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Subject: Re: [PATCH] x86/xen: Delay get_cpu_cap until stack canary is
 established
To:     Jason Andryuk <jandryuk@gmail.com>, Juergen Gross <jgross@suse.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
References: <20180319165804.10636-1-jandryuk@gmail.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <0726a193-6ce9-e835-9eef-01c56680d30a@oracle.com>
Date:   Mon, 19 Mar 2018 18:22:12 -0400
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On 03/19/2018 12:58 PM, Jason Andryuk wrote:
> Commit 2cc42bac1c79 ("x86-64/Xen: eliminate W+X mappings") introduced a
> call to get_cpu_cap, which is fstack-protected.  This is works on x86-64

s/This is works/This works/

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

Do we still need 4f277295e54?

-boris

> as commit 4f277295e54c ("x86/xen: init %gs very early to avoid page
> faults with stack protector") ensures the stack protector is configured,
> but it it did not cover x86-32.
>
> Delay calling get_cpu_cap until after xen_setup_gdt has initialized the
> stack canary.  Without this, a 32bit PV machine crashes early
> in boot.
> (XEN) Domain 0 (vcpu#0) crashed on cpu#0:
> (XEN) ----[ Xen-4.6.6-xc  x86_64  debug=n  Tainted:    C ]----
> (XEN) CPU:    0
> (XEN) RIP:    e019:[<00000000c10362f8>]
>
> And the PV kernel IP corresponds to init_scattered_cpuid_features
>    0xc10362f8 <+24>:    mov    %gs:0x14,%eax
>
> Fixes 2cc42bac1c79 ("x86-64/Xen: eliminate W+X mappings")
>
> Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
> ---
>  arch/x86/xen/enlighten_pv.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 3c2c2530737e..c36d23aa6c35 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1259,10 +1259,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
>  	 */
>  	__userpte_alloc_gfp &= ~__GFP_HIGHMEM;
>  
> -	/* Work out if we support NX */
> -	get_cpu_cap(&boot_cpu_data);
> -	x86_configure_nx();
> -
>  	/* Get mfn list */
>  	xen_build_dynamic_phys_to_machine();
>  
> @@ -1272,6 +1268,10 @@ asmlinkage __visible void __init xen_start_kernel(void)
>  	 */
>  	xen_setup_gdt(0);
>  
> +	/* Work out if we support NX */
> +	get_cpu_cap(&boot_cpu_data);
> +	x86_configure_nx();
> +
>  	xen_init_irq_ops();
>  
>  	/* Let's presume PV guests always boot on vCPU with id 0. */