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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v3 05/13] xen/arm: Add command line option to control SSBD mitigation
On Tue, 12 Jun 2018, Julien Grall wrote:
> On a system where the firmware implements ARCH_WORKAROUND_2, it may be
> useful to either permanently enable or disable the workaround for cases
> where the user decides that they'd rather not get a trap overhead, and
> keep the mitigation permanently on or off instead of switching it on
> exception entry/exit. In any case, default to mitigation being enabled.
>
> The new command line option is implemented as list of one option to
> follow x86 option and also allow to extend it more easily in the future.
>
> Note that for convenience, the full implemention of the workaround is
> done in the .matches callback.
>
> Lastly, a accessor is provided to know the state of the mitigation.
>
> After this patch, there are 3 methods complementing each other to find the
> state of the mitigation:
> - The capability ARM_SSBD indicates the platform is affected by the
> vulnerability. This will also return false if the user decide to force
> disabled the mitigation (spec-ctrl="ssbd=force-disable"). The
> capability is useful for putting shortcut in place using alternative.
> - ssbd_state indicates the global state of the mitigation (e.g
> unknown, force enable...). The global state is required to report
> the state to a guest.
> - The per-cpu ssbd_callback_required indicates whether a pCPU
> requires to call the SMC. This allows to shortcut SMC call
> and save an entry/exit to EL3.
>
> This is part of XSA-263.
>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> ---
> Changes in v3:
> - Fix typo in the name
>
> Changes in v2:
> - Move out some code to the previous patch.
> - Update the commit message with more background
> ---
> docs/misc/xen-command-line.markdown | 18 ++++++++
> xen/arch/arm/cpuerrata.c | 88
> ++++++++++++++++++++++++++++++++++---
> xen/include/asm-arm/cpuerrata.h | 21 +++++++++
> 3 files changed, 120 insertions(+), 7 deletions(-)
>
> diff --git a/docs/misc/xen-command-line.markdown
> b/docs/misc/xen-command-line.markdown
> index 8712a833a2..962028b6ed 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1756,6 +1756,24 @@ enforces the maximum theoretically necessary timeout
> of 670ms. Any number
> is being interpreted as a custom timeout in milliseconds. Zero or boolean
> false disable the quirk workaround, which is also the default.
>
> +### spec-ctrl (Arm)
> +> `= List of [ ssbd=force-disable|runtime|force-enable ]`
> +
> +Controls for speculative execution sidechannel mitigations.
> +
> +The option `ssbd=` is used to control the state of Speculative Store
> +Bypass Disable (SSBD) mitigation.
> +
> +* `ssbd=force-disable` will keep the mitigation permanently off. The guest
> +will not be able to control the state of the mitigation.
> +* `ssbd=runtime` will always turn on the mitigation when running in the
> +hypervisor context. The guest will be to turn on/off the mitigation for
> +itself by using the firmware interface ARCH\_WORKAROUND\_2.
> +* `ssbd=force-enable` will keep the mitigation permanently on. The guest will
> +not be able to control the state of the mitigation.
> +
> +By default SSBD will be mitigated at runtime (i.e `ssbd=runtime`).
> +
> ### spec-ctrl (x86)
> > `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb}=<bool>,
> > bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd}=<bool> ]`
> diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
> index 1a6130406c..4292008692 100644
> --- a/xen/arch/arm/cpuerrata.c
> +++ b/xen/arch/arm/cpuerrata.c
> @@ -237,6 +237,41 @@ static int enable_ic_inv_hardening(void *data)
>
> #ifdef CONFIG_ARM_SSBD
>
> +enum ssbd_state ssbd_state = ARM_SSBD_RUNTIME;
> +
> +static int __init parse_spec_ctrl(const char *s)
> +{
> + const char *ss;
> + int rc = 0;
> +
> + do {
> + ss = strchr(s, ',');
> + if ( !ss )
> + ss = strchr(s, '\0');
> +
> + if ( !strncmp(s, "ssbd=", 5) )
> + {
> + s += 5;
> +
> + if ( !strncmp(s, "force-disable", ss - s) )
> + ssbd_state = ARM_SSBD_FORCE_DISABLE;
> + else if ( !strncmp(s, "runtime", ss - s) )
> + ssbd_state = ARM_SSBD_RUNTIME;
> + else if ( !strncmp(s, "force-enable", ss - s) )
> + ssbd_state = ARM_SSBD_FORCE_ENABLE;
> + else
> + rc = -EINVAL;
> + }
> + else
> + rc = -EINVAL;
> +
> + s = ss + 1;
> + } while ( *ss );
> +
> + return rc;
> +}
> +custom_param("spec-ctrl", parse_spec_ctrl);
> +
> /*
> * Assembly code may use the variable directly, so we need to make sure
> * it fits in a register.
