x86/spec-ctrl: Mitigate the Special Register Buffer Data Sampling sidechannel

See patch documentation and comments.

This is part of XSA-320 / CVE-2020-0543

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 6a49b9a792)

docs/misc/xen-command-line.markdown

@@ -1858,7 +1858,7 @@ false disable the quirk workaround, which is also the default.
 ### spec-ctrl (x86)
 > `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb,md-clear}=<bool>,
 >              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,eager-fpu,
->              l1d-flush}=<bool> ]`
+>              l1d-flush,srb-lock}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -1930,6 +1930,12 @@ Irrespective of Xen's setting, the feature is virtualised for HVM guests to
 use.  By default, Xen will enable this mitigation on hardware believed to be
 vulnerable to L1TF.
 
+On hardware supporting SRBDS_CTRL, the `srb-lock=` option can be used to force
+or prevent Xen from protect the Special Register Buffer from leaking stale
+data. By default, Xen will enable this mitigation, except on parts where MDS
+is fixed and TAA is fixed/mitigated (in which case, there is believed to be no
+way for an attacker to obtain the stale data).
+
 ### sync\_console
 > `= <boolean>`
 

xen/arch/x86/acpi/power.c

@@ -266,6 +266,9 @@ static int enter_state(u32 state)
     ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
     spec_ctrl_exit_idle(ci);
 
+    if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+        wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
+
  done:
     spin_debug_enable();
     local_irq_restore(flags);

xen/arch/x86/smpboot.c

@@ -369,12 +369,14 @@ void start_secondary(void *unused)
         microcode_resume_cpu(cpu);
 
     /*
-     * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard
-     * any firmware settings.  Note: MSR_SPEC_CTRL may only become available
-     * after loading microcode.
+     * If any speculative control MSRs are available, apply Xen's default
+     * settings.  Note: These MSRs may only become available after loading
+     * microcode.
      */
     if ( boot_cpu_has(X86_FEATURE_IBRSB) )
         wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
+    if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+        wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
 
     tsx_init(); /* Needs microcode.  May change HLE/RTM feature bits. */
 

xen/arch/x86/spec_ctrl.c

@@ -63,6 +63,9 @@ static unsigned int __initdata l1d_maxphysaddr;
 static bool __initdata cpu_has_bug_msbds_only; /* => minimal HT impact. */
 static bool __initdata cpu_has_bug_mds; /* Any other M{LP,SB,FB}DS combination. */
 
+static int8_t __initdata opt_srb_lock = -1;
+uint64_t __read_mostly default_xen_mcu_opt_ctrl;
+
 static int __init parse_bti(const char *s)
 {
     const char *ss;
@@ -166,6 +169,7 @@ static int __init parse_spec_ctrl(const char *s)
             opt_ibpb = false;
             opt_ssbd = false;
             opt_l1d_flush = 0;
+            opt_srb_lock = 0;
         }
         else if ( val > 0 )
             rc = -EINVAL;
@@ -231,6 +235,8 @@ static int __init parse_spec_ctrl(const char *s)
             opt_eager_fpu = val;
         else if ( (val = parse_boolean("l1d-flush", s, ss)) >= 0 )
             opt_l1d_flush = val;
+        else if ( (val = parse_boolean("srb-lock", s, ss)) >= 0 )
+            opt_srb_lock = val;
         else
             rc = -EINVAL;
 
@@ -394,7 +400,7 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
                "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
-    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, Other:%s%s%s\n",
+    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, Other:%s%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
@@ -405,6 +411,8 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
            !(caps & ARCH_CAPS_TSX_CTRL)              ? "" :
            (opt_tsx & 1)                             ? " TSX+" : " TSX-",
+           !boot_cpu_has(X86_FEATURE_SRBDS_CTRL)     ? "" :
+           opt_srb_lock                              ? " SRB_LOCK+" : " SRB_LOCK-",
            opt_ibpb                                  ? " IBPB"  : "",
            opt_l1d_flush                             ? " L1D_FLUSH" : "",
            opt_md_clear_pv || opt_md_clear_hvm       ? " VERW"  : "");
@@ -1196,6 +1204,34 @@ void __init init_speculation_mitigations(void)
         tsx_init();
     }
 
+    /* Calculate suitable defaults for MSR_MCU_OPT_CTRL */
+    if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+    {
+        uint64_t val;
+
+        rdmsrl(MSR_MCU_OPT_CTRL, val);
+
+        /*
+         * On some SRBDS-affected hardware, it may be safe to relax srb-lock
+         * by default.
+         *
+         * On parts which enumerate MDS_NO and not TAA_NO, TSX is the only way
+         * to access the Fill Buffer.  If TSX isn't available (inc. SKU
+         * reasons on some models), or TSX is explicitly disabled, then there
+         * is no need for the extra overhead to protect RDRAND/RDSEED.
+         */
+        if ( opt_srb_lock == -1 &&
+             (caps & (ARCH_CAPS_MDS_NO|ARCH_CAPS_TAA_NO)) == ARCH_CAPS_MDS_NO &&
+             (!cpu_has_hle || ((caps & ARCH_CAPS_TSX_CTRL) && opt_tsx == 0)) )
+            opt_srb_lock = 0;
+
+        val &= ~MCU_OPT_CTRL_RNGDS_MITG_DIS;
+        if ( !opt_srb_lock )
+            val |= MCU_OPT_CTRL_RNGDS_MITG_DIS;
+
+        default_xen_mcu_opt_ctrl = val;
+    }
+
     print_details(thunk, caps);
 
     /*
@@ -1227,6 +1263,9 @@ void __init init_speculation_mitigations(void)
 
         wrmsrl(MSR_SPEC_CTRL, bsp_delay_spec_ctrl ? 0 : default_xen_spec_ctrl);
     }
+
+    if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+        wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
 }
 
 static void __init __maybe_unused build_assertions(void)

xen/include/asm-x86/spec_ctrl.h

@@ -46,6 +46,8 @@ extern int8_t opt_pv_l1tf_hwdom, opt_pv_l1tf_domu;
  */
 extern paddr_t l1tf_addr_mask, l1tf_safe_maddr;
 
+extern uint64_t default_xen_mcu_opt_ctrl;
+
 static inline void init_shadow_spec_ctrl_state(void)
 {
     struct cpu_info *info = get_cpu_info();