ACK/CMNT: [PATCH 1/3] arm64: KVM: Tighten guest core register access from userspace

[Tyler Hicks]

On 2019-02-19 14:02:33, Paolo Pisati wrote:
> From: Dave Martin <Dave.Martin at arm.com>
> 
> We currently allow userspace to access the core register file
> in about any possible way, including straddling multiple
> registers and doing unaligned accesses.
> 
> This is not the expected use of the ABI, and nobody is actually
> using it that way. Let's tighten it by explicitly checking
> the size and alignment for each field of the register file.
> 
> Cc: <stable at vger.kernel.org>
> Fixes: 2f4a07c5f9fe ("arm64: KVM: guest one-reg interface")
> Reviewed-by: Christoffer Dall <christoffer.dall at arm.com>
> Reviewed-by: Mark Rutland <mark.rutland at arm.com>
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> [maz: rewrote Dave's initial patch to be more easily backported]
> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
> Signed-off-by: Will Deacon <will.deacon at arm.com>

When applying this patch, the CVE id needs to be added to the commit
message:

CVE-2018-18021

With that change,

Acked-by: Tyler Hicks <tyhicks at canonical.com>

Tyler

> 
> (cherry picked from commit d26c25a9d19b5976b319af528886f89cf455692d)
> Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
> ---
>  arch/arm64/kvm/guest.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
> 
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 811f04c..3f4817d 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -57,6 +57,45 @@ static u64 core_reg_offset_from_id(u64 id)
>  	return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
>  }
>  
> +static int validate_core_offset(const struct kvm_one_reg *reg)
> +{
> +	u64 off = core_reg_offset_from_id(reg->id);
> +	int size;
> +
> +	switch (off) {
> +	case KVM_REG_ARM_CORE_REG(regs.regs[0]) ...
> +	     KVM_REG_ARM_CORE_REG(regs.regs[30]):
> +	case KVM_REG_ARM_CORE_REG(regs.sp):
> +	case KVM_REG_ARM_CORE_REG(regs.pc):
> +	case KVM_REG_ARM_CORE_REG(regs.pstate):
> +	case KVM_REG_ARM_CORE_REG(sp_el1):
> +	case KVM_REG_ARM_CORE_REG(elr_el1):
> +	case KVM_REG_ARM_CORE_REG(spsr[0]) ...
> +	     KVM_REG_ARM_CORE_REG(spsr[KVM_NR_SPSR - 1]):
> +		size = sizeof(__u64);
> +		break;
> +
> +	case KVM_REG_ARM_CORE_REG(fp_regs.vregs[0]) ...
> +	     KVM_REG_ARM_CORE_REG(fp_regs.vregs[31]):
> +		size = sizeof(__uint128_t);
> +		break;
> +
> +	case KVM_REG_ARM_CORE_REG(fp_regs.fpsr):
> +	case KVM_REG_ARM_CORE_REG(fp_regs.fpcr):
> +		size = sizeof(__u32);
> +		break;
> +
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	if (KVM_REG_SIZE(reg->id) == size &&
> +	    IS_ALIGNED(off, size / sizeof(__u32)))
> +		return 0;
> +
> +	return -EINVAL;
> +}
> +
>  static int get_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  {
>  	/*
> @@ -76,6 +115,9 @@ static int get_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  	    (off + (KVM_REG_SIZE(reg->id) / sizeof(__u32))) >= nr_regs)
>  		return -ENOENT;
>  
> +	if (validate_core_offset(reg))
> +		return -EINVAL;
> +
>  	if (copy_to_user(uaddr, ((u32 *)regs) + off, KVM_REG_SIZE(reg->id)))
>  		return -EFAULT;
>  
> @@ -98,6 +140,9 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  	    (off + (KVM_REG_SIZE(reg->id) / sizeof(__u32))) >= nr_regs)
>  		return -ENOENT;
>  
> +	if (validate_core_offset(reg))
> +		return -EINVAL;
> +
>  	if (KVM_REG_SIZE(reg->id) > sizeof(tmp))
>  		return -EINVAL;
>  
> -- 
> 2.7.4
> 
> 
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