linux-kernel - Re: efi boot failures due to PTI with 32 bit builds and Intel CPUs

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Date:   Thu, 30 Aug 2018 20:08:49 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...e.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Pavel Machek <pavel@....cz>
Subject: Re: efi boot failures due to PTI with 32 bit builds and Intel CPUs

On Thu, Aug 30, 2018 at 05:20:22AM -0700, Guenter Roeck wrote:
> Excellent.
> 
> In case it matters: I can use the same 32-bit efi image to boot a 64-bit kernel
> without problems.

Okay, I tracked it down. The 32-bit EFI code switches the CPU to the
read-write version of the GDT which is not mapped in the user
page-table. This obviously faults when the kernel entry/exit code
restores the segment registers.

Without a mapped GDT the #PF and #DF handlers also can't be started, so
the machine triple-faults. Below diff fixes it for me, I'll send a
proper patch tomorrow.

diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
index 324b93328b37..892edb4664fb 100644
--- a/arch/x86/platform/efi/efi_32.c
+++ b/arch/x86/platform/efi/efi_32.c
@@ -87,7 +87,7 @@ 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.address = (unsigned long)get_cpu_gdt_ro(0);
 	gdt_descr.size = GDT_SIZE - 1;
 	load_gdt(&gdt_descr);