x86/spec-ctrl: Mitigate the TSX Asynchronous Abort sidechannel

See patch documentation and comments.

This is part of XSA-305 / CVE-2019-11135

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

docs/misc/xen-command-line.pandoc

@@ -1973,7 +1973,7 @@ extreme care.**
 An overall boolean value, `spec-ctrl=no`, can be specified to turn off all
 mitigations, including pieces of infrastructure used to virtualise certain
 mitigation features for guests.  This also includes settings which `xpti`,
-`smt`, `pv-l1tf` control, unless the respective option(s) have been
+`smt`, `pv-l1tf`, `tsx` control, unless the respective option(s) have been
 specified earlier on the command line.
 
 Alternatively, a slightly more restricted `spec-ctrl=no-xen` can be used to
@@ -2083,7 +2083,7 @@ pages) must also be specified via the tbuf_size parameter.
     = <bool>
 
     Applicability: x86
-    Default: true
+    Default: false on parts vulnerable to TAA, true otherwise
 
 Controls for the use of Transactional Synchronization eXtensions.
 
@@ -2093,6 +2093,19 @@ a control has been introduced which allows TSX to be turned off.
 On systems with the ability to turn TSX off, this boolean offers system wide
 control of whether TSX is enabled or disabled.
 
+On parts vulnerable to CVE-2019-11135 / TSX Asynchronous Abort, the following
+logic applies:
+
+ * An explicit `tsx=` choice is honoured, even if it is `true` and would
+   result in a vulnerable system.
+
+ * When no explicit `tsx=` choice is given, parts vulnerable to TAA will be
+   mitigated by disabling TSX, as this is the lowest overhead option.
+
+ * If the use of TSX is important, the more expensive TAA mitigations can be
+   opted in to with `smt=0 spec-ctrl=md-clear`, at which point TSX will remain
+   active by default.
+
 ### ucode (x86)
 > `= List of [ <integer> | scan=<bool>, nmi=<bool> ]`
 

xen/arch/x86/spec_ctrl.c

@@ -99,6 +99,9 @@ static int __init parse_spec_ctrl(const char *s)
 
             opt_branch_harden = false;
 
+            if ( opt_tsx == -1 )
+                opt_tsx = -3;
+
         disable_common:
             opt_rsb_pv = false;
             opt_rsb_hvm = false;
@@ -310,7 +313,7 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
     printk("Speculative mitigation facilities:\n");
 
     /* Hardware features which pertain to speculative mitigations. */
-    printk("  Hardware features:%s%s%s%s%s%s%s%s%s%s%s%s\n",
+    printk("  Hardware features:%s%s%s%s%s%s%s%s%s%s%s%s%s%s\n",
            (_7d0 & cpufeat_mask(X86_FEATURE_IBRSB)) ? " IBRS/IBPB" : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_STIBP)) ? " STIBP"     : "",
            (_7d0 & cpufeat_mask(X86_FEATURE_L1D_FLUSH)) ? " L1D_FLUSH" : "",
@@ -322,7 +325,9 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (caps & ARCH_CAPS_RSBA)                  ? " RSBA"      : "",
            (caps & ARCH_CAPS_SKIP_L1DFL)            ? " SKIP_L1DFL": "",
            (caps & ARCH_CAPS_SSB_NO)                ? " SSB_NO"    : "",
-           (caps & ARCH_CAPS_MDS_NO)                ? " MDS_NO"    : "");
+           (caps & ARCH_CAPS_MDS_NO)                ? " MDS_NO"    : "",
+           (caps & ARCH_CAPS_TSX_CTRL)              ? " TSX_CTRL"  : "",
+           (caps & ARCH_CAPS_TAA_NO)                ? " TAA_NO"    : "");
 
     /* Compiled-in support which pertains to mitigations. */
     if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) || IS_ENABLED(CONFIG_SHADOW_PAGING) )
@@ -336,7 +341,7 @@ 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%s%s\n",
+    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, Other:%s%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
@@ -345,6 +350,8 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (default_xen_spec_ctrl & SPEC_CTRL_IBRS)  ? "IBRS+" :  "IBRS-",
            !boot_cpu_has(X86_FEATURE_SSBD)           ? "" :
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
+           !(caps & ARCH_CAPS_TSX_CTRL)              ? "" :
+           (opt_tsx & 1)                             ? " TSX+" : " TSX-",
            opt_ibpb                                  ? " IBPB"  : "",
            opt_l1d_flush                             ? " L1D_FLUSH" : "",
            opt_md_clear_pv || opt_md_clear_hvm       ? " VERW"  : "",
@@ -867,6 +874,7 @@ void __init init_speculation_mitigations(void)
 {
     enum ind_thunk thunk = THUNK_DEFAULT;
     bool use_spec_ctrl = false, ibrs = false, hw_smt_enabled;
+    bool cpu_has_bug_taa;
     uint64_t caps = 0;
 
