Subject: x86/spec-ctrl: Support for SRSO_U/S_NO and SRSO_MSR_FIX
From: Andrew Cooper <[email-masked]>

AMD have updated the SRSO whitepaper[1] with further information.  These
features exist on AMD Zen5 CPUs and are necessary for Xen to use.

The two features are in principle unrelated:

 * SRSO_U/S_NO is an enumeration saying that SRSO attacks can't cross the
   User(CPL3) / Supervisor(CPL<3) boundary.  i.e. Xen don't need to use
   IBPB-on-entry for PV64.  PV32 guests are explicitly unsupported for
   speculative issues, and excluded from consideration for simplicity.

 * SRSO_MSR_FIX is an enumeration identifying that the BP_SPEC_REDUCE bit is
   available in MSR_BP_CFG.  When set, SRSO attacks can't cross the host/guest
   boundary.  i.e. Xen don't need to use IBPB-on-entry for HVM.

Extend ibpb_calculations() to account for these when calculating
opt_ibpb_entry_{pv,hvm} defaults.  Add a `bp-spec-reduce=<bool>` option to
control the use of BP_SPEC_REDUCE, with it active by default.

Because MSR_BP_CFG is core-scoped with a race condition updating it, repurpose
amd_check_erratum_1485() into amd_check_bp_cfg() and calculate all updates at
once.

Xen also needs to to advertise SRSO_U/S_NO to guests to allow the guest kernel
to skip SRSO mitigations too:

 * This is trivial for HVM guests.  It is also is accurate for PV32 guests
   too, but we have already excluded them from consideration, and do so again
   here to simplify the policy logic.

 * As written, SRSO_U/S_NO does not help for the PV64 user->kernel boundary.
   However, after discussing with AMD, an implementation detail of having
   BP_SPEC_REDUCE active causes the PV64 user->kernel boundary to have the
   property described by SRSO_U/S_NO, so we can advertise SRSO_U/S_NO to
   guests when the BP_SPEC_REDUCE precondition is met.

Finally, fix a typo in the SRSO_NO's comment.

[1] https://www.amd.com/content/dam/amd/en/documents/corporate/cr/speculative-return-stack-overflow-whitepaper.pdf
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Roger Pau Monné <[email-masked]>
(cherry picked from commit a1746cd4434dd27ca2da8430dfb10edc76264bb3)

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 4f2907eb67f2..ad8027732665 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2330,7 +2330,7 @@ By default SSBD will be mitigated at runtime (i.e `ssbd=runtime`).
 >              {ibrs,ibpb,ssbd,psfd,
 >              eager-fpu,l1d-flush,branch-harden,srb-lock,
 >              unpriv-mmio,gds-mit,div-scrub,lock-harden,
->              bhi-dis-s}=<bool> ]`
+>              bhi-dis-s,bp-spec-reduce}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -2479,6 +2479,13 @@ boolean can be used to force or prevent Xen from using speculation barriers to
 protect lock critical regions.  This mitigation won't be engaged by default,
 and needs to be explicitly enabled on the command line.
 
+On hardware supporting SRSO_MSR_FIX, the `bp-spec-reduce=` option can be used
+to force or prevent Xen from using MSR_BP_CFG.BP_SPEC_REDUCE to mitigate the
+SRSO (Speculative Return Stack Overflow) vulnerability.  Xen will use
+bp-spec-reduce when available, as it is preferable to using `ibpb-entry=hvm`
+to mitigate SRSO for HVM guests, and because it is a prerequisite to advertise
+SRSO_U/S_NO to PV guests.
+
 ### sync_console
 > `= <boolean>`
 
diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index f259c77435ea..9224c36d81b5 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -14,6 +14,7 @@
 #include <asm/msr-index.h>
 #include <asm/paging.h>
 #include <asm/setup.h>
+#include <asm/spec_ctrl.h>
 #include <asm/xstate.h>
 
 struct cpu_policy __ro_after_init     raw_cpu_policy;
@@ -650,6 +651,26 @@ static void __init calculate_pv_max_policy(void)
         __clear_bit(X86_FEATURE_IBRS, fs);
     }
 
