linux-kernel - [PATCH] x86/efi: Load fixmap GDT in efi_call_phys

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Date:   Fri, 31 Aug 2018 10:05:38 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>, Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Pavel Machek <pavel@....cz>, hpa@...or.com, x86@...nel.org,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joerg Roedel <jroedel@...e.de>
Subject: [PATCH] x86/efi: Load fixmap GDT in efi_call_phys_epilog()

From: Joerg Roedel <jroedel@...e.de>

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@...ck-us.net>
Tested-by: Guenter Roeck <linux@...ck-us.net>
Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 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_cr3(save_pgd);
 	__flush_tlb_all();
+
+	load_fixmap_gdt(0);
 }

 void __init efi_runtime_update_mappings(void)
-- 
2.7.4