     if ( boot_cpu_has(X86_FEATURE_ARCH_CAPS) )
@@ -1094,6 +1102,53 @@ void __init init_speculation_mitigations(void)
             "enabled.  Mitigations will not be fully effective.  Please\n"
             "choose an explicit smt=<bool> setting.  See XSA-297.\n");
 
+    /*
+     * Vulnerability to TAA is a little complicated to quantify.
+     *
+     * In the pipeline, it is just another way to get speculative access to
+     * stale load port, store buffer or fill buffer data, and therefore can be
+     * considered a superset of MDS (on TSX-capable parts).  On parts which
+     * predate MDS_NO, the existing VERW flushing will mitigate this
+     * sidechannel as well.
+     *
+     * On parts which contain MDS_NO, the lack of VERW flushing means that an
+     * attacker can still use TSX to target microarchitectural buffers to leak
+     * secrets.  Therefore, we consider TAA to be the set of TSX-capable parts
+     * which have MDS_NO but lack TAA_NO.
+     *
+     * Note: cpu_has_rtm (== hle) could already be hidden by `tsx=0` on the
+     *       cmdline.  MSR_TSX_CTRL will only appear on TSX-capable parts, so
+     *       we check both to spot TSX in a microcode/cmdline independent way.
+     */
+    cpu_has_bug_taa =
+        (cpu_has_rtm || (caps & ARCH_CAPS_TSX_CTRL)) &&
+        (caps & (ARCH_CAPS_MDS_NO | ARCH_CAPS_TAA_NO)) == ARCH_CAPS_MDS_NO;
+
+    /*
+     * On TAA-affected hardware, disabling TSX is the preferred mitigation, vs
+     * the MDS mitigation of disabling HT and using VERW flushing.
+     *
+     * On CPUs which advertise MDS_NO, VERW has no flushing side effect until
+     * the TSX_CTRL microcode is loaded, despite the MD_CLEAR CPUID bit being
+     * advertised, and there isn't a MD_CLEAR_2 flag to use...
+     *
+     * If we're on affected hardware, able to do something about it (which
+     * implies that VERW now works), no explicit TSX choice and traditional
+     * MDS mitigations (no-SMT, VERW) not obviosuly in use (someone might
+     * plausibly value TSX higher than Hyperthreading...), disable TSX to
+     * mitigate TAA.
+     */
+    if ( opt_tsx == -1 && cpu_has_bug_taa && (caps & ARCH_CAPS_TSX_CTRL) &&
+         ((hw_smt_enabled && opt_smt) ||
+          !boot_cpu_has(X86_FEATURE_SC_VERW_IDLE)) )
+    {
+        setup_clear_cpu_cap(X86_FEATURE_HLE);
+        setup_clear_cpu_cap(X86_FEATURE_RTM);
+
+        opt_tsx = 0;
+        tsx_init();
+    }
+
     print_details(thunk, caps);
 
     /*

xen/arch/x86/tsx.c

@@ -5,7 +5,8 @@
  * Valid values:
  *   1 => Explicit tsx=1
  *   0 => Explicit tsx=0
- *  -1 => Default, implicit tsx=1
+ *  -1 => Default, implicit tsx=1, may change to 0 to mitigate TAA
+ *  -3 => Implicit tsx=1 (feed-through from spec-ctrl=0)
  *
  * This is arranged such that the bottom bit encodes whether TSX is actually
  * disabled, while identifying various explicit (>=0) and implicit (<0)

xen/include/asm-x86/msr-index.h

@@ -54,6 +54,7 @@
 #define ARCH_CAPS_MDS_NO		(_AC(1, ULL) << 5)
 #define ARCH_CAPS_IF_PSCHANGE_MC_NO	(_AC(1, ULL) << 6)
 #define ARCH_CAPS_TSX_CTRL		(_AC(1, ULL) << 7)
+#define ARCH_CAPS_TAA_NO		(_AC(1, ULL) << 8)
 
 #define MSR_FLUSH_CMD			0x0000010b
 #define FLUSH_CMD_L1D			(_AC(1, ULL) << 0)