+    /*
+     * SRSO_U/S_NO means that the CPU is not vulnerable to SRSO attacks across
+     * the User (CPL3) / Supervisor (CPL<3) boundary.
+     *
+     * PV32 guests are unsupported for speculative issues, and excluded from
+     * consideration for simplicity.
+     *
+     * The PV64 user/kernel boundary is CPL3 on both sides, so SRSO_U/S_NO
+     * won't convey the meaning that a PV kernel expects.
+     *
+     * After discussions with AMD, an implementation detail of having
+     * BP_SPEC_REDUCE active causes the PV64 user/kernel boundary to have a
+     * property compatible with the meaning of SRSO_U/S_NO.
+     *
+     * If BP_SPEC_REDUCE isn't active, remove SRSO_U/S_NO from the PV max
+     * policy, which will cause it to filter out of PV default too.
+     */
+    if ( !boot_cpu_has(X86_FEATURE_SRSO_MSR_FIX) || !opt_bp_spec_reduce )
+        __clear_bit(X86_FEATURE_SRSO_US_NO, fs);
+
     guest_common_max_feature_adjustments(fs);
     guest_common_feature_adjustments(fs);
 
diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 3312d896dd90..8c4e50de4536 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -1018,16 +1018,33 @@ static void amd_init_fp_cfg(const struct cpuinfo_x86 *c)
     wrmsrl(MSR_AMD64_FP_CFG, val | new);
 }
 
-static void amd_check_erratum_1485(void)
+static void amd_check_bp_cfg(void)
 {
-	uint64_t val, chickenbit = (1 << 5);
+	uint64_t val, new = 0;
 
-	if (cpu_has_hypervisor || boot_cpu_data.x86 != 0x19 || !is_zen4_uarch())
+	/*
+	 * AMD Erratum #1485.  Set bit 5, as instructed.
+	 */
+	if (!cpu_has_hypervisor && boot_cpu_data.x86 == 0x19 && is_zen4_uarch())
+		new |= (1 << 5);
+
+	/*
+	 * On hardware supporting SRSO_MSR_FIX, activate BP_SPEC_REDUCE by
+	 * default.  This lets us do two things:
+	 *
+	 * 1) Avoid IBPB-on-entry to mitigate SRSO attacks from HVM guests.
+	 * 2) Advertise SRSO_US_NO to PV guests.
+	 */
+	if (boot_cpu_has(X86_FEATURE_SRSO_MSR_FIX) && opt_bp_spec_reduce)
+		new |= BP_CFG_SPEC_REDUCE;
+
+	/* Avoid reading BP_CFG if we don't intend to change anything. */
+	if (!new)
 		return;
 
 	rdmsrl(MSR_AMD64_BP_CFG, val);
 
-	if (val & chickenbit)
+	if ((val & new) == new)
 		return;
 
 	/*
@@ -1036,7 +1053,7 @@ static void amd_check_erratum_1485(void)
 	 * same time before the chickenbit is set. It's benign because the
 	 * value being written is the same on both.
 	 */
-	wrmsrl(MSR_AMD64_BP_CFG, val | chickenbit);
+	wrmsrl(MSR_AMD64_BP_CFG, val | new);
 }
 
 static void cf_check init_amd(struct cpuinfo_x86 *c)
@@ -1314,7 +1331,7 @@ static void cf_check init_amd(struct cpuinfo_x86 *c)
 		disable_c1_ramping();
 
 	amd_check_zenbleed();
-	amd_check_erratum_1485();
+	amd_check_bp_cfg();
 
 	if (fam17_c6_disabled)
 		fam17_disable_c6(NULL);
diff --git a/xen/arch/x86/include/asm/msr-index.h b/xen/arch/x86/include/asm/msr-index.h
index bed0ab7b213c..db3db08a54a0 100644
--- a/xen/arch/x86/include/asm/msr-index.h
+++ b/xen/arch/x86/include/asm/msr-index.h
@@ -391,6 +391,7 @@
 #define AMD64_DE_CFG_LFENCE_SERIALISE	(_AC(1, ULL) << 1)
 #define MSR_AMD64_EX_CFG		0xc001102c
 #define MSR_AMD64_BP_CFG		0xc001102e
+#define  BP_CFG_SPEC_REDUCE		(_AC(1, ULL) <<  4)
 #define MSR_AMD64_DE_CFG2		0xc00110e3
 
 #define MSR_AMD64_DR0_ADDRESS_MASK	0xc0011027
diff --git a/xen/arch/x86/include/asm/spec_ctrl.h b/xen/arch/x86/include/asm/spec_ctrl.h
index 60844b755dff..b7349ac89852 100644
--- a/xen/arch/x86/include/asm/spec_ctrl.h
+++ b/xen/arch/x86/include/asm/spec_ctrl.h
@@ -102,6 +102,7 @@ extern int8_t opt_xpti_hwdom, opt_xpti_domu;
 
 extern bool cpu_has_bug_l1tf;
 extern int8_t opt_pv_l1tf_hwdom, opt_pv_l1tf_domu;
+extern bool opt_bp_spec_reduce;
 
