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Date:   Fri, 16 Oct 2020 13:17:03 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     x86@kernel.org, Joerg Roedel <jroedel@suse.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 5/5] x86/head/64: Disable stack protection for
 head$(BITS).o
Message-ID: <20201016111703.GD8483@zn.tnic>
References: <20201008191623.2881677-1-nivedita@alum.mit.edu>
 <20201008191623.2881677-6-nivedita@alum.mit.edu>
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On Thu, Oct 08, 2020 at 03:16:23PM -0400, Arvind Sankar wrote:
> On 64-bit, the startup_64_setup_env() function added in
>   866b556efa12 ("x86/head/64: Install startup GDT")
> has stack protection enabled because of set_bringup_idt_handler().

Where? I don't see it.

I have

CONFIG_STACKPROTECTOR=y
# CONFIG_STACKPROTECTOR_STRONG is not set

and a __stack_chk_fail call is nowhere to be found in the resulting
head64.s file.

startup_64_setup_env:
# arch/x86/kernel/head64.c:91: 	return ptr - (void *)_text + (void *)physaddr;
	leaq	startup_gdt(%rdi), %rax	#, tmp99
# arch/x86/kernel/head64.c:91: 	return ptr - (void *)_text + (void *)physaddr;
	subq	$_text, %rax	#, tmp101
	movq	%rax, startup_gdt_descr+2(%rip)	# tmp101, startup_gdt_descr.address
# ./arch/x86/include/asm/desc.h:209: 	asm volatile("lgdt %0"::"m" (*dtr));
#APP
# 209 "./arch/x86/include/asm/desc.h" 1
	lgdt startup_gdt_descr(%rip)		# startup_gdt_descr
# 0 "" 2
# arch/x86/kernel/head64.c:600: 	asm volatile("movl %%eax, %%ds\n"
#NO_APP
	movl	$24, %eax	#, tmp102
#APP
# 600 "arch/x86/kernel/head64.c" 1
	movl %eax, %ds
movl %eax, %ss
movl %eax, %es

# 0 "" 2
# arch/x86/kernel/head64.c:91: 	return ptr - (void *)_text + (void *)physaddr;
#NO_APP
	leaq	bringup_idt_table(%rdi), %r9	#, tmp105
	leaq	bringup_idt_descr(%rdi), %r8	#, tmp103
	leaq	vc_no_ghcb(%rdi), %rsi	#, tmp107
# arch/x86/kernel/head64.c:91: 	return ptr - (void *)_text + (void *)physaddr;
	subq	$_text, %r9	#, _11
	subq	$_text, %r8	#, _8
	subq	$_text, %rsi	#, tmp109
# arch/x86/kernel/head64.c:572: 		set_bringup_idt_handler(idt, X86_TRAP_VC, handler);
	movq	%r9, %rdi	# _11,
	call	set_bringup_idt_handler.constprop.0	#
# arch/x86/kernel/head64.c:575: 	desc->address = (unsigned long)idt;
	movq	%r9, 2(%r8)	# _11, MEM[(struct desc_ptr *)_8].address
# ./arch/x86/include/asm/desc.h:214: 	asm volatile("lidt %0"::"m" (*dtr));
#APP
# 214 "./arch/x86/include/asm/desc.h" 1
	lidt (%r8)		# MEM[(const struct desc_ptr *)_8]
# 0 "" 2
# arch/x86/kernel/head64.c:605: }
#NO_APP
	ret

-- 
Regards/Gruss,
    Boris.

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