[PATCH AUTOSEL 4.19 31/45] x86/boot/compressed/64: Fix boot on machines with broken E820 table

Sasha Levin
2019-08-29T18:16:46Z

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