Re: [PATCH v7 01/38] KVM: arm64: Introduce a validation function for an ID register

[Oliver Upton <oupton@google.com>]

On Mon, Apr 18, 2022 at 11:55:07PM -0700, Reiji Watanabe wrote:
> Introduce arm64_check_features(), which does a basic validity checking
> of an ID register value against the register's limit value, which is
> generally the host's sanitized value.
> 
> This function will be used by the following patches to check if an ID
> register value that userspace tries to set for a guest can be supported
> on the host.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  arch/arm64/include/asm/cpufeature.h |  1 +
>  arch/arm64/kernel/cpufeature.c      | 52 +++++++++++++++++++++++++++++
>  2 files changed, 53 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index c62e7e5e2f0c..7a009d4e18a6 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -634,6 +634,7 @@ void check_local_cpu_capabilities(void);
>  
>  u64 read_sanitised_ftr_reg(u32 id);
>  u64 __read_sysreg_by_encoding(u32 sys_id);
> +int arm64_check_features(const struct arm64_ftr_bits *ftrp, u64 val, u64 limit);
>  
>  static inline bool cpu_supports_mixed_endian_el0(void)
>  {
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index d72c4b4d389c..dbbc69745f22 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -3239,3 +3239,55 @@ ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr,
>  		return sprintf(buf, "Vulnerable\n");
>  	}
>  }
> +
> +/**
> + * arm64_check_features() - Check if a feature register value constitutes
> + * a subset of features indicated by @limit.
> + *
> + * @ftrp: Pointer to an array of arm64_ftr_bits. It must be terminated by
> + * an item whose width field is zero.
> + * @val: The feature register value to check
> + * @limit: The limit value of the feature register
> + *
> + * This function will check if each feature field of @val is the "safe" value
> + * against @limit based on @ftrp[], each of which specifies the target field
> + * (shift, width), whether or not the field is for a signed value (sign),
> + * how the field is determined to be "safe" (type), and the safe value
> + * (safe_val) when type == FTR_EXACT (safe_val won't be used by this
> + * function when type != FTR_EXACT). Any other fields in arm64_ftr_bits
> + * won't be used by this function. If a field value in @val is the same
> + * as the one in @limit, it is always considered the safe value regardless
> + * of the type. For register fields that are not in @ftrp[], only the value
> + * in @limit is considered the safe value.
> + *
> + * Return: 0 if all the fields are safe. Otherwise, return negative errno.
> + */
> +int arm64_check_features(const struct arm64_ftr_bits *ftrp, u64 val, u64 limit)
> +{
> +	u64 mask = 0;
> +
> +	for (; ftrp->width; ftrp++) {
> +		s64 f_val, f_lim, safe_val;
> +
> +		f_val = arm64_ftr_value(ftrp, val);
> +		f_lim = arm64_ftr_value(ftrp, limit);
> +		mask |= arm64_ftr_mask(ftrp);
> +
> +		if (f_val == f_lim)
> +			safe_val = f_val;
> +		else
> +			safe_val = arm64_ftr_safe_value(ftrp, f_val, f_lim);
> +
> +		if (safe_val != f_val)
> +			return -E2BIG;
> +	}
> +
> +	/*
> +	 * For fields that are not indicated in ftrp, values in limit are the
> +	 * safe values.
> +	 */
> +	if ((val & ~mask) != (limit & ~mask))
> +		return -E2BIG;

This bit is interesting. Apologies if I paged out relevant context. What
features are we trying to limit that exist outside of an arm64_ftr_bits
definition? I'll follow the series and see if I figure out later :-P

Generally speaking, though, it seems to me that we'd prefer to have an
arm64_ftr_bits struct plumbed up for whatever hits this case.

--
Thanks,
Oliver

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