# Commit 8c4330818f6ee70cbf7428a40a28a73df1272d10
# Date 2019-11-12 17:12:54 +0000
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
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>

--- xen-4.12.1-testing.orig/docs/misc/xen-command-line.pandoc
+++ xen-4.12.1-testing/docs/misc/xen-command-line.pandoc
@@ -1935,7 +1935,7 @@ 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
@@ -2052,7 +2052,7 @@ Xen version.
     = <bool>
 
     Applicability: x86
-    Default: true
+    Default: false on parts vulnerable to TAA, true otherwise
 
 Controls for the use of Transactional Synchronization eXtensions.
 
@@ -2062,6 +2062,19 @@ a control has been introduced which allo
 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)
 > `= [<integer> | scan]`
 
--- xen-4.12.1-testing.orig/xen/arch/x86/spec_ctrl.c
+++ xen-4.12.1-testing/xen/arch/x86/spec_ctrl.c
@@ -96,6 +96,9 @@ static int __init parse_spec_ctrl(const
             if ( opt_pv_l1tf_domu < 0 )
                 opt_pv_l1tf_domu = 0;
 
+            if ( opt_tsx == -1 )
+                opt_tsx = -3;
+
         disable_common:
             opt_rsb_pv = false;
             opt_rsb_hvm = false;
@@ -306,7 +309,7 @@ static void __init print_details(enum in
     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" : "",
@@ -318,7 +321,9 @@ static void __init print_details(enum in
            (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) )
@@ -332,7 +337,7 @@ static void __init print_details(enum in
                "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
-    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s, Other:%s%s%s\n",
+    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, Other:%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
@@ -341,6 +346,8 @@ static void __init print_details(enum in
            (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"  : "");
@@ -862,6 +869,7 @@ void __init init_speculation_mitigations
 {
     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) )
@@ -1086,6 +1094,53 @@ void __init init_speculation_mitigations
             "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-4.12.1-testing.orig/xen/arch/x86/tsx.c
+++ xen-4.12.1-testing/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-4.12.1-testing.orig/xen/include/asm-x86/msr-index.h
+++ xen-4.12.1-testing/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)
