x86/efi: Load fixmap GDT in efi_call_phys_epilog()

joergroedel · KAGA-KOKO · commit eeb89e2bb1ac · 2018-08-31T17:45:54.000+02:00
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: 7757d60 x86/pti: ('Allow CONFIG_PAGE_TABLE_ISOLATION for x86_32') Reported-by: Guenter Roeck <linux@roeck-us.net> Signed-off-by: Joerg Roedel <jroedel@suse.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Guenter Roeck <linux@roeck-us.net> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: hpa@zytor.com Cc: linux-efi@vger.kernel.org Link: https://lkml.kernel.org/r/1535702738-10971-1-git-send-email-joro@8bytes.org
diff --git 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)
