# Commit 858cba0d4c6b6b45180afcb41561fd6585ad51a3
# Date 2018-01-16 17:45:50 +0000
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86: Introduce alternative indirect thunks

Depending on hardware and microcode availability, we will want to replace
IND_THUNK_REPOLINE with other implementations.

For AMD hardware, choose IND_THUNK_LFENCE in preference to retpoline if lfence
is known to be (or was successfully made) dispatch serialising.

This is part of XSA-254.

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

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -237,6 +237,22 @@ and not running softirqs. Reduce this if
 enough. Setting this to a high value may cause boot failure, particularly if
 the NMI watchdog is also enabled.
 
+### bti (x86)
+> `= List of [ thunk=retpoline|lfence|jmp ]`
+
+Branch Target Injection controls.  By default, Xen will pick the most
+appropriate BTI mitigations based on compiled in support, loaded microcode,
+and hardware details.
+
+**WARNING: Any use of this option may interfere with heuristics.  Use with
+extreme care.**
+
+If Xen was compiled with INDIRECT_THUNK support, `thunk=` can be used to
+select which of the thunks gets patched into the `__x86_indirect_thunk_%reg`
+locations.  The default thunk is `retpoline` (generally preferred for Intel
+hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
+overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
+
 ### xenheap\_megabytes (arm32)
 > `= <size>`
 
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -348,6 +348,15 @@ void __cpuinit identify_cpu(struct cpuin
 	 * executed, c == &boot_cpu_data.
 	 */
 	if ( c != &boot_cpu_data ) {
+		/* Inherit certain bits from the boot CPU. */
+		if (test_bit(X86_FEATURE_IND_THUNK_LFENCE,
+		             boot_cpu_data.x86_capability))
+			__set_bit(X86_FEATURE_IND_THUNK_LFENCE,
+			          c->x86_capability);
+		if (test_bit(X86_FEATURE_IND_THUNK_JMP,
+			     boot_cpu_data.x86_capability))
+			__set_bit(X86_FEATURE_IND_THUNK_JMP, c->x86_capability);
+
 		/* AND the already accumulated flags with these */
 		for ( i = 0 ; i < NCAPINTS ; i++ )
 			boot_cpu_data.x86_capability[i] &= c->x86_capability[i];
--- a/xen/arch/x86/indirect-thunk.S
+++ b/xen/arch/x86/indirect-thunk.S
@@ -21,15 +21,26 @@
         ret
 .endm
 
+.macro IND_THUNK_LFENCE reg:req
+        lfence
+        jmp *%\reg
+.endm
+
+.macro IND_THUNK_JMP reg:req
+        jmp *%\reg
+.endm
+
 /*
- * Build the __x86_indirect_thunk_* symbols.  Currently implement the
- * retpoline thunk only.
+ * Build the __x86.indirect_thunk.* symbols.  Execution lands on an
+ * alternative patch point which implements one of the above THUNK_*'s
  */
 .macro GEN_INDIRECT_THUNK reg:req
         .section .text.__x86_indirect_thunk_\reg, "ax", @progbits
 
 ENTRY(__x86_indirect_thunk_\reg)
-        IND_THUNK_RETPOLINE \reg
+        ALTERNATIVE_2 __stringify(IND_THUNK_RETPOLINE \reg),              \
+        __stringify(IND_THUNK_LFENCE \reg), X86_FEATURE_IND_THUNK_LFENCE, \
+        __stringify(IND_THUNK_JMP \reg),    X86_FEATURE_IND_THUNK_JMP
 .endm
 
 /* Instantiate GEN_INDIRECT_THUNK for each register except %rsp. */
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -16,18 +16,54 @@
  *
  * Copyright (c) 2017-2018 Citrix Systems Ltd.
  */
+#include <xen/errno.h>
 #include <xen/init.h>
 #include <xen/lib.h>
 
 #include <asm/processor.h>
 #include <asm/spec_ctrl.h>
 
-enum ind_thunk {
+static enum ind_thunk {
     THUNK_DEFAULT, /* Decide which thunk to use at boot time. */
     THUNK_NONE,    /* Missing compiler support for thunks. */
 
