Re: [PATCH v2 11/15] xen/arm64: Add ARM_SMCCC_ARCH_WORKAROUND_1 BP hardening support

[Volodymyr Babchuk <volodymyr_babchuk@epam.com>]

Hi Julien,

On 08.02.18 21:21, Julien Grall wrote:
> Add the detection and runtime code for ARM_SMCCC_ARCH_WORKAROUND_1.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> 
> ---
>      Changes in v2:
>          - Patch added
> ---
>   xen/arch/arm/arm64/bpi.S    | 12 ++++++++++++
>   xen/arch/arm/cpuerrata.c    | 32 +++++++++++++++++++++++++++++++-
>   xen/include/asm-arm/smccc.h |  1 +
>   3 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/arm64/bpi.S b/xen/arch/arm/arm64/bpi.S
> index 4b7f1dc21f..ef237de7bd 100644
> --- a/xen/arch/arm/arm64/bpi.S
> +++ b/xen/arch/arm/arm64/bpi.S
> @@ -16,6 +16,8 @@
>    * along with this program.  If not, see <http://www.gnu.org/licenses/>.
>    */
>   
> +#include <asm/smccc.h>
> +
>   .macro ventry target
>       .rept 31
>       nop
> @@ -81,6 +83,16 @@ ENTRY(__psci_hyp_bp_inval_start)
>       add     sp, sp, #(8 * 18)
>   ENTRY(__psci_hyp_bp_inval_end)
>   
> +ENTRY(__smccc_workaround_1_smc_start)
> +    sub     sp, sp, #(8 * 4)
> +    stp     x2, x3, [sp, #(8 * 0)]
> +    stp     x0, x1, [sp, #(8 * 2)]
> +    mov     w0, #ARM_SMCCC_ARCH_WORKAROUND_1_FID
> +    ldp     x2, x3, [sp, #(8 * 0)]
> +    ldp     x0, x1, [sp, #(8 * 2)]
> +    add     sp, sp, #(8 * 4)
> +ENTRY(__smccc_workaround_1_smc_end)
> +

This code confuses me. You allocate 32 bytes on stack, save x0-x4 there, 
then you load ARM_SMCCC_ARCH_WORKAROUND_1_FID into w0 and restore values 
of x0-x4, overwriting value written into w0. Am I missing something?

Btw, you can use something like stp	x0, x1, [sp, #-16]! to avoid manual 
adjustment of sp. This will save you two instructions.

>   /*
>    * Local variables:
>    * mode: ASM
> diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
> index 6704648b26..6557577bcb 100644
> --- a/xen/arch/arm/cpuerrata.c
> +++ b/xen/arch/arm/cpuerrata.c
> @@ -147,6 +147,34 @@ install_bp_hardening_vec(const struct arm_cpu_capabilities *entry,
>       return ret;
>   }
>   
> +extern char __smccc_workaround_1_smc_start[], __smccc_workaround_1_smc_end[];
> +
> +static bool
> +check_smccc_arch_workaround_1(const struct arm_cpu_capabilities *entry)
> +{
> +    struct arm_smccc_res res;
> +
> +    /*
> +     * Enable callbacks are called on every CPU based on the
> +     * capabilities. So double-check whether the CPU matches the
> +     * entry.
> +     */
> +    if ( !entry->matches(entry) )
> +        return false;
> +
> +    if ( smccc_ver < SMCCC_VERSION(1, 1) )
> +        return false;
> +
> +    arm_smccc_1_1_smc(ARM_SMCCC_ARCH_FEATURES_FID,
> +                      ARM_SMCCC_ARCH_WORKAROUND_1_FID, &res);
> +    if ( res.a0 != ARM_SMCCC_SUCCESS )
> +        return false;
> +
> +    return install_bp_hardening_vec(entry,__smccc_workaround_1_smc_start,
> +                                    __smccc_workaround_1_smc_end,
> +                                    "call ARM_SMCCC_ARCH_WORKAROUND_1");
> +}
> +
>   extern char __psci_hyp_bp_inval_start[], __psci_hyp_bp_inval_end[];
>   
>   static int enable_psci_bp_hardening(void *data)
> @@ -154,12 +182,14 @@ static int enable_psci_bp_hardening(void *data)
>       bool ret = true;
>       static bool warned = false;
>   
> +    if ( check_smccc_arch_workaround_1(data) )
> +        return 0;
>       /*
>        * The mitigation is using PSCI version function to invalidate the
>        * branch predictor. This function is only available with PSCI 0.2
>        * and later.
>        */
> -    if ( psci_ver >= PSCI_VERSION(0, 2) )
> +    else if ( psci_ver >= PSCI_VERSION(0, 2) )
>           ret = install_bp_hardening_vec(data, __psci_hyp_bp_inval_start,
>                                          __psci_hyp_bp_inval_end,
>                                          "call PSCI get version");
> diff --git a/xen/include/asm-arm/smccc.h b/xen/include/asm-arm/smccc.h
> index 154772b728..8342cc33fe 100644
> --- a/xen/include/asm-arm/smccc.h
> +++ b/xen/include/asm-arm/smccc.h
> @@ -261,6 +261,7 @@ struct arm_smccc_res {
>   /* SMCCC error codes */
>   #define ARM_SMCCC_ERR_UNKNOWN_FUNCTION  (-1)
>   #define ARM_SMCCC_NOT_SUPPORTED         (-1)
> +#define ARM_SMCCC_SUCCESS               (0)
>   
>   /* SMCCC function identifier range which is reserved for existing APIs */
>   #define ARM_SMCCC_RESERVED_RANGE_START  0x0
> 

-- 
Volodymyr Babchuk

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