Subject: [PATCH v2 01/11] KVM: arm: plug guest debug exploit

From: [email-masked] (zichao)
Date: Sun, 07 Jun 2015 21:40:58 +0800

Hi, Marc,

On 2015/6/1 18:56, Marc Zyngier wrote:
> Hi Zhichao,
> 
> On 31/05/15 05:27, Zhichao Huang wrote:
>> Hardware debugging in guests is not intercepted currently, it means
>> that a malicious guest can bring down the entire machine by writing
>> to the debug registers.
>>
>> This patch enable trapping of all debug registers, preventing the guests
>> to mess with the host state.
>>
>> However, it is a precursor for later patches which will need to do
>> more to world switch debug states while necessary.
>>
>> Cc: <[email-masked]>
>> Signed-off-by: Zhichao Huang <[email-masked]>
>> ---
>>  arch/arm/include/asm/kvm_coproc.h |  3 +-
>>  arch/arm/kvm/coproc.c             | 60 +++++++++++++++++++++++++++++++++++----
>>  arch/arm/kvm/handle_exit.c        |  4 +--
>>  arch/arm/kvm/interrupts_head.S    |  2 +-
>>  4 files changed, 59 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_coproc.h b/arch/arm/include/asm/kvm_coproc.h
>> index 4917c2f..e74ab0f 100644
>> --- a/arch/arm/include/asm/kvm_coproc.h
>> +++ b/arch/arm/include/asm/kvm_coproc.h
>> @@ -31,7 +31,8 @@ void kvm_register_target_coproc_table(struct kvm_coproc_target_table *table);
>>  int kvm_handle_cp10_id(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>  int kvm_handle_cp_0_13_access(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>  int kvm_handle_cp14_load_store(struct kvm_vcpu *vcpu, struct kvm_run *run);
>> -int kvm_handle_cp14_access(struct kvm_vcpu *vcpu, struct kvm_run *run);
>> +int kvm_handle_cp14_32(struct kvm_vcpu *vcpu, struct kvm_run *run);
>> +int kvm_handle_cp14_64(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>  int kvm_handle_cp15_32(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>  int kvm_handle_cp15_64(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>  
>> diff --git a/arch/arm/kvm/coproc.c b/arch/arm/kvm/coproc.c
>> index f3d88dc..2e12760 100644
>> --- a/arch/arm/kvm/coproc.c
>> +++ b/arch/arm/kvm/coproc.c
>> @@ -91,12 +91,6 @@ int kvm_handle_cp14_load_store(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  	return 1;
>>  }
>>  
>> -int kvm_handle_cp14_access(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> -{
>> -	kvm_inject_undefined(vcpu);
>> -	return 1;
>> -}
>> -
>>  static void reset_mpidr(struct kvm_vcpu *vcpu, const struct coproc_reg *r)
>>  {
>>  	/*
>> @@ -519,6 +513,60 @@ int kvm_handle_cp15_32(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  	return emulate_cp15(vcpu, &params);
>>  }
>>  
>> +/**
>> + * kvm_handle_cp14_64 -- handles a mrrc/mcrr trap on a guest CP14 access
>> + * @vcpu: The VCPU pointer
>> + * @run:  The kvm_run struct
>> + */
>> +int kvm_handle_cp14_64(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> +{
>> +	struct coproc_params params;
>> +
>> +	params.CRn = (kvm_vcpu_get_hsr(vcpu) >> 1) & 0xf;
>> +	params.Rt1 = (kvm_vcpu_get_hsr(vcpu) >> 5) & 0xf;
>> +	params.is_write = ((kvm_vcpu_get_hsr(vcpu) & 1) == 0);
>> +	params.is_64bit = true;
>> +
>> +	params.Op1 = (kvm_vcpu_get_hsr(vcpu) >> 16) & 0xf;
>> +	params.Op2 = 0;
>> +	params.Rt2 = (kvm_vcpu_get_hsr(vcpu) >> 10) & 0xf;
>> +	params.CRm = 0;
>> +
>> +	/* raz_wi */
>> +	(void)pm_fake(vcpu, &params, NULL);
