[PATCH for-4.17?] x86: support data operand independent timing mode

[Jan Beulich <jbeulich@xxxxxxxx>]

[1] specifies a long list of instructions which are intended to exhibit
timing behavior independent of the data they operate on. On certain
hardware this independence is optional, controlled by a bit in a new
MSR. Provide a command line option to control the mode Xen and its
guests are to operate in, with a build time control over the default.
Longer term we may want to allow guests to control this.

Since Arm64 supposedly also has such a control, put command line option
and Kconfig control in common files.

[1] 
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/data-operand-independent-timing-isa-guidance.html

Requested-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
This may be viewed as a new feature, and hence be too late for 4.17. It
may, however, also be viewed as security relevant, which is why I'd like
to propose to at least consider it.

Slightly RFC, in particular for whether the Kconfig option should
default to Y or N.

I would have wanted to invoke setup_doitm() from cpu_init(), but that
works only on the BSP. On APs cpu_init() runs before ucode loading.

--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -746,6 +746,14 @@ Specify the size of the console debug tr
 additionally a trace buffer of the specified size is allocated per cpu.
 The debug trace feature is only enabled in debugging builds of Xen.
 
+### dit (x86)
+> `= <boolean>`
+
+> Default: `CONFIG_DIT_DEFAULT`
+
+Specify whether Xen and guests should operate in Data Independent Timing
+mode.
+
 ### dma_bits
 > `= <integer>`
 
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -14,6 +14,7 @@ config X86
        select HAS_ALTERNATIVE
        select HAS_COMPAT
        select HAS_CPUFREQ
+       select HAS_DIT
        select HAS_EHCI
        select HAS_EX_TABLE
        select HAS_FAST_MULTIPLY
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -209,6 +209,24 @@ void ctxt_switch_levelling(const struct
                alternative_vcall(ctxt_switch_masking, next);
 }
 
+static void setup_doitm(void)
+{
+    uint64_t msr;
+
+    if ( !cpu_has_arch_caps )
+        return;
+
+    rdmsrl(MSR_ARCH_CAPABILITIES, msr);
+    if ( !(msr & ARCH_CAPS_DOITM) )
+        return;
+
+    rdmsrl(MSR_UARCH_MISC_CTRL, msr);
+    msr |= UARCH_CTRL_DOITM;
+    if ( !opt_dit )
+        msr &= ~UARCH_CTRL_DOITM;
+    wrmsrl(MSR_UARCH_MISC_CTRL, msr);
+}
+
 bool_t opt_cpu_info;
 boolean_param("cpuinfo", opt_cpu_info);
 
@@ -581,6 +599,8 @@ void identify_cpu(struct cpuinfo_x86 *c)
 
                mtrr_bp_init();
        }
+
+       setup_doitm();
 }
 
 /* leaf 0xb SMT level */
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -37,6 +37,9 @@ config HAS_COMPAT
 config HAS_DEVICE_TREE
        bool
 
+config HAS_DIT # Data Independent Timing
+       bool
+
 config HAS_EX_TABLE
        bool
 
@@ -171,6 +174,18 @@ config SPECULATIVE_HARDEN_GUEST_ACCESS
 
 endmenu
 
+config DIT_DEFAULT
+       bool "Data Independent Timing default"
+       depends on HAS_DIT
+       help
+         Hardware often surfaces instructions the timing of which is dependent
+         on the data they process.  Some of these instructions may be used in
+         timing sensitive environments, e.g. cryptography.  When such
+         instructions exist, hardware may further surface a control allowing
+         to make the behavior of such instructions independent of the data
+         they act upon.  Choose the default here when no "dit" command line
+         option is present.
+
 config HYPFS
        bool "Hypervisor file system support"
        default y
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -463,6 +463,11 @@ static int __init cf_check param_init(vo
 __initcall(param_init);
 #endif
 
+#ifdef CONFIG_HAS_DIT
+bool __ro_after_init opt_dit = IS_ENABLED(CONFIG_DIT_DEFAULT);
+boolean_param("dit", opt_dit);
+#endif
+
 # define DO(fn) long do_##fn
 
 #endif
--- a/xen/include/xen/param.h
+++ b/xen/include/xen/param.h
@@ -184,6 +184,8 @@ extern struct param_hypfs __paramhypfs_s
     string_param(_name, _var); \
     string_runtime_only_param(_name, _var)
 
+extern bool opt_dit;
+
 static inline void no_config_param(const char *cfg, const char *param,
                                    const char *s, const char *e)
 {