linux-kernel - Re: [PATCH] KVM: x86: Fix CR3 and LDT sel should not be saved in TSS

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Message-ID: <535E3249.2010000@redhat.com>
Date:	Mon, 28 Apr 2014 12:49:45 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	Nadav Amit <namit@...technion.ac.il>
CC:	gleb@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Fix CR3 and LDT sel should not be saved in
 TSS

Il 16/04/2014 21:20, Marcelo Tosatti ha scritto:
> On Mon, Apr 07, 2014 at 06:37:47PM +0300, Nadav Amit wrote:
>> According to Intel specifications, only general purpose registers and segment
>> selectors should are saved in the old TSS during 32-bit task-switch.
>
> should be
>
>> Signed-off-by: Nadav Amit <namit@...technion.ac.il>
>> ---
>>  arch/x86/kvm/emulate.c |   10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index 205b17e..0dec502 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -2496,7 +2496,7 @@ static int task_switch_16(struct x86_emulate_ctxt *ctxt,
>>  static void save_state_to_tss32(struct x86_emulate_ctxt *ctxt,
>>  				struct tss_segment_32 *tss)
>>  {
>> -	tss->cr3 = ctxt->ops->get_cr(ctxt, 3);
>> +	/* CR3 and ldt selector are not saved intentionally */
>>  	tss->eip = ctxt->_eip;
>>  	tss->eflags = ctxt->eflags;
>>  	tss->eax = reg_read(ctxt, VCPU_REGS_RAX);
>> @@ -2514,7 +2514,6 @@ static void save_state_to_tss32(struct x86_emulate_ctxt *ctxt,
>>  	tss->ds = get_segment_selector(ctxt, VCPU_SREG_DS);
>>  	tss->fs = get_segment_selector(ctxt, VCPU_SREG_FS);
>>  	tss->gs = get_segment_selector(ctxt, VCPU_SREG_GS);
>> -	tss->ldt_selector = get_segment_selector(ctxt, VCPU_SREG_LDTR);
>>  }
>>
>>  static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
>
> Only this hunk is enough ?

I guess there could be a corner case where the beginning or tail of the 
TSS is in a read-only page but EIP...LDT is all writable.

Paolo

>> @@ -2604,6 +2603,8 @@ static int task_switch_32(struct x86_emulate_ctxt *ctxt,
>>  	struct tss_segment_32 tss_seg;
>>  	int ret;
>>  	u32 new_tss_base = get_desc_base(new_desc);
>> +	u32 eip_offset = offsetof(struct tss_segment_32, eip);
>> +	u32 ldt_sel_offset = offsetof(struct tss_segment_32, ldt_selector);
>>
>>  	ret = ops->read_std(ctxt, old_tss_base, &tss_seg, sizeof tss_seg,
>>  			    &ctxt->exception);
>> @@ -2613,8 +2614,9 @@ static int task_switch_32(struct x86_emulate_ctxt *ctxt,
>>
>>  	save_state_to_tss32(ctxt, &tss_seg);
>>
>> -	ret = ops->write_std(ctxt, old_tss_base, &tss_seg, sizeof tss_seg,
>> -			     &ctxt->exception);
>> +	/* Only GP registers and segment selectors are saved */
>> +	ret = ops->write_std(ctxt, old_tss_base + eip_offset, &tss_seg.eip,
>> +			     ldt_sel_offset - eip_offset, &ctxt->exception);
>>  	if (ret != X86EMUL_CONTINUE)
>>  		/* FIXME: need to provide precise fault address */
>>  		return ret;
>> --
>> 1.7.10.4
>>
>> --
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