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    Subject[PATCH AUTOSEL 5.2 51/76] x86/boot/compressed/64: Fix boot on machines with broken E820 table
    Date
    From: "Kirill A. Shutemov" <kirill@shutemov.name>

    [ Upstream commit 0a46fff2f9108c2c44218380a43a736cf4612541 ]

    BIOS on Samsung 500C Chromebook reports very rudimentary E820 table that
    consists of 2 entries:

    BIOS-e820: [mem 0x0000000000000000-0x0000000000000fff] usable
    BIOS-e820: [mem 0x00000000fffff000-0x00000000ffffffff] reserved

    It breaks logic in find_trampoline_placement(): bios_start lands on the
    end of the first 4k page and trampoline start gets placed below 0.

    Detect underflow and don't touch bios_start for such cases. It makes
    kernel ignore E820 table on machines that doesn't have two usable pages
    below BIOS_START_MAX.

    Fixes: 1b3a62643660 ("x86/boot/compressed/64: Validate trampoline placement against E820")
    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: x86-ml <x86@kernel.org>
    Link: https://bugzilla.kernel.org/show_bug.cgi?id=203463
    Link: https://lkml.kernel.org/r/20190813131654.24378-1-kirill.shutemov@linux.intel.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/x86/boot/compressed/pgtable_64.c | 13 ++++++++++---
    1 file changed, 10 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
    index f8debf7aeb4c1..f0537a1f7fc25 100644
    --- a/arch/x86/boot/compressed/pgtable_64.c
    +++ b/arch/x86/boot/compressed/pgtable_64.c
    @@ -73,6 +73,8 @@ static unsigned long find_trampoline_placement(void)

    /* Find the first usable memory region under bios_start. */
    for (i = boot_params->e820_entries - 1; i >= 0; i--) {
    + unsigned long new;
    +
    entry = &boot_params->e820_table[i];

    /* Skip all entries above bios_start. */
    @@ -85,15 +87,20 @@ static unsigned long find_trampoline_placement(void)

    /* Adjust bios_start to the end of the entry if needed. */
    if (bios_start > entry->addr + entry->size)
    - bios_start = entry->addr + entry->size;
    + new = entry->addr + entry->size;

    /* Keep bios_start page-aligned. */
    - bios_start = round_down(bios_start, PAGE_SIZE);
    + new = round_down(new, PAGE_SIZE);

    /* Skip the entry if it's too small. */
    - if (bios_start - TRAMPOLINE_32BIT_SIZE < entry->addr)
    + if (new - TRAMPOLINE_32BIT_SIZE < entry->addr)
    continue;

    + /* Protect against underflow. */
    + if (new - TRAMPOLINE_32BIT_SIZE > bios_start)
    + break;
    +
    + bios_start = new;
    break;
    }

    --
    2.20.1
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