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 <[email protected]>
Signed-off-by: Joerg Roedel <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
index 324b93328b3746f4bb8010780ee85915707b8095..05ca1422246336c6d3c923c22f06ad87113ac1d3 100644 (file)
--- 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)