linux-kernel - Re: [PATCH v3 1/2] xen/PVH: Set up GS segment for stack canary

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Date:   Thu, 17 May 2018 13:47:12 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     xen-devel <xen-devel@...ts.xenproject.org>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] xen/PVH: Set up GS segment for stack canary

On 05/17/2018 11:02 AM, Jan Beulich wrote:
>>>> On 17.05.18 at 16:47, <boris.ostrovsky@...cle.com> wrote:
>> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen)
>>  	mov %eax,%es
>>  	mov %eax,%ss
>>  
>> +	mov $PVH_CANARY_SEL,%eax
>> +	mov %eax,%gs
> I doubt this is needed for 64-bit (you could equally well load zero or leave
> in place what's there in that case),

I don't understand this.


>  and loading the selector before setting
> the base address in the descriptor won't have the intended effect.


I wasn't sure about this either but then I noticed that
secondary_startup_64() does it in the same order (although not using the
MSR).


>
>> @@ -150,9 +170,12 @@ gdt_start:
>>  	.quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* __KERNEL_CS */
>>  #endif
>>  	.quad GDT_ENTRY(0xc092, 0, 0xfffff) /* __KERNEL_DS */
>> +	.quad GDT_ENTRY(0x4090, 0, 0x18)    /* PVH_CANARY_SEL */
>>  gdt_end:
>>  
>> -	.balign 4
>> +	.balign 16
>> +canary:
>> +	.fill 24, 1, 0
> This is too little space for 64-bit afaict (the canary lives at offset 40 there
> if I can trust asm/processor.h).

Yes, should be 48. I didn't realize the two modes use different offsets.

-boris