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From: Joerg Roedel <>
To: Ard Biesheuvel <>,
	Thomas Gleixner <>,
	Ingo Molnar <>
Cc: Michal Hocko <>, Andi Kleen <>,
	Linus Torvalds <>,
	Dave Hansen <>, Pavel Machek <>,,,,, Joerg Roedel <>
Subject: [PATCH] x86/efi: Load fixmap GDT in efi_call_phys_epilog()
Date: Fri, 31 Aug 2018 10:05:38 +0200
Message-ID: <> (raw)

From: Joerg Roedel <>

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 <>
Tested-by: Guenter Roeck <>
Signed-off-by: Joerg Roedel <>
 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 324b9332..05ca142 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_fixmap_gdt(0);
 void __init efi_runtime_update_mappings(void)

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