source: bootcd/isolinux/syslinux-6.03/gpxe/src/arch/i386/firmware/pcbios/hidemem.c

/* Copyright (C) 2006 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., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

#include <assert.h>
#include <realmode.h>
#include <biosint.h>
#include <basemem.h>
#include <fakee820.h>
#include <gpxe/init.h>
#include <gpxe/memmap.h>
#include <gpxe/hidemem.h>

/** Set to true if you want to test a fake E820 map */
#define FAKE_E820 0

/** Alignment for hidden memory regions */
#define ALIGN_HIDDEN 4096   /* 4kB page alignment should be enough */

/**
 * A hidden region of gPXE
 *
 * This represents a region that will be edited out of the system's
 * memory map.
 *
 * This structure is accessed by assembly code, so must not be
 * changed.
 */
struct hidden_region {
        /** Physical start address */
        uint64_t start;
        /** Physical end address */
        uint64_t end;
};

/** Hidden base memory */
extern struct hidden_region __data16 ( hidemem_base );
#define hidemem_base __use_data16 ( hidemem_base )

/** Hidden umalloc memory */
extern struct hidden_region __data16 ( hidemem_umalloc );
#define hidemem_umalloc __use_data16 ( hidemem_umalloc )

/** Hidden text memory */
extern struct hidden_region __data16 ( hidemem_textdata );
#define hidemem_textdata __use_data16 ( hidemem_textdata )

/** Assembly routine in e820mangler.S */
extern void int15();

/** Vector for storing original INT 15 handler */
extern struct segoff __text16 ( int15_vector );
#define int15_vector __use_text16 ( int15_vector )

/* The linker defines these symbols for us */
extern char _textdata[];
extern char _etextdata[];
extern char _text16_memsz[];
#define _text16_memsz ( ( unsigned int ) _text16_memsz )
extern char _data16_memsz[];
#define _data16_memsz ( ( unsigned int ) _data16_memsz )

/**
 * Hide region of memory from system memory map
 *
 * @v region            Hidden memory region
 * @v start             Start of region
 * @v end               End of region
 */
static void hide_region ( struct hidden_region *region,
                          physaddr_t start, physaddr_t end ) {

        /* Some operating systems get a nasty shock if a region of the
         * E820 map seems to start on a non-page boundary.  Make life
         * safer by rounding out our edited region.
         */
        region->start = ( start & ~( ALIGN_HIDDEN - 1 ) );
        region->end = ( ( end + ALIGN_HIDDEN - 1 ) & ~( ALIGN_HIDDEN - 1 ) );

        DBG ( "Hiding region [%llx,%llx)\n", region->start, region->end );
}

/**
 * Hide used base memory
 *
 */
void hide_basemem ( void ) {
        /* Hide from the top of free base memory to 640kB.  Don't use
         * hide_region(), because we don't want this rounded to the
         * nearest page boundary.
         */
        hidemem_base.start = ( get_fbms() * 1024 );
}

/**
 * Hide umalloc() region
 *
 */
void hide_umalloc ( physaddr_t start, physaddr_t end ) {
        assert ( end <= virt_to_phys ( _textdata ) );
        hide_region ( &hidemem_umalloc, start, end );
}

/**
 * Hide .text and .data
 *
 */
void hide_textdata ( void ) {
        hide_region ( &hidemem_textdata, virt_to_phys ( _textdata ),
                      virt_to_phys ( _etextdata ) );
}

/**
 * Hide Etherboot
 *
 * Installs an INT 15 handler to edit Etherboot out of the memory map
 * returned by the BIOS.
 */
static void hide_etherboot ( void ) {
        struct memory_map memmap;
        unsigned int rm_ds_top;
        unsigned int rm_cs_top;
        unsigned int fbms;

        /* Dump memory map before mangling */
        DBG ( "Hiding gPXE from system memory map\n" );
        get_memmap ( &memmap );

        /* Hook in fake E820 map, if we're testing one */
        if ( FAKE_E820 ) {
                DBG ( "Hooking in fake E820 map\n" );
                fake_e820();
                get_memmap ( &memmap );
        }

        /* Initialise the hidden regions */
        hide_basemem();
        hide_umalloc ( virt_to_phys ( _textdata ), virt_to_phys ( _textdata ) );
        hide_textdata();

        /* Some really moronic BIOSes bring up the PXE stack via the
         * UNDI loader entry point and then don't bother to unload it
         * before overwriting the code and data segments.  If this
         * happens, we really don't want to leave INT 15 hooked,
         * because that will cause any loaded OS to die horribly as
         * soon as it attempts to fetch the system memory map.
         *
         * We use a heuristic to guess whether or not we are being
         * loaded sensibly.
         */
        rm_cs_top = ( ( ( rm_cs << 4 ) + _text16_memsz + 1024 - 1 ) >> 10 );
        rm_ds_top = ( ( ( rm_ds << 4 ) + _data16_memsz + 1024 - 1 ) >> 10 );
        fbms = get_fbms();
        if ( ( rm_cs_top < fbms ) && ( rm_ds_top < fbms ) ) {
                DBG ( "Detected potentially unsafe UNDI load at CS=%04x "
                      "DS=%04x FBMS=%dkB\n", rm_cs, rm_ds, fbms );
                DBG ( "Disabling INT 15 memory hiding\n" );
                return;
        }

        /* Hook INT 15 */
        hook_bios_interrupt ( 0x15, ( unsigned int ) int15,
                              &int15_vector );

        /* Dump memory map after mangling */
        DBG ( "Hidden gPXE from system memory map\n" );
        get_memmap ( &memmap );
}

/**
 * Unhide Etherboot
 *
 * Uninstalls the INT 15 handler installed by hide_etherboot(), if
 * possible.
 */
static void unhide_etherboot ( int flags __unused ) {

        /* If we have more than one hooked interrupt at this point, it
         * means that some other vector is still hooked, in which case
         * we can't safely unhook INT 15 because we need to keep our
         * memory protected.  (We expect there to be at least one
         * hooked interrupt, because INT 15 itself is still hooked).
         */
        if ( hooked_bios_interrupts > 1 ) {
                DBG ( "Cannot unhide: %d interrupt vectors still hooked\n",
                      hooked_bios_interrupts );
                return;
        }

        /* Try to unhook INT 15.  If it fails, then just leave it
         * hooked; it takes care of protecting itself.  :)
         */
        unhook_bios_interrupt ( 0x15, ( unsigned int ) int15,
                                &int15_vector );

        /* Unhook fake E820 map, if used */
        if ( FAKE_E820 )
                unfake_e820();
}

/** Hide Etherboot startup function */
struct startup_fn hide_etherboot_startup_fn __startup_fn ( STARTUP_EARLY ) = {
        .startup = hide_etherboot,
        .shutdown = unhide_etherboot,
};