Commit aa4bc3a3 authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Greg Kroah-Hartman

x86/boot/compressed/64: Fix boot on machines with broken E820 table

[ Upstream commit 0a46fff2 ]

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

Fixes: 1b3a6264 ("x86/boot/compressed/64: Validate trampoline placement against E820")
Signed-off-by: default avatarKirill A. Shutemov <>
Signed-off-by: default avatarBorislav Petkov <>
Cc: "H. Peter Anvin" <>
Cc: Ingo Molnar <>
Cc: Thomas Gleixner <>
Cc: x86-ml <>
Link: default avatarSasha Levin <>
parent 3091859b
......@@ -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)
/* Protect against underflow. */
if (new - TRAMPOLINE_32BIT_SIZE > bios_start)
bios_start = new;
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