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Date:   Wed, 25 Aug 2021 17:07:26 +0200
From:   Joerg Roedel <jroedel@suse.de>
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
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        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
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Subject: Re: [PATCH Part1 v5 23/38] x86/head/64: set up a startup %gs for
 stack protector
Message-ID: <YSZcrtqExehVwvhf@suse.de>
References: <20210820151933.22401-1-brijesh.singh@amd.com>
 <20210820151933.22401-24-brijesh.singh@amd.com>
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In-Reply-To: <20210820151933.22401-24-brijesh.singh@amd.com>
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On Fri, Aug 20, 2021 at 10:19:18AM -0500, Brijesh Singh wrote:
>  void __head startup_64_setup_env(unsigned long physbase)
>  {
> +	u64 gs_area = (u64)fixup_pointer(startup_gs_area, physbase);
> +

This breaks as soon as the compiler decides that startup_64_setup_env()
needs stack protection too.

And the startup_gs_area is also not needed, there is initial_gs for
that. 

What you need is something along these lines (untested):

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index d8b3ebd2bb85..3c7c59bc9903 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -65,6 +65,16 @@ SYM_CODE_START_NOALIGN(startup_64)
 	leaq	(__end_init_task - FRAME_SIZE)(%rip), %rsp
 
 	leaq	_text(%rip), %rdi
+
+	movl	$MSR_GS_BASE, %ecx
+	movq	initial_gs(%rip), %rax
+	movq	$_text, %rdx
+	subq	%rdx, %rax
+	addq	%rdi, %rax
+	movq	%rax, %rdx
+	shrq	$32,  %rdx
+	wrmsr
+
 	pushq	%rsi
 	call	startup_64_setup_env
 	popq	%rsi


It loads the initial_gs pointer, applies the fixup on it and loads it
into MSR_GS_BASE.