[Xen-changelog] [xen stable-4.6] x86/AMD: Mitigations for GPZ SP4 - Speculative Store Bypass

[patchbot@xxxxxxx]

commit 4f0509d9bf143bd12f43319345c26c235042edb3
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue May 29 11:08:10 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 29 11:08:10 2018 +0200

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 8c0e338086f060eba31d37b83fbdb883928aa085
    master date: 2018-05-21 14:20:06 +0100
---
 docs/misc/xen-command-line.markdown |  7 ++++++-
 xen/arch/x86/cpu/amd.c              | 20 ++++++++++++++++++++
 xen/arch/x86/spec_ctrl.c            |  3 +++
 xen/include/asm-x86/spec_ctrl.h     |  1 +
 4 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 903eab7c9f..e2f4ae35d5 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1399,7 +1399,7 @@ 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,
@@ -1443,6 +1443,11 @@ 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 9d27e2afcb..efd943ae13 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -10,6 +10,7 @@
 #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>
 
@@ -496,6 +497,25 @@ static void __devinit 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);
+               }
+       }
+
        switch(c->x86)
        {
        case 0xf ... 0x17:
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 582d5714ca..ae9f830b51 100644
--- 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_t __read_mostly opt_ibpb = 1;
+bool_t __read_mostly opt_ssbd = 0;
 
 bool_t __initdata bsp_delay_spec_ctrl;
 uint8_t __read_mostly default_xen_spec_ctrl;
@@ -164,6 +165,8 @@ static int __init parse_spec_ctrl(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 d36f0e92a8..dd084d2d4d 100644
--- 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_t opt_ibpb;
+extern bool_t opt_ssbd;
 
 extern bool_t bsp_delay_spec_ctrl;
 extern uint8_t default_xen_spec_ctrl;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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