Subject: [PATCH v3 01/11] KVM: arm: plug guest debug exploit

From: [email-masked] (Christoffer Dall)
Date: Mon, 29 Jun 2015 17:49:53 +0200

On Mon, Jun 22, 2015 at 06:41:24PM +0800, 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 access the debug registers.
> 
> This patch also disable the debug mode(DBGDSCR) in the guest world all
> the time, 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    | 13 ++++++++-
>  4 files changed, 70 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;

this is a complete duplicate of kvm_handle_cp15_64, can you share this
code somehow?

> +
> +	/* 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;

this is a complete duplicate of kvm_handle_cp15_32, can you share this
code somehow?

> +
> +	/* 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..f85c447 100644
> --- a/arch/arm/kvm/interrupts_head.S
> +++ b/arch/arm/kvm/interrupts_head.S
> @@ -97,6 +97,10 @@ vcpu	.req	r0		@ vcpu pointer always in r0
>  	mrs	r8, LR_fiq
>  	mrs	r9, SPSR_fiq
>  	push	{r2-r9}
> +
> +	/* DBGDSCR reg */
> +	mrc	p14, 0, r2, c0, c1, 0
> +	push	{r2}

this feels like it should belong in read_cp15_state and not the gp regs
portion ?

>  .endm
>  
>  .macro pop_host_regs_mode mode
> @@ -111,6 +115,9 @@ vcpu	.req	r0		@ vcpu pointer always in r0
>   * Clobbers all registers, in all modes, except r0 and r1.
>   */
>  .macro restore_host_regs
> +	pop	{r2}
> +	mcr	p14, 0, r2, c0, c2, 2
> +

Why are we reading the DBGDSCRint and writing the DBGDSCRext ?

>  	pop	{r2-r9}
>  	msr	r8_fiq, r2
>  	msr	r9_fiq, r3
> @@ -159,6 +166,10 @@ vcpu	.req	r0		@ vcpu pointer always in r0
>   * Clobbers *all* registers.
>   */
>  .macro restore_guest_regs
> +	/* reset DBGDSCR to disable debug mode */
> +	mov	r2, #0
> +	mcr	p14, 0, r2, c0, c2, 2

Is it valid to write 0 in all all fields of this register?

I thought Will expressed concern about accessing this register?  Why is
it safe in this context and not before?  It seems from the spec that
this can still raise an undefined exception if an external debugger
lowers the software debug enable signal.

> +
>  	restore_guest_regs_mode svc, #VCPU_SVC_REGS
>  	restore_guest_regs_mode abt, #VCPU_ABT_REGS
>  	restore_guest_regs_mode und, #VCPU_UND_REGS
> @@ -607,7 +618,7 @@ 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)
>  	.if \operation == vmentry
>  	orr	r2, r2, r3		@ Trap some perfmon accesses
>  	.else
> -- 
> 1.7.12.4
> 

Thanks,
-Christoffer