to the client that a user entry
 * does not exist; ideally, the external failure mode is identical to that
 * of an incorrect password.  Mechanisms may instead use the logdetail
 * output parameter to internally differentiate between failure cases and
 * assist debugging by the server admin.
 *
 * A mechanism is not required to utilize a shadow entry, or even a password
 * system at all; for these cases, shadow_pass may be ignored and the caller
 * should just pass NULL.
 */
int
CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
              const char **logdetail, bool *abandoned)
{
    StringInfoData sasl_mechs;
    int         mtype;
    StringInfoData buf;
    void       *opaq = NULL;
    char       *output = NULL;
    int         outputlen = 0;
    const char *input;
    int         inputlen;
    int         result;
    bool        initial;
 
    /*
     * Send the SASL authentication request to user.  It includes the list of
     * authentication mechanisms that are supported.
     */
    initStringInfo(&sasl_mechs);
 
    mech->get_mechanisms(port, &sasl_mechs);
    /* Put another '\0' to mark that list is finished. */
    appendStringInfoChar(&sasl_mechs, '\0');
 
    sendAuthRequest(port, AUTH_REQ_SASL, sasl_mechs.data, sasl_mechs.len);
    pfree(sasl_mechs.data);
 
    /*
     * Loop through SASL message exchange.  This exchange can consist of
     * multiple messages sent in both directions.  First message is always
     * from the client.  All messages from client to server are password
     * packets (type 'p').
     */
    initial = true;
    do
    {
        pq_startmsgread();
        mtype = pq_getbyte();
        if (mtype != PqMsg_SASLResponse)
        {
            /* Only log error if client didn't disconnect. */
            if (mtype != EOF)
            {
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("expected SASL response, got message type %d",
                                mtype)));
            }
            else
                return STATUS_EOF;
        }
 
        /* Get the actual SASL message */
        initStringInfo(&buf);
        if (pq_getmessage(&buf, mech->max_message_length))
        {
            /* EOF - pq_getmessage already logged error */
            pfree(buf.data);
            return STATUS_ERROR;
        }
 
        elog(DEBUG4, "processing received SASL response of length %d", buf.len);
 
        /*
         * The first SASLInitialResponse message is different from the others.
         * It indicates which SASL mechanism the client selected, and contains
         * an optional Initial Client Response payload.  The subsequent
         * SASLResponse messages contain just the SASL payload.
         */
        if (initial)
        {
            const char *selected_mech;
 
            selected_mech = pq_getmsgrawstring(&buf);
 
            /*
             * Initialize the status tracker for message exchanges.
             *
             * If the user doesn't exist, or doesn't have a valid password, or
             * it's expired, we still go through the motions of SASL
             * authentication, but tell the authentication method that the
             * authentication is "doomed". That is, it's going to fail, no
             * matter what.
             *
             * This is because we don't want to reveal to an attacker what
             * usernames are valid, nor which users have a valid password.
             */
            opaq = mech->init(port, selected_mech, shadow_pass);
 
            inputlen = pq_getmsgint(&buf, 4);
            if (inputlen == -1)
                input = NULL;
            else
                input = pq_getmsgbytes(&buf, inputlen);
 
            initial = false;
        }
        else
        {
            inputlen = buf.len;
            input = pq_getmsgbytes(&buf, buf.len);
        }
        pq_getmsgend(&buf);
 
        /*
         * The StringInfo guarantees that there's a \0 byte after the
         * response.
         */
        Assert(input == NULL || input[inputlen] == '\0');
 
        /*
         * Hand the incoming message to the mechanism implementation.
         */
        result = mech->exchange(opaq, input, inputlen,
                                &output, &outputlen,
                                logdetail);
 
        /* input buffer no longer used */
        pfree(buf.data);
 
        if (output)
        {
            /*
             * PG_SASL_EXCHANGE_FAILURE with some output is forbidden by SASL.
             * Make sure here that the mechanism used got that right.
             */
            if (result == PG_SASL_EXCHANGE_FAILURE || result == PG_SASL_EXCHANGE_ABANDONED)
                elog(ERROR, "output message found after SASL exchange failure");
 
            /*
             * Negotiation generated data to be sent to the client.
             */
            elog(DEBUG4, "sending SASL challenge of length %d", outputlen);
 
            if (result == PG_SASL_EXCHANGE_SUCCESS)
                sendAuthRequest(port, AUTH_REQ_SASL_FIN, output, outputlen);
            else
                sendAuthRequest(port, AUTH_REQ_SASL_CONT, output, outputlen);
 
            pfree(output);
        }
    } while (result == PG_SASL_EXCHANGE_CONTINUE);
 
    if (result == PG_SASL_EXCHANGE_ABANDONED)
    {
        if (!abandoned)
        {
            /*
             * Programmer error: caller needs to track the abandoned state for
             * this mechanism.
             */
            elog(ERROR, "SASL exchange was abandoned, but CheckSASLAuth isn't tracking it");
        }
 
        *abandoned = true;
    }
 
    /* Oops, Something bad happened */
    if (result != PG_SASL_EXCHANGE_SUCCESS)
    {
        return STATUS_ERROR;
    }
 
    return STATUS_OK;
}