rtc_entropy.c

Go to the documentation of this file.

/*
 * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

/** @file
 *
 * RTC-based entropy source
 *
 */

#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <biosint.h>
#include <pic8259.h>
#include <rtc.h>
#include <ipxe/cpuid.h>
#include <ipxe/entropy.h>

struct entropy_source rtc_entropy __entropy_source ( ENTROPY_NORMAL );

/** Maximum time to wait for an RTC interrupt, in milliseconds */
#define RTC_MAX_WAIT_MS 100

/** Number of RTC interrupts to check for */
#define RTC_CHECK_COUNT 3

/** RTC interrupt handler */
extern void rtc_isr ( void );

/** Previous RTC interrupt handler */
static struct segoff rtc_old_handler;

/** Flag set by RTC interrupt handler */
extern volatile uint8_t __text16 ( rtc_flag );
#define rtc_flag __use_text16 ( rtc_flag )

/**
 * Hook RTC interrupt handler
 *
 */
static void rtc_hook_isr ( void ) {

        /* RTC interrupt handler */
        __asm__ __volatile__ (
                TEXT16_CODE ( "\nrtc_isr:\n\t"
                              /* Preserve registers */
                              "pushw %%ax\n\t"
                              /* Set "interrupt triggered" flag */
                              "movb $0x01, %%cs:rtc_flag\n\t"
                              /* Read RTC status register C to
                               * acknowledge interrupt
                               */
                              "movb %2, %%al\n\t"
                              "outb %%al, %0\n\t"
                              "inb %1\n\t"
                              /* Send EOI */
                              "movb $0x20, %%al\n\t"
                              "outb %%al, $0xa0\n\t"
                              "outb %%al, $0x20\n\t"
                              /* Restore registers and return */
                              "popw %%ax\n\t"
                              "iret\n\t"
                              "\nrtc_flag:\n\t"
                              ".byte 0\n\t" )
                :
                : "i" ( CMOS_ADDRESS ), "i" ( CMOS_DATA ),
                  "i" ( RTC_STATUS_C ) );

        hook_bios_interrupt ( RTC_INT, ( intptr_t ) rtc_isr, &rtc_old_handler );
}

/**
 * Unhook RTC interrupt handler
 *
 */
static void rtc_unhook_isr ( void ) {
        int rc;

        rc = unhook_bios_interrupt ( RTC_INT, ( intptr_t ) rtc_isr,
                                     &rtc_old_handler );
        assert ( rc == 0 ); /* Should always be able to unhook */
}

/**
 * Enable RTC interrupts
 *
 */
static void rtc_enable_int ( void ) {
        uint8_t status_b;

        /* Clear any stale pending interrupts via status register C */
        outb ( ( RTC_STATUS_C | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
        inb ( CMOS_DATA );

        /* Set Periodic Interrupt Enable bit in status register B */
        outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
        status_b = inb ( CMOS_DATA );
        outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
        outb ( ( status_b | RTC_STATUS_B_PIE ), CMOS_DATA );

        /* Re-enable NMI and reset to default address */
        outb ( CMOS_DEFAULT_ADDRESS, CMOS_ADDRESS );
        inb ( CMOS_DATA ); /* Discard; may be needed on some platforms */
}

/**
 * Disable RTC interrupts
 *
 */
static void rtc_disable_int ( void ) {
        uint8_t status_b;

        /* Clear Periodic Interrupt Enable bit in status register B */
        outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
        status_b = inb ( CMOS_DATA );
        outb ( ( RTC_STATUS_B | CMOS_DISABLE_NMI ), CMOS_ADDRESS );
        outb ( ( status_b & ~RTC_STATUS_B_PIE ), CMOS_DATA );

        /* Re-enable NMI and reset to default address */
        outb ( CMOS_DEFAULT_ADDRESS, CMOS_ADDRESS );
        inb ( CMOS_DATA ); /* Discard; may be needed on some platforms */
}

/**
 * Check that entropy gathering is functional
 *
 * @ret rc              Return status code
 */
static int rtc_entropy_check ( void ) {
        unsigned int count = 0;
        unsigned int i;

        /* Check that RTC interrupts are working */
        rtc_flag = 0;
        for ( i = 0 ; i < RTC_MAX_WAIT_MS ; i++ ) {

