x86/spec-ctrl: Introduce an option to control L1TF mitigation for PV guests

Shadowing a PV guest is only available when shadow paging is compiled in.
When shadow paging isn't available, guests can be crashed instead as
mitigation from Xen's point of view.

Ideally, dom0 would also be potentially-shadowed-by-default, but dom0 has
never been shadowed before, and there are some stability issues under
investigation.

This is part of XSA-273 / CVE-2018-3620.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 66a4e986819a86ba66ca2fe9d925e62a4fd30114)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 014476c57dd1d86934d08c4b9d615881220a769e..9a3cb3886950537fd4d029f8ff999644cd919b83 100644 (file)
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1475,6 +1475,27 @@ do; there may be other custom operating systems which do.  If you're
 certain you don't plan on having PV guests which use this feature,
 turning it off can reduce the attack surface.
 
+### pv-l1tf (x86)
+> `= List of [ <bool>, dom0=<bool>, domu=<bool> ]`
+
+> Default: `false` on believed-unaffected hardware.
+>          `domu`  on believed-affected hardware.
+
+Mitigations for L1TF / XSA-273 / CVE-2018-3620 for PV guests.
+
+For backwards compatibility, we may not alter an architecturally-legitimate
+pagetable entry a PV guest chooses to write.  We can however force such a
+guest into shadow mode so that Xen controls the PTEs which are reachable by
+the CPU pagewalk.
+
+Shadowing is performed at the point where a PV guest first tries to write an
+L1TF-vulnerable PTE.  Therefore, a PV guest kernel which has been updated with
+its own L1TF mitigations will not trigger shadow mode if it is well behaved.
+
+If CONFIG\_SHADOW\_PAGING is not compiled in, this mitigation instead crashes
+the guest when an L1TF-vulnerable PTE is written, which still allows updated,
+well-behaved PV guests to run, despite Shadow being compiled out.
+
 ### reboot
 > `= t[riple] | k[bd] | a[cpi] | p[ci] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]`
 

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index ee75fd51f5b32278d4b659b55922eccac8f28e64..ad7c22f8489a4a1c533df436a6d74cfc444e9e77 100644 (file)
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -70,6 +70,7 @@ config SHADOW_PAGING
             * Running HVM guests on hardware lacking hardware paging support
               (First-generation Intel VT-x or AMD SVM).
             * Live migration of PV guests.
+            * L1TF sidechannel mitigation for PV guests.
 
           Under a small number of specific workloads, shadow paging may be
           deliberately used as a performance optimisation.

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 7dfb6ed644191b86e308018c7544a2b8a7b66bf6..73ac11ee8eda246efc27fe0bc23e4ecedf44faa3 100644 (file)
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -187,6 +187,55 @@ static int __init parse_spec_ctrl(char *s)
 }
 custom_param("spec-ctrl", parse_spec_ctrl);
 
+int8_t __read_mostly opt_pv_l1tf = -1;
+
+static __init int parse_pv_l1tf(char *s)
+{
+    char *ss;
+    int val, rc = 0;
+
+    /* Inhibit the defaults as an explicit choice has been given. */
+    if ( opt_pv_l1tf == -1 )
+        opt_pv_l1tf = 0;
+
+    /* Interpret 'pv-l1tf' alone in its positive boolean form. */
+    if ( *s == '\0' )
+        opt_xpti = OPT_PV_L1TF_DOM0 | OPT_PV_L1TF_DOMU;
+
+    do {
+        ss = strchr(s, ',');
+        if ( ss )
+            *ss = '\0';
+
+        switch ( parse_bool(s) )
+        {
+        case 0:
+            opt_pv_l1tf = 0;
+            break;
+
+        case 1:
+            opt_pv_l1tf = OPT_PV_L1TF_DOM0 | OPT_PV_L1TF_DOMU;
+            break;
+
+        default:
+            if ( (val = parse_boolean("dom0", s, ss)) >= 0 )
+                opt_pv_l1tf = ((opt_pv_l1tf & ~OPT_PV_L1TF_DOM0) |
+                               (val ? OPT_PV_L1TF_DOM0 : 0));
+            else if ( (val = parse_boolean("domu", s, ss)) >= 0 )
+                opt_pv_l1tf = ((opt_pv_l1tf & ~OPT_PV_L1TF_DOMU) |
+                               (val ? OPT_PV_L1TF_DOMU : 0));
+            else
+                rc = -EINVAL;
+            break;
+        }
+
+        s = ss + 1;
+    } while ( ss );
+
+    return rc;
+}
+custom_param("pv-l1tf", parse_pv_l1tf);
+
 static void __init print_details(enum ind_thunk thunk, uint64_t caps)
 {
     unsigned int _7d0 = 0, e8b = 0, tmp;
@@ -210,9 +259,16 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (caps & ARCH_CAPS_RSBA)                  ? " RSBA"      : "",
            (caps & ARCH_CAPS_SSB_NO)                ? " SSB_NO"    : "");
 
