[tip:x86/urgent] x86/efi: Load fixmap GDT in efi_call_phys_epilog() before setting %cr3

[tip-bot for Guenter Roeck <tipbot@zytor.com>]

Commit-ID:  cf40361ede6cf9dc09349e4c049dc0d166ca2d8b
Gitweb:     https://git.kernel.org/tip/cf40361ede6cf9dc09349e4c049dc0d166ca2d8b
Author:     Guenter Roeck <linux@roeck-us.net>
AuthorDate: Tue, 11 Sep 2018 11:18:12 -0700
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 12 Sep 2018 21:53:34 +0200

x86/efi: Load fixmap GDT in efi_call_phys_epilog() before setting %cr3

Commit eeb89e2bb1ac ("x86/efi: Load fixmap GDT in efi_call_phys_epilog()")
moved loading the fixmap in efi_call_phys_epilog() after load_cr3() since
it was assumed to be more logical.

Turns out this is incorrect: In efi_call_phys_prolog(), the gdt with its
physical address is loaded first, and when the %cr3 is reloaded in _epilog
from initial_page_table to swapper_pg_dir again the gdt is no longer
mapped.  This results in a triple fault if an interrupt occurs after
load_cr3() and before load_fixmap_gdt(0). Calling load_fixmap_gdt(0) first
restores the execution order prior to commit eeb89e2bb1ac and fixes the
problem.

Fixes: eeb89e2bb1ac ("x86/efi: Load fixmap GDT in efi_call_phys_epilog()")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: linux-efi@vger.kernel.org
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Joerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/1536689892-21538-1-git-send-email-linux@roeck-us.net

---
 arch/x86/platform/efi/efi_32.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
index 05ca14222463..9959657127f4 100644
--- a/arch/x86/platform/efi/efi_32.c
+++ b/arch/x86/platform/efi/efi_32.c
@@ -85,10 +85,9 @@ pgd_t * __init efi_call_phys_prolog(void)
 
 void __init efi_call_phys_epilog(pgd_t *save_pgd)
 {
+	load_fixmap_gdt(0);
 	load_cr3(save_pgd);
 	__flush_tlb_all();
-
-	load_fixmap_gdt(0);
 }
 
 void __init efi_runtime_update_mappings(void)