x86/AMD: Mitigations for GPZ SP4 - Speculative Store Bypass

AMD processors will execute loads and stores with the same base register in
program order, which is typically how a compiler emits code.

Therefore, by default no mitigating actions are taken, despite there being
corner cases which are vulnerable to the issue.

For performance testing, or for users with particularly sensitive workloads,
the `spec-ctrl=ssbd` command line option is available to force Xen to disable
Memory Disambiguation on applicable hardware.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index b6b1530a25e94843a36fdb8e00f85295ac27dff2..da570b4ab1a82abd6df1a8a0c1b996e2ab24fd98 100644 (file)
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1757,7 +1757,7 @@ false disable the quirk workaround, which is also the default.
 
 ### spec-ctrl (x86)
 > `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb}=<bool>,
->              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb}=<bool> ]`
+>              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -1801,6 +1801,11 @@ 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.
 
+On hardware supporting SSBD (Speculative Store Bypass Disable), the `ssbd=`
+option can be used to force or prevent Xen using the feature itself.  On AMD
+hardware, this is a global option applied at boot, and not virtualised for
+guest use.
+
 ### sync\_console
 > `= <boolean>`
 

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index fc9677f0200aaa51e71b045dd441050c87564928..458a3fe60c646fd487e7ed1545ac5955e89f5717 100644 (file)
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -9,6 +9,7 @@
 #include <asm/amd.h>
 #include <asm/hvm/support.h>
 #include <asm/setup.h> /* amd_init_cpu */
+#include <asm/spec_ctrl.h>
 #include <asm/acpi.h>
 #include <asm/apic.h>
 
@@ -594,6 +595,25 @@ static void init_amd(struct cpuinfo_x86 *c)
                                  c->x86_capability);
        }
 
+       /*
+        * If the user has explicitly chosen to disable Memory Disambiguation
+        * to mitigiate Speculative Store Bypass, poke the appropriate MSR.
+        */
+       if (opt_ssbd) {
+               int bit = -1;
+
+               switch (c->x86) {
+               case 0x15: bit = 54; break;
+               case 0x16: bit = 33; break;
+               case 0x17: bit = 10; break;
+               }
+
+               if (bit >= 0 && !rdmsr_safe(MSR_AMD64_LS_CFG, value)) {
+                       value |= 1ull << bit;
+                       wrmsr_safe(MSR_AMD64_LS_CFG, value);
+               }
+       }
+
        /* MFENCE stops RDTSC speculation */
        if (!cpu_has_lfence_dispatch)
                __set_bit(X86_FEATURE_MFENCE_RDTSC, c->x86_capability);

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 33733697eef083f1331a4d3f41a731feac47b907..20b215e31c113bfe18171afe040263e04de00465 100644 (file)
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -43,6 +43,7 @@ static enum ind_thunk {
 } opt_thunk __initdata = THUNK_DEFAULT;
 static int8_t __initdata opt_ibrs = -1;
 bool __read_mostly opt_ibpb = true;
+bool __read_mostly opt_ssbd = false;
 
 bool __initdata bsp_delay_spec_ctrl;
 uint8_t __read_mostly default_xen_spec_ctrl;
@@ -180,6 +181,8 @@ static int __init parse_spec_ctrl(const char *s)
             opt_ibrs = val;
         else if ( (val = parse_boolean("ibpb", s, ss)) >= 0 )
             opt_ibpb = val;
+        else if ( (val = parse_boolean("ssbd", s, ss)) >= 0 )
+            opt_ssbd = val;
         else
             rc = -EINVAL;
 

diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index 993b958cdfa8899a411693ac9033deabd3d921f7..91bed1b4764ef4d13d1813054229832200bb655f 100644 (file)
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -27,6 +27,7 @@
 void init_speculation_mitigations(void);
 
 extern bool opt_ibpb;
+extern bool opt_ssbd;
 
 extern bool bsp_delay_spec_ctrl;
 extern uint8_t default_xen_spec_ctrl;