[Xen-devel] [PATCH SpectreV1+L1TF v6 4/9] spec: add l1tf-barrier

[Norbert Manthey <nmanthey@xxxxxxxxx>]

To control the runtime behavior on L1TF vulnerable platforms better, the
command line option l1tf-barrier is introduced. This option controls
whether on vulnerable x86 platforms the lfence instruction is used to
prevent speculative execution from bypassing the evaluation of
conditionals that are protected with the evaluate_nospec macro.

By now, Xen is capable of identifying L1TF vulnerable hardware. However,
this information cannot be used for alternative patching, as a CPU feature
is required. To control alternative patching with the command line option,
a new x86 feature "X86_FEATURE_SC_L1TF_VULN" is introduced. This feature
is used to patch the lfence instruction into the arch_barrier_nospec_true
function. The feature is enabled only if L1TF vulnerable hardware is
detected and the command line option does not prevent using this feature.

The status of hyperthreading is not considered when automatically enabling
adding the lfence instruction, because platforms without hyperthreading
can still be vulnerable to L1TF in case the L1 cache is not flushed
properly.

Signed-off-by: Norbert Manthey <nmanthey@xxxxxxxxx>

---

Notes:
  v6: Move disabling l1tf-barrier into spec-ctrl=no
      Use gap in existing flags
      Force barrier based on commandline, independently of L1TF detection

 docs/misc/xen-command-line.pandoc | 14 ++++++++++----
 xen/arch/x86/spec_ctrl.c          | 17 +++++++++++++++--
 xen/include/asm-x86/cpufeatures.h |  1 +
 xen/include/asm-x86/spec_ctrl.h   |  1 +
 4 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -483,9 +483,9 @@ accounting for hardware capabilities as enumerated via 
CPUID.
 
 Currently accepted:
 
-The Speculation Control hardware features `ibrsb`, `stibp`, `ibpb`,
-`l1d-flush` and `ssbd` are used by default if available and applicable.  They 
can
-be ignored, e.g. `no-ibrsb`, at which point Xen won't use them itself, and
+The Speculation Control hardware features `ibrsb`, `stibp`, `ibpb`, 
`l1d-flush`,
+`l1tf-barrier` and `ssbd` are used by default if available and applicable.  
They
+can be ignored, e.g. `no-ibrsb`, at which point Xen won't use them itself, and
 won't offer them to guests.
 
 ### cpuid_mask_cpu
@@ -1896,7 +1896,7 @@ By default SSBD will be mitigated at runtime (i.e 
`ssbd=runtime`).
 ### spec-ctrl (x86)
 > `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb}=<bool>,
 >              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,eager-fpu,
->              l1d-flush}=<bool> ]`
+>              l1d-flush,l1tf-barrier}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -1962,6 +1962,12 @@ 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 vulnerable to L1TF, the `l1tf-barrier=` option can be used to force
+or prevent Xen from protecting evaluations inside the hypervisor with a barrier
+instruction to not load potentially secret information into L1 cache.  By
+default, Xen will enable this mitigation on hardware believed to be vulnerable
+to L1TF.
+
 ### sync_console
 > `= <boolean>`
 
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -21,6 +21,7 @@
 #include <xen/lib.h>
 #include <xen/warning.h>
 
+#include <asm/cpuid.h>
 #include <asm/microcode.h>
 #include <asm/msr.h>
 #include <asm/processor.h>
@@ -50,6 +51,7 @@ bool __read_mostly opt_ibpb = true;
 bool __read_mostly opt_ssbd = false;
 int8_t __read_mostly opt_eager_fpu = -1;
 int8_t __read_mostly opt_l1d_flush = -1;
+int8_t __read_mostly opt_l1tf_barrier = -1;
 
 bool __initdata bsp_delay_spec_ctrl;
 uint8_t __read_mostly default_xen_spec_ctrl;
@@ -91,6 +93,8 @@ static int __init parse_spec_ctrl(const char *s)
             if ( opt_pv_l1tf_domu < 0 )
                 opt_pv_l1tf_domu = 0;
 
+            opt_l1tf_barrier = 0;
+
         disable_common:
             opt_rsb_pv = false;
             opt_rsb_hvm = false;
@@ -157,6 +161,8 @@ 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("l1tf-barrier", s, ss)) >= 0 )
+            opt_l1tf_barrier = val;
         else
             rc = -EINVAL;
 
@@ -248,7 +254,7 @@ 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, Other:%s%s\n",
+    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s, Other:%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
@@ -258,7 +264,8 @@ static void __init print_details(enum ind_thunk thunk, 
uint64_t caps)
            !boot_cpu_has(X86_FEATURE_SSBD)           ? "" :
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
            opt_ibpb                                  ? " IBPB"  : "",
-           opt_l1d_flush                             ? " L1D_FLUSH" : "");
+           opt_l1d_flush                             ? " L1D_FLUSH" : "",
+           opt_l1tf_barrier                          ? " L1TF_BARRIER" : "");
 
     /* L1TF diagnostics, printed if vulnerable or PV shadowing is in use. */
     if ( cpu_has_bug_l1tf || opt_pv_l1tf_hwdom || opt_pv_l1tf_domu )
@@ -842,6 +849,12 @@ void __init init_speculation_mitigations(void)
     else if ( opt_l1d_flush == -1 )
         opt_l1d_flush = cpu_has_bug_l1tf && !(caps & ARCH_CAPS_SKIP_L1DFL);
 
+    /* By default, enable L1TF_VULN on L1TF-vulnerable hardware */
+    if ( opt_l1tf_barrier == -1 )
+        opt_l1tf_barrier = cpu_has_bug_l1tf;
+    if ( opt_l1tf_barrier > 0)
+        setup_force_cpu_cap(X86_FEATURE_SC_L1TF_VULN);
+
     /*
      * We do not disable HT by default on affected hardware.
      *
diff --git a/xen/include/asm-x86/cpufeatures.h 
b/xen/include/asm-x86/cpufeatures.h
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -25,6 +25,7 @@ XEN_CPUFEATURE(XEN_SMAP,        (FSCAPINTS+0)*32+11) /* SMAP 
gets used by Xen it
 XEN_CPUFEATURE(LFENCE_DISPATCH, (FSCAPINTS+0)*32+12) /* lfence set as Dispatch 
Serialising */
 XEN_CPUFEATURE(IND_THUNK_LFENCE,(FSCAPINTS+0)*32+13) /* Use IND_THUNK_LFENCE */
 XEN_CPUFEATURE(IND_THUNK_JMP,   (FSCAPINTS+0)*32+14) /* Use IND_THUNK_JMP */
+XEN_CPUFEATURE(SC_L1TF_VULN,    (FSCAPINTS+0)*32+15) /* L1TF protection 
required */
 XEN_CPUFEATURE(SC_MSR_PV,       (FSCAPINTS+0)*32+16) /* MSR_SPEC_CTRL used by 
Xen for PV */
 XEN_CPUFEATURE(SC_MSR_HVM,      (FSCAPINTS+0)*32+17) /* MSR_SPEC_CTRL used by 
Xen for HVM */
 XEN_CPUFEATURE(SC_RSB_PV,       (FSCAPINTS+0)*32+18) /* RSB overwrite needed 
for PV */
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -37,6 +37,7 @@ extern bool opt_ibpb;
 extern bool opt_ssbd;
 extern int8_t opt_eager_fpu;
 extern int8_t opt_l1d_flush;
+extern int8_t opt_l1tf_barrier;
 
 extern bool bsp_delay_spec_ctrl;
 extern uint8_t default_xen_spec_ctrl;
-- 
2.7.4




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