 /*
  * The L1D address mask, which might be wider than reported in CPUID, and the
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 5005f0acdde9..b71fe0a4ece1 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -95,6 +95,7 @@ static bool __initdata opt_unpriv_mmio;
 static bool __ro_after_init opt_verw_mmio;
 static int8_t __initdata opt_gds_mit = -1;
 static int8_t __initdata opt_div_scrub = -1;
+bool __ro_after_init opt_bp_spec_reduce = true;
 
 static int __init cf_check parse_spec_ctrl(const char *s)
 {
@@ -155,6 +156,7 @@ static int __init cf_check parse_spec_ctrl(const char *s)
             opt_unpriv_mmio = false;
             opt_gds_mit = 0;
             opt_div_scrub = 0;
+            opt_bp_spec_reduce = false;
         }
         else if ( val > 0 )
             rc = -EINVAL;
@@ -375,6 +377,8 @@ static int __init cf_check parse_spec_ctrl(const char *s)
             opt_gds_mit = val;
         else if ( (val = parse_boolean("div-scrub", s, ss)) >= 0 )
             opt_div_scrub = val;
+        else if ( (val = parse_boolean("bp-spec-reduce", s, ss)) >= 0 )
+            opt_bp_spec_reduce = val;
         else
             rc = -EINVAL;
 
@@ -517,7 +521,7 @@ static void __init print_details(enum ind_thunk thunk)
      * Hardware read-only information, stating immunity to certain issues, or
      * suggestions of which mitigation to use.
      */
-    printk("  Hardware hints:%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s\n",
+    printk("  Hardware hints:%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s\n",
            (caps & ARCH_CAPS_RDCL_NO)                        ? " RDCL_NO"        : "",
            (caps & ARCH_CAPS_EIBRS)                          ? " EIBRS"          : "",
            (caps & ARCH_CAPS_RSBA)                           ? " RSBA"           : "",
@@ -542,11 +546,12 @@ static void __init print_details(enum ind_thunk thunk)
            (e8b  & cpufeat_mask(X86_FEATURE_IBPB_RET))       ? " IBPB_RET"       : "",
            (e21a & cpufeat_mask(X86_FEATURE_IBPB_BRTYPE))    ? " IBPB_BRTYPE"    : "",
            (e21a & cpufeat_mask(X86_FEATURE_SRSO_NO))        ? " SRSO_NO"        : "",
+           (e21a & cpufeat_mask(X86_FEATURE_SRSO_US_NO))     ? " SRSO_US_NO"     : "",
            (e21c & cpufeat_mask(X86_FEATURE_TSA_SQ_NO))      ? " TSA_SQ_NO"      : "",
            (e21c & cpufeat_mask(X86_FEATURE_TSA_L1_NO))      ? " TSA_L1_NO"      : "");
 
     /* Hardware features which need driving to mitigate issues. */
-    printk("  Hardware features:%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s\n",
+    printk("  Hardware features:%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s%s\n",
            (e8b  & cpufeat_mask(X86_FEATURE_IBPB)) ||
            (_7d0 & cpufeat_mask(X86_FEATURE_IBRSB))          ? " IBPB"           : "",
            (e8b  & cpufeat_mask(X86_FEATURE_IBRS)) ||
@@ -566,6 +571,7 @@ static void __init print_details(enum ind_thunk thunk)
            (caps & ARCH_CAPS_GDS_CTRL)                       ? " GDS_CTRL"       : "",
            (caps & ARCH_CAPS_RFDS_CLEAR)                     ? " RFDS_CLEAR"     : "",
            (e21a & cpufeat_mask(X86_FEATURE_SBPB))           ? " SBPB"           : "",
+           (e21a & cpufeat_mask(X86_FEATURE_SRSO_MSR_FIX))   ? " SRSO_MSR_FIX"   : "",
            (e21a & cpufeat_mask(X86_FEATURE_VERW_CLEAR))     ? " VERW_CLEAR"     : "");
 
     /* Compiled-in support which pertains to mitigations. */
@@ -1138,7 +1144,7 @@ static void __init div_calculations(bool hw_smt_enabled)
 
 static void __init ibpb_calculations(void)
 {
-    bool def_ibpb_entry = false;
+    bool def_ibpb_entry_pv = false, def_ibpb_entry_hvm = false;
 