-    /* Compiled-in support which pertains to BTI mitigations. */
-    if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) )
-        printk("  Compiled-in support: INDIRECT_THUNK\n");
+    /* Compiled-in support which pertains to mitigations. */
+    if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) || IS_ENABLED(CONFIG_SHADOW_PAGING) )
+        printk("  Compiled-in support:"
+#ifdef CONFIG_INDIRECT_THUNK
+               " INDIRECT_THUNK"
+#endif
+#ifdef CONFIG_SHADOW_PAGING
+               " SHADOW_PAGING"
+#endif
+               "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
     printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s, Other:%s\n",
@@ -226,6 +282,13 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
            opt_ibpb                                  ? " IBPB"  : "");
 
+    /* L1TF diagnostics, printed if vulnerable or PV shadowing is in use. */
+    if ( cpu_has_bug_l1tf || opt_pv_l1tf )
+        printk("  L1TF: believed%s vulnerable, maxphysaddr L1D %u, CPUID %u"
+               ", Safe address %"PRIx64"\n",
+               cpu_has_bug_l1tf ? "" : " not",
+               l1d_maxphysaddr, paddr_bits, l1tf_safe_maddr);
+
     /*
      * Alternatives blocks for protecting against and/or virtualising
      * mitigation support for guests.
@@ -247,6 +310,10 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
     printk("  XPTI (64-bit PV only): Dom0 %s, DomU %s\n",
            opt_xpti & OPT_XPTI_DOM0 ? "enabled" : "disabled",
            opt_xpti & OPT_XPTI_DOMU ? "enabled" : "disabled");
+
+    printk("  PV L1TF shadowing: Dom0 %s, DomU %s\n",
+           opt_pv_l1tf & OPT_PV_L1TF_DOM0  ? "enabled"  : "disabled",
+           opt_pv_l1tf & OPT_PV_L1TF_DOMU  ? "enabled"  : "disabled");
 }
 
 /* Calculate whether Retpoline is known-safe on this CPU. */
@@ -770,6 +837,18 @@ void __init init_speculation_mitigations(void)
 
     l1tf_calculations(caps);
 
+    /*
+     * By default, enable PV domU L1TF mitigations on all L1TF-vulnerable
+     * hardware.
+     */
+    if ( opt_pv_l1tf == -1 )
+    {
+        if ( !cpu_has_bug_l1tf )
+            opt_pv_l1tf = 0;
+        else
+            opt_pv_l1tf = OPT_PV_L1TF_DOMU;
+    }
+
     print_details(thunk, caps);
 
     /*

diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index d7e8ed0f5f05e9152a14df62baa366d8030cf1eb..cdf5737dc204470ff5f57f818d9f021b3d4d32d7 100644 (file)
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -38,6 +38,10 @@ extern int8_t opt_xpti;
 #define OPT_XPTI_DOM0  0x01
 #define OPT_XPTI_DOMU  0x02
 
+extern int8_t opt_pv_l1tf;
+#define OPT_PV_L1TF_DOM0  0x01
+#define OPT_PV_L1TF_DOMU  0x02
+
 /*
  * The L1D address mask, which might be wider than reported in CPUID, and the
  * system physical address above which there are believed to be no cacheable