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    Subject[ 106/153] x86: Make sure we can boot in the case the BDA contains pure garbage
    3.2-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: "H. Peter Anvin" <hpa@linux.intel.com>

    commit 7c10093692ed2e6f318387d96b829320aa0ca64c upstream.

    On non-BIOS platforms it is possible that the BIOS data area contains
    garbage instead of being zeroed or something equivalent (firmware
    people: we are talking of 1.5K here, so please do the sane thing.)

    We need on the order of 20-30K of low memory in order to boot, which
    may grow up to < 64K in the future. We probably want to avoid the
    lowest of the low memory. At the same time, it seems extremely
    unlikely that a legitimate EBDA would ever reach down to the 128K
    (which would require it to be over half a megabyte in size.) Thus,
    pick 128K as the cutoff for "this is insane, ignore." We may still
    end up reserving a bunch of extra memory on the low megabyte, but that
    is not really a major issue these days. In the worst case we lose
    512K of RAM.

    This code really should be merged with trim_bios_range() in
    arch/x86/kernel/setup.c, but that is a bigger patch for a later merge
    window.

    Reported-by: Darren Hart <dvhart@linux.intel.com>
    Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
    Cc: Matt Fleming <matt.fleming@intel.com>
    Link: http://lkml.kernel.org/n/tip-oebml055yyfm8yxmria09rja@git.kernel.org
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/x86/kernel/head.c | 53 +++++++++++++++++++++++++++++++-----------------
    1 file changed, 34 insertions(+), 19 deletions(-)

    --- a/arch/x86/kernel/head.c
    +++ b/arch/x86/kernel/head.c
    @@ -5,8 +5,6 @@
    #include <asm/setup.h>
    #include <asm/bios_ebda.h>

    -#define BIOS_LOWMEM_KILOBYTES 0x413
    -
    /*
    * The BIOS places the EBDA/XBDA at the top of conventional
    * memory, and usually decreases the reported amount of
    @@ -16,17 +14,30 @@
    * chipset: reserve a page before VGA to prevent PCI prefetch
    * into it (errata #56). Usually the page is reserved anyways,
    * unless you have no PS/2 mouse plugged in.
    + *
    + * This functions is deliberately very conservative. Losing
    + * memory in the bottom megabyte is rarely a problem, as long
    + * as we have enough memory to install the trampoline. Using
    + * memory that is in use by the BIOS or by some DMA device
    + * the BIOS didn't shut down *is* a big problem.
    */
    +
    +#define BIOS_LOWMEM_KILOBYTES 0x413
    +#define LOWMEM_CAP 0x9f000U /* Absolute maximum */
    +#define INSANE_CUTOFF 0x20000U /* Less than this = insane */
    +
    void __init reserve_ebda_region(void)
    {
    unsigned int lowmem, ebda_addr;

    - /* To determine the position of the EBDA and the */
    - /* end of conventional memory, we need to look at */
    - /* the BIOS data area. In a paravirtual environment */
    - /* that area is absent. We'll just have to assume */
    - /* that the paravirt case can handle memory setup */
    - /* correctly, without our help. */
    + /*
    + * To determine the position of the EBDA and the
    + * end of conventional memory, we need to look at
    + * the BIOS data area. In a paravirtual environment
    + * that area is absent. We'll just have to assume
    + * that the paravirt case can handle memory setup
    + * correctly, without our help.
    + */
    if (paravirt_enabled())
    return;

    @@ -37,19 +48,23 @@ void __init reserve_ebda_region(void)
    /* start of EBDA area */
    ebda_addr = get_bios_ebda();

    - /* Fixup: bios puts an EBDA in the top 64K segment */
    - /* of conventional memory, but does not adjust lowmem. */
    - if ((lowmem - ebda_addr) <= 0x10000)
    - lowmem = ebda_addr;
    -
    - /* Fixup: bios does not report an EBDA at all. */
    - /* Some old Dells seem to need 4k anyhow (bugzilla 2990) */
    - if ((ebda_addr == 0) && (lowmem >= 0x9f000))
    - lowmem = 0x9f000;
    -
    - /* Paranoia: should never happen, but... */
    - if ((lowmem == 0) || (lowmem >= 0x100000))
    - lowmem = 0x9f000;
    + /*
    + * Note: some old Dells seem to need 4k EBDA without
    + * reporting so, so just consider the memory above 0x9f000
    + * to be off limits (bugzilla 2990).
    + */
    +
    + /* If the EBDA address is below 128K, assume it is bogus */
    + if (ebda_addr < INSANE_CUTOFF)
    + ebda_addr = LOWMEM_CAP;
    +
    + /* If lowmem is less than 128K, assume it is bogus */
    + if (lowmem < INSANE_CUTOFF)
    + lowmem = LOWMEM_CAP;
    +
    + /* Use the lower of the lowmem and EBDA markers as the cutoff */
    + lowmem = min(lowmem, ebda_addr);
    + lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */

    /* reserve all memory between lowmem and the 1MB mark */
    memblock_x86_reserve_range(lowmem, 0x100000, "* BIOS reserved");



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