     /* Check we have hardware IBPB support before using it... */
     if ( !boot_cpu_has(X86_FEATURE_IBRSB) && !boot_cpu_has(X86_FEATURE_IBPB) )
@@ -1163,22 +1169,43 @@ static void __init ibpb_calculations(void)
          * Confusion.  Mitigate with IBPB-on-entry.
          */
         if ( !boot_cpu_has(X86_FEATURE_BTC_NO) )
-            def_ibpb_entry = true;
+            def_ibpb_entry_pv = def_ibpb_entry_hvm = true;
 
         /*
-         * Further to BTC, Zen3/4 CPUs suffer from Speculative Return Stack
-         * Overflow in most configurations.  Mitigate with IBPB-on-entry if we
-         * have the microcode that makes this an effective option.
+         * In addition to BTC, Zen3 and later CPUs suffer from Speculative
+         * Return Stack Overflow in most configurations.  If we have microcode
+         * that makes IBPB-on-entry an effective mitigation, see about using
+         * it.
          */
         if ( !boot_cpu_has(X86_FEATURE_SRSO_NO) &&
              boot_cpu_has(X86_FEATURE_IBPB_BRTYPE) )
-            def_ibpb_entry = true;
+        {
+            /*
+             * SRSO_U/S_NO is a subset of SRSO_NO, identifying that SRSO isn't
+             * possible across the User (CPL3) / Supervisor (CPL<3) boundary.
+             *
+             * Ignoring PV32 (not security supported for speculative issues),
+             * this means we only need to use IBPB-on-entry for PV guests on
+             * hardware which doesn't enumerate SRSO_US_NO.
+             */
+            if ( !boot_cpu_has(X86_FEATURE_SRSO_US_NO) )
+                def_ibpb_entry_pv = true;
+
+            /*
+             * SRSO_MSR_FIX enumerates that we can use MSR_BP_CFG.SPEC_REDUCE
+             * to mitigate SRSO across the host/guest boundary.  We only need
+             * to use IBPB-on-entry for HVM guests if we haven't enabled this
+             * control.
+             */
+            if ( !boot_cpu_has(X86_FEATURE_SRSO_MSR_FIX) || !opt_bp_spec_reduce )
+                def_ibpb_entry_hvm = true;
+        }
     }
 
     if ( opt_ibpb_entry_pv == -1 )
-        opt_ibpb_entry_pv = IS_ENABLED(CONFIG_PV) && def_ibpb_entry;
+        opt_ibpb_entry_pv = IS_ENABLED(CONFIG_PV) && def_ibpb_entry_pv;
     if ( opt_ibpb_entry_hvm == -1 )
-        opt_ibpb_entry_hvm = IS_ENABLED(CONFIG_HVM) && def_ibpb_entry;
+        opt_ibpb_entry_hvm = IS_ENABLED(CONFIG_HVM) && def_ibpb_entry_hvm;
 
     if ( opt_ibpb_entry_pv )
     {
diff --git a/xen/include/public/arch-x86/cpufeatureset.h b/xen/include/public/arch-x86/cpufeatureset.h
index 42db132b4c2f..12ca3efc5ce9 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -289,7 +289,9 @@ XEN_CPUFEATURE(NSCB,               11*32+ 6) /*A  Null Selector Clears Base (and
 XEN_CPUFEATURE(AUTO_IBRS,          11*32+ 8) /*   Automatic IBRS */
 XEN_CPUFEATURE(SBPB,               11*32+27) /*A  Selective Branch Predictor Barrier */
 XEN_CPUFEATURE(IBPB_BRTYPE,        11*32+28) /*A  IBPB flushes Branch Type predictions too */
-XEN_CPUFEATURE(SRSO_NO,            11*32+29) /*A  Hardware not vulenrable to Speculative Return Stack Overflow */
+XEN_CPUFEATURE(SRSO_NO,            11*32+29) /*A  Hardware not vulnerable to Speculative Return Stack Overflow */
+XEN_CPUFEATURE(SRSO_US_NO,         11*32+30) /*A! Hardware not vulnerable to SRSO across the User/Supervisor boundary */
+XEN_CPUFEATURE(SRSO_MSR_FIX,       11*32+31) /*   MSR_BP_CFG.BP_SPEC_REDUCE available */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1.ebx, word 12 */
 XEN_CPUFEATURE(INTEL_PPIN,         12*32+ 0) /*   Protected Processor Inventory Number */