     THUNK_RETPOLINE,
-};
+    THUNK_LFENCE,
+    THUNK_JMP,
+} opt_thunk __initdata = THUNK_DEFAULT;
+
+static int __init parse_bti(const char *s)
+{
+    const char *ss;
+    int rc = 0;
+
+    do {
+        ss = strchr(s, ',');
+        if ( !ss )
+            ss = strchr(s, '\0');
+
+        if ( !strncmp(s, "thunk=", 6) )
+        {
+            s += 6;
+
+            if ( !strncmp(s, "retpoline", ss - s) )
+                opt_thunk = THUNK_RETPOLINE;
+            else if ( !strncmp(s, "lfence", ss - s) )
+                opt_thunk = THUNK_LFENCE;
+            else if ( !strncmp(s, "jmp", ss - s) )
+                opt_thunk = THUNK_JMP;
+            else
+                rc = -EINVAL;
+        }
+        else
+            rc = -EINVAL;
+
+        s = ss + 1;
+    } while ( *ss );
+
+    return rc;
+}
+custom_param("bti", parse_bti);
 
 static void __init print_details(enum ind_thunk thunk)
 {
@@ -41,7 +77,9 @@ static void __init print_details(enum in
     printk(XENLOG_INFO
            "BTI mitigations: Thunk %s\n",
            thunk == THUNK_NONE      ? "N/A" :
-           thunk == THUNK_RETPOLINE ? "RETPOLINE" : "?");
+           thunk == THUNK_RETPOLINE ? "RETPOLINE" :
+           thunk == THUNK_LFENCE    ? "LFENCE" :
+           thunk == THUNK_JMP       ? "JMP" : "?");
 }
 
 void __init init_speculation_mitigations(void)
@@ -49,6 +87,30 @@ void __init init_speculation_mitigations
     enum ind_thunk thunk = THUNK_DEFAULT;
 
     /*
+     * Has the user specified any custom BTI mitigations?  If so, follow their
+     * instructions exactly and disable all heuristics.
+     */
+    if ( opt_thunk != THUNK_DEFAULT )
+    {
+        thunk = opt_thunk;
+    }
+    else
+    {
+        /*
+         * Evaluate the safest Branch Target Injection mitigations to use.
+         * First, begin with compiler-aided mitigations.
+         */
+#ifdef CONFIG_INDIRECT_THUNK
+        /*
+         * AMD's recommended mitigation is to set lfence as being dispatch
+         * serialising, and to use IND_THUNK_LFENCE.
+         */
+        if ( cpu_has_lfence_dispatch )
+            thunk = THUNK_LFENCE;
+#endif
+    }
+
+    /*
      * Supplimentary minor adjustments.  Without compiler support, there are
      * no thunks.
      */
@@ -63,6 +125,12 @@ void __init init_speculation_mitigations
     if ( thunk == THUNK_DEFAULT )
         thunk = THUNK_RETPOLINE;
 
+    /* Apply the chosen settings. */
+    if ( thunk == THUNK_LFENCE )
+        __set_bit(X86_FEATURE_IND_THUNK_LFENCE, boot_cpu_data.x86_capability);
+    else if ( thunk == THUNK_JMP )
+        __set_bit(X86_FEATURE_IND_THUNK_JMP, boot_cpu_data.x86_capability);
+
     print_details(thunk);
 }
 
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -64,6 +64,8 @@
 /* Other features, Linux-defined mapping, word 3 */
 /* This range is used for feature bits which conflict or are synthesized */
 #define X86_FEATURE_LFENCE_DISPATCH (3*32+ 0) /* lfence set as Dispatch Serialising */
+#define X86_FEATURE_IND_THUNK_LFENCE (3*32+ 1) /* Use IND_THUNK_LFENCE */
+#define X86_FEATURE_IND_THUNK_JMP   (3*32+ 2) /* Use IND_THUNK_JMP */
 #define X86_FEATURE_MFENCE_RDTSC (3*32+ 7) /* MFENCE synchronizes RDTSC */
 #define X86_FEATURE_CONSTANT_TSC (3*32+ 8) /* TSC ticks at a constant rate */
 #define X86_FEATURE_NONSTOP_TSC	(3*32+ 9) /* TSC does not stop in C states */