> @@ -251,20 +286,17 @@ static bool has_ssbd_mitigation(const struct
> arm_cpu_capabilities *entry)
> if ( smccc_ver < SMCCC_VERSION(1, 1) )
> return false;
>
> - /*
> - * The probe function return value is either negative (unsupported
> - * or mitigated), positive (unaffected), or zero (requires
> - * mitigation). We only need to do anything in the last case.
> - */
> arm_smccc_1_1_smc(ARM_SMCCC_ARCH_FEATURES_FID,
> ARM_SMCCC_ARCH_WORKAROUND_2_FID, &res);
>
> switch ( (int)res.a0 )
> {
> case ARM_SMCCC_NOT_SUPPORTED:
> + ssbd_state = ARM_SSBD_UNKNOWN;
> return false;
>
> case ARM_SMCCC_NOT_REQUIRED:
> + ssbd_state = ARM_SSBD_MITIGATED;
> return false;
>
> case ARM_SMCCC_SUCCESS:
> @@ -280,8 +312,49 @@ static bool has_ssbd_mitigation(const struct
> arm_cpu_capabilities *entry)
> return false;
> }
>
> - if ( required )
> - this_cpu(ssbd_callback_required) = 1;
> + switch ( ssbd_state )
> + {
> + case ARM_SSBD_FORCE_DISABLE:
> + {
> + static bool once = true;
> +
> + if ( once )
> + printk("%s disabled from command-line\n", entry->desc);
> + once = false;
> +
> + arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2_FID, 0, NULL);
> + required = false;
> +
> + break;
> + }
> +
> + case ARM_SSBD_RUNTIME:
> + if ( required )
> + {
> + this_cpu(ssbd_callback_required) = 1;
> + arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2_FID, 1, NULL);
> + }
> +
> + break;
> +
> + case ARM_SSBD_FORCE_ENABLE:
> + {
> + static bool once = true;
> +
> + if ( once )
> + printk("%s forced from command-line\n", entry->desc);
> + once = false;
> +
> + arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2_FID, 1, NULL);
> + required = true;
> +
> + break;
> + }
> +
> + default:
> + ASSERT_UNREACHABLE();
> + return false;
> + }
>
> return required;
> }
> @@ -390,6 +463,7 @@ static const struct arm_cpu_capabilities arm_errata[] = {
> #endif
> #ifdef CONFIG_ARM_SSBD
> {
> + .desc = "Speculative Store Bypass Disabled",
> .capability = ARM_SSBD,
> .matches = has_ssbd_mitigation,
> },
> diff --git a/xen/include/asm-arm/cpuerrata.h b/xen/include/asm-arm/cpuerrata.h
> index e628d3ff56..55ddfda272 100644
> --- a/xen/include/asm-arm/cpuerrata.h
> +++ b/xen/include/asm-arm/cpuerrata.h
> @@ -31,10 +31,26 @@ CHECK_WORKAROUND_HELPER(ssbd, ARM_SSBD, CONFIG_ARM_SSBD)
>
> #undef CHECK_WORKAROUND_HELPER
>
> +enum ssbd_state
> +{
> + ARM_SSBD_UNKNOWN,
> + ARM_SSBD_FORCE_DISABLE,
> + ARM_SSBD_RUNTIME,
> + ARM_SSBD_FORCE_ENABLE,
> + ARM_SSBD_MITIGATED,
> +};
> +
> #ifdef CONFIG_ARM_SSBD
>
> #include <asm/current.h>
>
> +extern enum ssbd_state ssbd_state;
> +
> +static inline enum ssbd_state get_ssbd_state(void)
> +{
> + return ssbd_state;
> +}
> +
> DECLARE_PER_CPU(register_t, ssbd_callback_required);
>
> static inline bool cpu_require_ssbd_mitigation(void)
> @@ -49,6 +65,11 @@ static inline bool cpu_require_ssbd_mitigation(void)
> return false;
> }
>
> +static inline enum ssbd_state get_ssbd_state(void)
> +{
> + return ARM_SSBD_UNKNOWN;
> +}
> +
> #endif
>
> #endif /* __ARM_CPUERRATA_H__ */
> --
> 2.11.0
>
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