[xen master] x86/spec-ctrl: Support BHI_DIS_S in order to mitigate BHI

[patchbot@xxxxxxx]

commit 62a1106415c5e8a49b45147ca84d54a58d471343
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Mar 26 19:01:37 2024 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Apr 9 16:37:30 2024 +0100

    x86/spec-ctrl: Support BHI_DIS_S in order to mitigate BHI
    
    Introduce a "bhi-dis-s" boolean to match the other options we have for
    MSR_SPEC_CTRL values.  Also introduce bhi_calculations().
    
    Use BHI_DIS_S whenever possible.
    
    Guests which are levelled to be migration compatible with older CPUs can't 
see
    BHI_DIS_S, and Xen must fill in the difference to make the guest safe.  Use
    the virt MSR_SPEC_CTRL infrastructure to force BHI_DIS_S behind the guest's
    back.
    
    This is part of XSA-456 / CVE-2024-2201.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 docs/misc/xen-command-line.pandoc    |  8 +++++++-
 xen/arch/x86/hvm/vmx/vmx.c           | 17 +++++++++++++++++
 xen/arch/x86/include/asm/spec_ctrl.h |  1 +
 xen/arch/x86/spec_ctrl.c             | 24 +++++++++++++++++++++++-
 4 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 14cafb462a..b87e7ebc35 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2377,7 +2377,8 @@ By default SSBD will be mitigated at runtime (i.e 
`ssbd=runtime`).
 >              {msr-sc,rsb,verw,ibpb-entry}=<bool>|{pv,hvm}=<bool>,
 >              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,psfd,
 >              eager-fpu,l1d-flush,branch-harden,srb-lock,
->              unpriv-mmio,gds-mit,div-scrub,lock-harden}=<bool> ]`
+>              unpriv-mmio,gds-mit,div-scrub,lock-harden,
+>              bhi-dis-s}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -2458,6 +2459,11 @@ option can be used to force or prevent Xen using the 
feature itself.  By
 default, Xen will not use PSFD.  PSFD is implied by SSBD, and SSBD is off by
 default.
 
+On hardware supporting BHI_DIS_S (Branch History Injection Disable
+Supervisor), the `bhi-dis-s=` option can be used to force or prevent Xen using
+the feature itself.  By default Xen will use BHI_DIS_S on hardware susceptible
+to Branch History Injection.
+
 On hardware supporting IBPB (Indirect Branch Prediction Barrier), the `ibpb=`
 option can be used to force (the default) or prevent Xen from issuing branch
 prediction barriers on vcpu context switches.
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index c886331006..0935762378 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -48,6 +48,7 @@
 #include <asm/mce.h>
 #include <asm/monitor.h>
 #include <asm/prot-key.h>
+#include <asm/spec_ctrl.h>
 #include <public/arch-x86/cpuid.h>
 
 static bool __initdata opt_force_ept;
@@ -842,6 +843,22 @@ static void cf_check vmx_cpuid_policy_changed(struct vcpu 
*v)
             vmx_del_msr(v, MSR_SPEC_CTRL, VMX_MSR_GUEST);
     }
 
+    if ( cpu_has_vmx_virt_spec_ctrl )
+    {
+        /*
+         * If we're on BHI_DIS_S capable hardware, the short loop sequence is
+         * not sufficient to mitigate Native-BHI.  If the VM can't see it
+         * (i.e. it's levelled with older hardware), force it behind the
+         * guests back for safey.
+         *
+         * Because there's not a real Host/Guest split of the MSR_SPEC_CTRL
+         * value, this only works as expected when Xen is using BHI_DIS_S too.
+         */
+        bool force_bhi_dis_s = opt_bhi_dis_s && !cp->feat.bhi_ctrl;
+
+        __vmwrite(SPEC_CTRL_MASK, force_bhi_dis_s ? SPEC_CTRL_BHI_DIS_S : 0);
+    }
+
     /* MSR_PRED_CMD is safe to pass through if the guest knows about it. */
     if ( cp->feat.ibrsb || cp->extd.ibpb )
         vmx_clear_msr_intercept(v, MSR_PRED_CMD,  VMX_MSR_RW);
diff --git a/xen/arch/x86/include/asm/spec_ctrl.h 
b/xen/arch/x86/include/asm/spec_ctrl.h
index 95dd081f30..b2d2c25842 100644
--- a/xen/arch/x86/include/asm/spec_ctrl.h
+++ b/xen/arch/x86/include/asm/spec_ctrl.h
@@ -77,6 +77,7 @@ static always_inline void spec_ctrl_new_guest_context(void)
 
