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Subject: Re: [PATCH] x86/xen: init %gs very early to avoid page faults with
 stack protector
To:     Andrew Cooper <andrew.cooper3@citrix.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Cc:     boris.ostrovsky@oracle.com, stable@vger.kernel.org
References: <20180201121601.1268-1-jgross@suse.com>
 <66ad81d7-0230-d0a5-f4fb-9dbbd8162884@citrix.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <24614db8-7369-8178-f690-7c1614e2c0c9@suse.com>
Date:   Thu, 1 Feb 2018 13:24:39 +0100
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On 01/02/18 13:22, Andrew Cooper wrote:
> On 01/02/18 12:16, Juergen Gross wrote:
>> When running as Xen pv guest %gs is initialized some time after
>> C code is started. Depending on stack protector usage this might be
>> too late, resulting in page faults.
>>
>> So setup %gs and MSR_GS_BASE in assembly code already.
>>
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>  arch/x86/xen/xen-head.S | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
>> index 497cc55a0c16..b47d87076efb 100644
>> --- a/arch/x86/xen/xen-head.S
>> +++ b/arch/x86/xen/xen-head.S
>> @@ -9,7 +9,9 @@
>>  
>>  #include <asm/boot.h>
>>  #include <asm/asm.h>
>> +#include <asm/msr.h>
>>  #include <asm/page_types.h>
>> +#include <asm/percpu.h>
>>  #include <asm/unwind_hints.h>
>>  
>>  #include <xen/interface/elfnote.h>
>> @@ -35,6 +37,18 @@ ENTRY(startup_xen)
>>  	mov %_ASM_SI, xen_start_info
>>  	mov $init_thread_union+THREAD_SIZE, %_ASM_SP
>>  
>> +	/* Set up %gs.
>> +	 *
>> +	 * The base of %gs always points to the bottom of the irqstack
>> +	 * union.  If the stack protector canary is enabled, it is
>> +	 * located at %gs:40.  Note that, on SMP, the boot cpu uses
>> +	 * init data section till per cpu areas are set up.
>> +	 */
>> +	movl	$MSR_GS_BASE,%ecx
>> +	movq	$INIT_PER_CPU_VAR(irq_stack_union),%rax
>> +	cdq
>> +	wrmsr
> 
> You surely want a #ifdef __x86_64__ ?  This path is common to the 32bit
> entry as well?

Oh, indeed! Thanks for noticing.

V2 coming soon...


Juergen