x86: support data operand independent timing mode

[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 <[email protected]>
Signed-off-by: Jan Beulich <[email protected]>
Acked-by: Roger Pau Monné <[email protected]>
Release-acked-by: Henry Wang <[email protected]>

CHANGELOG.md

@@ -33,6 +33,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    nodes using a device tree overlay binary (.dtbo).
  - Introduce two new hypercalls to map the vCPU runstate and time areas by
    physical rather than linear/virtual addresses.
+ - On x86, support for enforcing system-wide operation in Data Operand
+   Independent Timing Mode.
 
 ### Removed
  - On x86, the "pku" command line option has been removed.  It has never

docs/misc/xen-command-line.pandoc

@@ -788,6 +788,17 @@ Specify the size of the console debug trace buffer. By specifying `cpu:`
 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/Intel)
+> `= <boolean>`
+
+> Default: `CONFIG_DIT_DEFAULT`
+
+Specify whether Xen and guests should operate in Data Independent Timing
+mode (Intel calls this DOITM, Data Operand Independent Timing Mode). Note
+that enabling this option cannot guarantee anything beyond what underlying
+hardware guarantees (with, where available and known to Xen, respective
+tweaks applied).
+
 ### dma_bits
 > `= <integer>`
 

xen/arch/x86/Kconfig

@@ -15,6 +15,7 @@ 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

xen/arch/x86/cpu/common.c

@@ -204,6 +204,28 @@ void ctxt_switch_levelling(const struct vcpu *next)
 		alternative_vcall(ctxt_switch_masking, next);
 }
 
+static void setup_doitm(void)
+{
+    uint64_t msr;
+
+    if ( !cpu_has_doitm )
+        return;
+
+    /*
+     * We don't currently enumerate DOITM to guests.  As a conseqeuence, guest
+     * kernels will believe they're safe even when they are not.
+     *
+     * For now, set it unilaterally.  This prevents otherwise-correct crypto
+     * code from becoming vulnerable to timing sidechannels.
+     */
+
+    rdmsrl(MSR_UARCH_MISC_CTRL, msr);
+    msr |= UARCH_CTRL_DOITM;
+    if ( !opt_dit )
+        msr &= ~UARCH_CTRL_DOITM;
+    wrmsrl(MSR_UARCH_MISC_CTRL, msr);
+}
+
 bool opt_cpu_info;
 boolean_param("cpuinfo", opt_cpu_info);
 
@@ -599,6 +621,8 @@ void identify_cpu(struct cpuinfo_x86 *c)
 
 		mtrr_bp_init();
 	}
+
+	setup_doitm();
 }
 
 /* leaf 0xb SMT level */

xen/arch/x86/include/asm/cpufeature.h

@@ -202,6 +202,7 @@ static inline bool boot_cpu_has(unsigned int feat)
 #define cpu_has_tsx_ctrl        boot_cpu_has(X86_FEATURE_TSX_CTRL)
 #define cpu_has_taa_no          boot_cpu_has(X86_FEATURE_TAA_NO)
 #define cpu_has_mcu_ctrl        boot_cpu_has(X86_FEATURE_MCU_CTRL)
+#define cpu_has_doitm           boot_cpu_has(X86_FEATURE_DOITM)
 #define cpu_has_fb_clear        boot_cpu_has(X86_FEATURE_FB_CLEAR)
 #define cpu_has_rrsba           boot_cpu_has(X86_FEATURE_RRSBA)
 #define cpu_has_gds_ctrl        boot_cpu_has(X86_FEATURE_GDS_CTRL)

xen/common/Kconfig

@@ -56,6 +56,9 @@ config HAS_COMPAT
 config HAS_DEVICE_TREE
 	bool
 
+config HAS_DIT # Data Independent Timing
+	bool
+
 config HAS_EX_TABLE
 	bool
 
@@ -187,6 +190,21 @@ 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.  Note the build time value can be overridden at runtime
+	  using the "dit" command line option.
+
+	  NB: Intel calls the feature DOITM (Data Operand Independent Timing
+	      Mode).
+
 config HYPFS
 	bool "Hypervisor file system support"
 	default y

xen/common/kernel.c

@@ -28,6 +28,11 @@ CHECK_feature_info;
 
 enum system_state system_state = SYS_STATE_early_boot;
 
+#ifdef CONFIG_HAS_DIT
+bool __ro_after_init opt_dit = IS_ENABLED(CONFIG_DIT_DEFAULT);
+boolean_param("dit", opt_dit);
+#endif
+
 static xen_commandline_t saved_cmdline;
 static const char __initconst opt_builtin_cmdline[] = CONFIG_CMDLINE;
 

xen/include/xen/param.h

@@ -184,6 +184,8 @@ extern struct param_hypfs __paramhypfs_start[], __paramhypfs_end[];
     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)
 {