>> +
>> +	/* handled */
>> +	kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
>> +	return 1;
>> +}
>> +
>> +/**
>> + * kvm_handle_cp14_32 -- handles a mrc/mcr trap on a guest CP14 access
>> + * @vcpu: The VCPU pointer
>> + * @run:  The kvm_run struct
>> + */
>> +int kvm_handle_cp14_32(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> +{
>> +	struct coproc_params params;
>> +
>> +	params.CRm = (kvm_vcpu_get_hsr(vcpu) >> 1) & 0xf;
>> +	params.Rt1 = (kvm_vcpu_get_hsr(vcpu) >> 5) & 0xf;
>> +	params.is_write = ((kvm_vcpu_get_hsr(vcpu) & 1) == 0);
>> +	params.is_64bit = false;
>> +
>> +	params.CRn = (kvm_vcpu_get_hsr(vcpu) >> 10) & 0xf;
>> +	params.Op1 = (kvm_vcpu_get_hsr(vcpu) >> 14) & 0x7;
>> +	params.Op2 = (kvm_vcpu_get_hsr(vcpu) >> 17) & 0x7;
>> +	params.Rt2 = 0;
>> +
>> +	/* raz_wi */
>> +	(void)pm_fake(vcpu, &params, NULL);
>> +
>> +	/* handled */
>> +	kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
>> +	return 1;
>> +}
>> +
>>  /******************************************************************************
>>   * Userspace API
>>   *****************************************************************************/
>> diff --git a/arch/arm/kvm/handle_exit.c b/arch/arm/kvm/handle_exit.c
>> index 95f12b2..357ad1b 100644
>> --- a/arch/arm/kvm/handle_exit.c
>> +++ b/arch/arm/kvm/handle_exit.c
>> @@ -104,9 +104,9 @@ static exit_handle_fn arm_exit_handlers[] = {
>>  	[HSR_EC_WFI]		= kvm_handle_wfx,
>>  	[HSR_EC_CP15_32]	= kvm_handle_cp15_32,
>>  	[HSR_EC_CP15_64]	= kvm_handle_cp15_64,
>> -	[HSR_EC_CP14_MR]	= kvm_handle_cp14_access,
>> +	[HSR_EC_CP14_MR]	= kvm_handle_cp14_32,
>>  	[HSR_EC_CP14_LS]	= kvm_handle_cp14_load_store,
>> -	[HSR_EC_CP14_64]	= kvm_handle_cp14_access,
>> +	[HSR_EC_CP14_64]	= kvm_handle_cp14_64,
>>  	[HSR_EC_CP_0_13]	= kvm_handle_cp_0_13_access,
>>  	[HSR_EC_CP10_ID]	= kvm_handle_cp10_id,
>>  	[HSR_EC_SVC_HYP]	= handle_svc_hyp,
>> diff --git a/arch/arm/kvm/interrupts_head.S b/arch/arm/kvm/interrupts_head.S
>> index 35e4a3a..a9f3a56 100644
>> --- a/arch/arm/kvm/interrupts_head.S
>> +++ b/arch/arm/kvm/interrupts_head.S
>> @@ -607,7 +607,7 @@ ARM_BE8(rev	r6, r6  )
>>   * (hardware reset value is 0) */
>>  .macro set_hdcr operation
>>  	mrc	p15, 4, r2, c1, c1, 1
>> -	ldr	r3, =(HDCR_TPM|HDCR_TPMCR)
>> +	ldr	r3, =(HDCR_TPM|HDCR_TPMCR|HDCR_TDRA|HDCR_TDOSA|HDCR_TDA)
> 
> There is a small problem here. Imagine the host has programmed a
> watchpoint on some VA. We switch to the guest, and then access the same
> VA. At that stage, the guest will take the debug exception. That's
> really not pretty.
> 

I've thought about it and I think there maybe OK, because when we switch from the
host to the guest, the context_switch() in host must be called first, and then
the host will switch debug registers, and the guest will not see the watchpoint
the host programmed before.

Or am I missing some circumstances here?

> I think using HDCR_TDE as well should sort it, effectively preventing
> the exception from being delivered to the guest, but you will need to
> handle this on the HYP side. Performance wise, this is also really horrible.
> 
> A better way would be to disable the host's BPs/WPs if any is enabled.
> 
>>  	.if \operation == vmentry
>>  	orr	r2, r2, r3		@ Trap some perfmon accesses
>>  	.else
>>
> 
> Thanks,
> 
> 	M.
>