From eeb89e2bb1ac45b0836d4170e97a988c3a746c62 Mon Sep 17 00:00:00 2001
From: Joerg Roedel <jroedel@suse.de>
Date: Fri, 31 Aug 2018 10:05:38 +0200
Subject: [PATCH] x86/efi: Load fixmap GDT in efi_call_phys_epilog()
When PTI is enabled on x86-32 the kernel uses the GDT mapped in the fixmap
for the simple reason that this address is also mapped for user-space.
The efi_call_phys_prolog()/efi_call_phys_epilog() wrappers change the GDT
to call EFI runtime services and switch back to the kernel GDT when they
return. But the switch-back uses the writable GDT, not the fixmap GDT.
When that happened and and the CPU returns to user-space it switches to the
user %cr3 and tries to restore user segment registers. This fails because
the writable GDT is not mapped in the user page-table, and without a GDT
the fault handlers also can't be launched. The result is a triple fault and
reboot of the machine.
Fix that by restoring the GDT back to the fixmap GDT which is also mapped
in the user page-table.
Fixes: 7757d607c6b3 x86/pti: ('Allow CONFIG_PAGE_TABLE_ISOLATION for x86_32')
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: hpa@zytor.com
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/1535702738-10971-1-git-send-email-joro@8bytes.org
---
arch/x86/platform/efi/efi_32.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
index 324b93328b374..05ca142224633 100644
--- a/arch/x86/platform/efi/efi_32.c
+++ b/arch/x86/platform/efi/efi_32.c
@@ -85,14 +85,10 @@ pgd_t * __init efi_call_phys_prolog(void)
void __init efi_call_phys_epilog(pgd_t *save_pgd)
{
- struct desc_ptr gdt_descr;
-
- gdt_descr.address = (unsigned long)get_cpu_gdt_rw(0);
- gdt_descr.size = GDT_SIZE - 1;
- load_gdt(&gdt_descr);
-
load_cr3(save_pgd);
__flush_tlb_all();
+
+ load_fixmap_gdt(0);
}
void __init efi_runtime_update_mappings(void)
--
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