[tip:x86/urgent] x86/efi: Load fixmap GDT in efi_call_phys_epilog()

[tip-bot for Joerg Roedel]

Commit-ID:  eeb89e2bb1ac45b0836d4170e97a988c3a746c62
Gitweb:     https://git.kernel.org/tip/eeb89e2bb1ac45b0836d4170e97a988c3a746c62
Author:     Joerg Roedel <jroedel@xxxxxxx>
AuthorDate: Fri, 31 Aug 2018 10:05:38 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 31 Aug 2018 17:45:54 +0200

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@xxxxxxxxxxxx>
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxx>
Cc: hpa@xxxxxxxxx
Cc: linux-efi@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/1535702738-10971-1-git-send-email-joro@xxxxxxxxxx

---
 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 324b93328b37..05ca14222463 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)