Re: [PATCH 2/3] x86/HVM: limit writes to outgoing TSS during task switch

From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
	Wei Liu <wei.liu2@citrix.com>
Subject: Re: [PATCH 2/3] x86/HVM: limit writes to outgoing TSS during task switch
Date: Wed, 23 Nov 2016 01:30:19 -0700	[thread overview]
Message-ID: <583561AB020000780012125C@prv-mh.provo.novell.com> (raw)
In-Reply-To: <c580a9a2-4dec-a650-3e5f-d19dfdf88b6f@citrix.com>

>>> On 22.11.16 at 17:46, <andrew.cooper3@citrix.com> wrote:
> On 22/11/16 13:55, Jan Beulich wrote:
>> The only fields modified are EIP, EFLAGS, GPRs, and segment selectors.
>> CR3 in particular is not supposed to be updated.
>>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>
>> --- a/xen/arch/x86/hvm/hvm.c
>> +++ b/xen/arch/x86/hvm/hvm.c
>> @@ -2952,7 +2952,6 @@ void hvm_task_switch(
>>      if ( taskswitch_reason == TSW_iret )
>>          eflags &= ~X86_EFLAGS_NT;
>>  
>> -    tss.cr3    = v->arch.hvm_vcpu.guest_cr[3];
>>      tss.eip    = regs->eip;
>>      tss.eflags = eflags;
>>      tss.eax    = regs->eax;
>> @@ -2979,8 +2978,10 @@ void hvm_task_switch(
>>      hvm_get_segment_register(v, x86_seg_ldtr, &segr);
>>      tss.ldt = segr.sel;
>>  
>> -    rc = hvm_copy_to_guest_virt(
>> -        prev_tr.base, &tss, sizeof(tss), PFEC_page_present);
>> +    rc = hvm_copy_to_guest_virt(prev_tr.base + offsetof(typeof(tss), eip),
>> +                                &tss.eip,
>> +                                (void *)&tss.trace - (void *)&tss.eip,
> 
> How about offsetof(typeof(tss), trace) - offsetof(typeof(tss), eip)?
> 
> It avoids some casts and void pointer arithmetic.

Oh, yes, that's better.

> Either way, Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Thanks, Jan

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