 extern int8_t opt_ibpb_ctxt_switch;
 extern bool opt_ssbd;
+extern int8_t opt_bhi_dis_s;
 extern int8_t opt_eager_fpu;
 extern int8_t opt_l1d_flush;
 
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index ed52ce3cc5..a5096ab4fb 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -47,6 +47,7 @@ static int8_t __initdata opt_ibrs = -1;
 static int8_t __initdata opt_stibp = -1;
 bool __ro_after_init opt_ssbd;
 static int8_t __initdata opt_psfd = -1;
+int8_t __ro_after_init opt_bhi_dis_s = -1;
 
 int8_t __ro_after_init opt_ibpb_ctxt_switch = -1;
 int8_t __ro_after_init opt_eager_fpu = -1;
@@ -269,6 +270,8 @@ static int __init cf_check parse_spec_ctrl(const char *s)
             opt_ssbd = val;
         else if ( (val = parse_boolean("psfd", s, ss)) >= 0 )
             opt_psfd = val;
+        else if ( (val = parse_boolean("bhi-dis-s", s, ss)) >= 0 )
+            opt_bhi_dis_s = val;
 
         /* Misc settings. */
         else if ( (val = parse_boolean("ibpb", s, ss)) >= 0 )
@@ -524,7 +527,7 @@ static void __init print_details(enum ind_thunk thunk)
                "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
-    printk("  Xen settings: %s%sSPEC_CTRL: %s%s%s%s%s, Other:%s%s%s%s%s%s%s\n",
+    printk("  Xen settings: %s%sSPEC_CTRL: %s%s%s%s%s%s, 
Other:%s%s%s%s%s%s%s\n",
            thunk != THUNK_NONE      ? "BTI-Thunk: " : "",
            thunk == THUNK_NONE      ? "" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE, " :
@@ -542,6 +545,8 @@ static void __init print_details(enum ind_thunk thunk)
            (!boot_cpu_has(X86_FEATURE_PSFD) &&
             !boot_cpu_has(X86_FEATURE_INTEL_PSFD))   ? "" :
            (default_xen_spec_ctrl & SPEC_CTRL_PSFD)  ? " PSFD+" : " PSFD-",
+           !boot_cpu_has(X86_FEATURE_BHI_CTRL)       ? "" :
+           (default_xen_spec_ctrl & SPEC_CTRL_BHI_DIS_S) ? " BHI_DIS_S+" : " 
BHI_DIS_S-",
            !(caps & ARCH_CAPS_TSX_CTRL)              ? "" :
            (opt_tsx & 1)                             ? " TSX+" : " TSX-",
            !cpu_has_srbds_ctrl                       ? "" :
@@ -1596,6 +1601,21 @@ static void __init gds_calculations(void)
     }
 }
 
+/*
+ * 
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/branch-history-injection.html
+ */
+static void __init bhi_calculations(void)
+{
+    if ( opt_bhi_dis_s == -1 )
+        opt_bhi_dis_s = !boot_cpu_has(X86_FEATURE_BHI_NO);
+
+    if ( !boot_cpu_has(X86_FEATURE_BHI_CTRL) )
+        opt_bhi_dis_s = false;
+
+    if ( opt_bhi_dis_s )
+        default_xen_spec_ctrl |= SPEC_CTRL_BHI_DIS_S;
+}
+
 void spec_ctrl_init_domain(struct domain *d)
 {
     bool pv = is_pv_domain(d);
@@ -2140,6 +2160,8 @@ void __init init_speculation_mitigations(void)
 
     gds_calculations();
 
+    bhi_calculations();
+
     print_details(thunk);
 
     /*
--
generated by git-patchbot for /home/xen/git/xen.git#master