                /* Allow interrupts to occur */
                __asm__ __volatile__ ( "sti\n\t"
                                       "nop\n\t"
                                       "nop\n\t"
                                       "cli\n\t" );

                /* Check for RTC interrupt flag */
                if ( rtc_flag ) {
                        rtc_flag = 0;
                        if ( ++count >= RTC_CHECK_COUNT )
                                return 0;
                }

                /* Delay */
                mdelay ( 1 );
        }

        DBGC ( &rtc_flag, "RTC timed out waiting for interrupt %d/%d\n",
               ( count + 1 ), RTC_CHECK_COUNT );
        return -ETIMEDOUT;
}

/**
 * Enable entropy gathering
 *
 * @ret rc              Return status code
 */
static int rtc_entropy_enable ( void ) {
        struct x86_features features;
        int rc;

        /* Check that TSC is supported */
        x86_features ( &features );
        if ( ! ( features.intel.edx & CPUID_FEATURES_INTEL_EDX_TSC ) ) {
                DBGC ( &rtc_flag, "RTC has no TSC\n" );
                rc = -ENOTSUP;
                goto err_no_tsc;
        }

        /* Hook ISR and enable RTC interrupts */
        rtc_hook_isr();
        enable_irq ( RTC_IRQ );
        rtc_enable_int();

        /* Check that RTC interrupts are working */
        if ( ( rc = rtc_entropy_check() ) != 0 )
                goto err_check;

        /* The min-entropy has been measured on several platforms
         * using the entropy_sample test code.  Modelling the samples
         * as independent, and using a confidence level of 99.99%, the
         * measurements were as follows:
         *
         *    qemu-kvm          : 7.38 bits
         *    VMware            : 7.46 bits
         *    Physical hardware : 2.67 bits
         *
         * We choose the lowest of these (2.67 bits) and apply a 50%
         * safety margin to allow for some potential non-independence
         * of samples.
         */
        entropy_init ( &rtc_entropy, MIN_ENTROPY ( 1.3 ) );

        return 0;

 err_check:
        rtc_disable_int();
        disable_irq ( RTC_IRQ );
        rtc_unhook_isr();
 err_no_tsc:
        return rc;
}

/**
 * Disable entropy gathering
 *
 */
static void rtc_entropy_disable ( void ) {

        /* Disable RTC interrupts and unhook ISR */
        rtc_disable_int();
        disable_irq ( RTC_IRQ );
        rtc_unhook_isr();
}

/**
 * Get noise sample
 *
 * @ret noise           Noise sample
 * @ret rc              Return status code
 */
static int rtc_get_noise ( noise_sample_t *noise ) {
        uint32_t before;
        uint32_t after;
        uint32_t temp;

        __asm__ __volatile__ (
                REAL_CODE ( /* Enable interrupts */
                            "sti\n\t"
                            /* Wait for RTC interrupt */
                            "movb %b2, %%cs:rtc_flag\n\t"
                            "\n1:\n\t"
                            "xchgb %b2, %%cs:rtc_flag\n\t" /* Serialize */
                            "testb %b2, %b2\n\t"
                            "jz 1b\n\t"
                            /* Read "before" TSC */
                            "rdtsc\n\t"
                            /* Store "before" TSC on stack */
                            "pushl %0\n\t"
                            /* Wait for another RTC interrupt */
                            "xorb %b2, %b2\n\t"
                            "movb %b2, %%cs:rtc_flag\n\t"
                            "\n1:\n\t"
                            "xchgb %b2, %%cs:rtc_flag\n\t" /* Serialize */
                            "testb %b2, %b2\n\t"
                            "jz 1b\n\t"
                            /* Read "after" TSC */
                            "rdtsc\n\t"
                            /* Retrieve "before" TSC on stack */
                            "popl %1\n\t"
                            /* Disable interrupts */
                            "cli\n\t"
                            )
                : "=a" ( after ), "=d" ( before ), "=Q" ( temp )
                : "2" ( 0 ) );

        *noise = ( after - before );
        return 0;
}

/** RTC entropy source */
struct entropy_source rtc_entropy __entropy_source ( ENTROPY_NORMAL ) = {
        .name = "rtc",
        .enable = rtc_entropy_enable,
        .disable = rtc_entropy_disable,
        .get_noise = rtc_get_noise,
};