linux-kernel - Re: [PATCH] x86/efi: Restore Firmware IDT in before ExitBootServices()

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Message-ID: <YR91KKJ1Bq/KNBnY@zn.tnic>
Date:   Fri, 20 Aug 2021 11:26:00 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Joerg Roedel <jroedel@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Uros Bizjak <ubizjak@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
        Fabio Aiuto <fabioaiuto83@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/efi: Restore Firmware IDT in before
 ExitBootServices()

On Fri, Aug 20, 2021 at 09:34:29AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Commit 79419e13e808 ("x86/boot/compressed/64: Setup IDT in startup_32
> boot path") introduced an IDT into the 32 bit boot path of the
> decompressor stub.  But the IDT is set up before ExitBootServices() is
> called and some UEFI firmwares rely on their own IDT.
> 
> Save the firmware IDT on boot and restore it before calling into EFI
> functions to fix boot failures introduced by above commit.
> 
> Reported-by: Fabio Aiuto <fabioaiuto83@...il.com>
> Fixes: 79419e13e808 ("x86/boot/compressed/64: Setup IDT in startup_32 boot path")
> Cc: stable@...r.kernel.org # 5.13+
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/boot/compressed/efi_thunk_64.S | 23 ++++++++++++++++++-----
>  arch/x86/boot/compressed/head_64.S      |  3 +++
>  2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
> index 95a223b3e56a..99cfd5dea23c 100644
> --- a/arch/x86/boot/compressed/efi_thunk_64.S
> +++ b/arch/x86/boot/compressed/efi_thunk_64.S
> @@ -39,7 +39,7 @@ SYM_FUNC_START(__efi64_thunk)
>  	/*
>  	 * Convert x86-64 ABI params to i386 ABI
>  	 */
> -	subq	$32, %rsp
> +	subq	$64, %rsp
>  	movl	%esi, 0x0(%rsp)
>  	movl	%edx, 0x4(%rsp)
>  	movl	%ecx, 0x8(%rsp)
> @@ -49,14 +49,19 @@ SYM_FUNC_START(__efi64_thunk)
>  	leaq	0x14(%rsp), %rbx
>  	sgdt	(%rbx)
>  
> +	addq	$16, %rbx
> +	sidt	(%rbx)
> +
>  	/*
> -	 * Switch to gdt with 32-bit segments. This is the firmware GDT
> -	 * that was installed when the kernel started executing. This
> -	 * pointer was saved at the EFI stub entry point in head_64.S.
> +	 * Switch to idt and gdt with 32-bit segments. This is the firmware GDT

IDT and GDT, both capitalized pls. Also, the comment at the top of the
file needs adjusting.

> +	 * and IDT that was installed when the kernel started executing. The
> +	 * pointers were saved at the EFI stub entry point in head_64.S.
>  	 *
>  	 * Pass the saved DS selector to the 32-bit code, and use far return to
>  	 * restore the saved CS selector.
>  	 */
> +	leaq	efi32_boot_idt(%rip), %rax
> +	lidt	(%rax)
>  	leaq	efi32_boot_gdt(%rip), %rax
>  	lgdt	(%rax)
>  
> @@ -67,7 +72,7 @@ SYM_FUNC_START(__efi64_thunk)
>  	pushq	%rax
>  	lretq
>  
> -1:	addq	$32, %rsp
> +1:	addq	$64, %rsp
>  	movq	%rdi, %rax
>  
>  	pop	%rbx
> @@ -132,6 +137,9 @@ SYM_FUNC_START_LOCAL(efi_enter32)
>  	 */

Can you pls also extend this comment here to say "IDT" for faster
pinpointing when someone like me is looking for the place where the
kernel IDT/GDT get restored again.

In any case, those are all minor nitpicks, other than that LGTM.

Lemme go test it on whatever I can - if others wanna run this, it would
be very much appreciated!

Thx.

-- 
Regards/Gruss,
    Boris.

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