From mboxrd@z Thu Jan 1 00:00:00 1970 From: 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: <1535702738-10971-1-git-send-email-joro@8bytes.org> Return-path: Sender: linux-kernel-owner@vger.kernel.org To: Ard Biesheuvel , Thomas Gleixner , Ingo Molnar Cc: Michal Hocko , Andi Kleen , Linus Torvalds , Dave Hansen , Pavel Machek , hpa@zytor.com, x86@kernel.org, linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, Joerg Roedel List-Id: linux-efi@vger.kernel.org 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_cr3(save_pgd); __flush_tlb_all(); + + load_fixmap_gdt(0); } void __init efi_runtime_update_mappings(void) -- 2.7.4