linux-kernel - Re: [Xen-devel] [PATCH 3/4] xen/PVH: Set up GS segment for stack canary

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Message-ID: <615b0e30-c360-3ad4-f1b3-0e907d790643@oracle.com>
Date:   Wed, 2 May 2018 11:00:30 -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,
        stable@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH 3/4] xen/PVH: Set up GS segment for stack
 canary

On 05/02/2018 04:16 AM, Jan Beulich wrote:
>>>> On 30.04.18 at 18:23, <boris.ostrovsky@...cle.com> wrote:
>> --- a/arch/x86/xen/xen-pvh.S
>> +++ b/arch/x86/xen/xen-pvh.S
>> @@ -54,6 +54,9 @@
>>   * charge of setting up it's own stack, GDT and IDT.
>>   */
>>  
>> +#define PVH_GDT_ENTRY_CANARY    4
>> +#define PVH_CANARY_SEL          (PVH_GDT_ENTRY_CANARY * 8)
> I can only advise against doing it this way: There's no safeguard against
> someone changing asm/segment.h without changing this value (in fact
> this applies to all of the GDT selectors populated in this file). At the very
> least tie this to GDT_ENTRY_BOOT_TSS / __BOOT_TSS?
>
>> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen)
>>  	mov %eax,%es
>>  	mov %eax,%ss
>>  
>> +	mov $(PVH_CANARY_SEL),%eax
>> +	mov %eax,%gs
>> +
>>  	/* Stash hvm_start_info. */
>>  	mov $_pa(pvh_start_info), %edi
>>  	mov %ebx, %esi
>> @@ -150,6 +156,7 @@ gdt_start:
>>  	.quad 0x00cf9a000000ffff            /* __BOOT_CS */
>>  #endif
>>  	.quad 0x00cf92000000ffff            /* __BOOT_DS */
>> +	.quad 0x0040900000000018            /* PVH_CANARY_SEL */
> Without any further code before loading the selector, this points at
> physical address 0. Don't you need to add in the base address of
> the per-CPU stack_canary?

This GDT is gone soon after we jump into generic x86 startup code.That
code will load its own GDT (and then set up per-cpu segments and all that).

-boris