x86/entry: Avoid using alternatives in NMI/#MC paths

This patch is deliberately arranged to be easy to revert if/when alternatives
patching becomes NMI/#MC safe.

For safety, there must be a dispatch serialising instruction in (what is
logically) DO_SPEC_CTRL_ENTRY so that, in the case that Xen needs IBRS set in
context, an attacker can't speculate around the WRMSR and reach an indirect
branch within the speculation window.

Using conditionals opens this attack vector up, so the else clause gets an
LFENCE to force the pipeline to catch up before continuing.  This also covers
the safety of RSB conditional, as execution it is guaranteed to either hit the
WRMSR or LFENCE.

One downside of not using alternatives is that there unconditionally an LFENCE
in the IST path in cases where we are not using the features from IBRS-capable
microcode.

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

xen/arch/x86/spec_ctrl.c

@@ -20,8 +20,10 @@
 #include <xen/init.h>
 #include <xen/lib.h>
 
+#include <asm/msr-index.h>
 #include <asm/processor.h>
 #include <asm/spec_ctrl.h>
+#include <asm/spec_ctrl_asm.h>
 
 static enum ind_thunk {
     THUNK_DEFAULT, /* Decide which thunk to use at boot time. */
@@ -150,6 +152,12 @@ void __init init_speculation_mitigations(void)
     print_details(thunk);
 }
 
+static void __init __maybe_unused build_assertions(void)
+{
+    /* The optimised assembly relies on this alias. */
+    BUILD_BUG_ON(BTI_IST_IBRS != SPEC_CTRL_IBRS);
+}
+
 /*
  * Local variables:
  * mode: C

xen/arch/x86/x86_64/asm-offsets.c

@@ -142,6 +142,7 @@ void __dummy__(void)
     OFFSET(CPUINFO_pv_cr3, struct cpu_info, pv_cr3);
     OFFSET(CPUINFO_shadow_spec_ctrl, struct cpu_info, shadow_spec_ctrl);
     OFFSET(CPUINFO_use_shadow_spec_ctrl, struct cpu_info, use_shadow_spec_ctrl);
+    OFFSET(CPUINFO_bti_ist_info, struct cpu_info, bti_ist_info);
     DEFINE(CPUINFO_sizeof, sizeof(struct cpu_info));
     BLANK();
 

xen/arch/x86/x86_64/entry.S

@@ -126,7 +126,7 @@ UNLIKELY_START(g, exit_cr3)
 UNLIKELY_END(exit_cr3)
 
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
-        SPEC_CTRL_EXIT_TO_XEN /* Req: %rbx=end, Clob: acd */
+        SPEC_CTRL_EXIT_TO_XEN_IST /* Req: %rbx=end, Clob: acd */
 
         RESTORE_ALL adj=8
         iretq
@@ -732,7 +732,7 @@ ENTRY(double_fault)
 
         GET_STACK_END(14)
 
-        SPEC_CTRL_ENTRY_FROM_INTR /* Req: %rsp=regs, %r14=end, Clob: acd */
+        SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
@@ -765,7 +765,7 @@ handle_ist_exception:
 
         GET_STACK_END(14)
 
-        SPEC_CTRL_ENTRY_FROM_INTR /* Req: %rsp=regs, %r14=end, Clob: acd */
+        SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rcx

xen/include/asm-x86/current.h

@@ -57,6 +57,7 @@ struct cpu_info {
     /* See asm-x86/spec_ctrl_asm.h for usage. */
     unsigned int shadow_spec_ctrl;
     bool         use_shadow_spec_ctrl;
+    uint8_t      bti_ist_info;
 
     unsigned long __pad;
     /* get_stack_bottom() must be 16-byte aligned */

xen/include/asm-x86/spec_ctrl.h

@@ -29,6 +29,7 @@ static inline void init_shadow_spec_ctrl_state(void)
     struct cpu_info *info = get_cpu_info();
 
     info->shadow_spec_ctrl = info->use_shadow_spec_ctrl = 0;
+    info->bti_ist_info = 0;
 }
 
 #endif /* !__X86_SPEC_CTRL_H__ */

xen/include/asm-x86/spec_ctrl_asm.h

@@ -20,6 +20,11 @@
 #ifndef __X86_SPEC_CTRL_ASM_H__
 #define __X86_SPEC_CTRL_ASM_H__
 
+/* Encoding of the bottom bits in cpuinfo.bti_ist_info */
+#define BTI_IST_IBRS  (1 << 0)
+#define BTI_IST_WRMSR (1 << 1)
+#define BTI_IST_RSB   (1 << 2)
+
 #ifdef __ASSEMBLY__
 #include <asm/msr-index.h>
 
@@ -255,6 +260,68 @@
         DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_XEN_IBRS_SET,           \
         DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_XEN_IBRS_CLEAR
 
+/* TODO: Drop these when the alternatives infrastructure is NMI/#MC safe. */
+.macro SPEC_CTRL_ENTRY_FROM_INTR_IST
+/*
+ * Requires %rsp=regs, %r14=stack_end
+ * Clobbers %rax, %rcx, %rdx
+ *
+ * This is logical merge of DO_OVERWRITE_RSB and DO_SPEC_CTRL_ENTRY
+ * maybexen=1, but with conditionals rather than alternatives.
+ */
+    movzbl STACK_CPUINFO_FIELD(bti_ist_info)(%r14), %eax
+
+    testb $BTI_IST_RSB, %al
+    jz .L\@_skip_rsb
+
+    DO_OVERWRITE_RSB
+
+.L\@_skip_rsb:
+
+    testb $BTI_IST_WRMSR, %al
+    jz .L\@_skip_wrmsr
+
+    xor %edx, %edx
+    testb $3, UREGS_cs(%rsp)
+    setz %dl
+    and %dl, STACK_CPUINFO_FIELD(use_shadow_spec_ctrl)(%r14)
+
+.L\@_entry_from_xen:
+    /*
+     * Load Xen's intended value.  SPEC_CTRL_IBRS vs 0 is encoded in the
+     * bottom bit of bti_ist_info, via a deliberate alias with BTI_IST_IBRS.
+     */
+    mov $MSR_SPEC_CTRL, %ecx
+    and $BTI_IST_IBRS, %eax
+    wrmsr
+
+    /* Opencoded UNLIKELY_START() with no condition. */
+UNLIKELY_DISPATCH_LABEL(\@_serialise):
+    .subsection 1
+    /*
+     * In the case that we might need to set SPEC_CTRL.IBRS for safety, we
+     * need to ensure that an attacker can't poison the `jz .L\@_skip_wrmsr`
+     * to speculate around the WRMSR.  As a result, we need a dispatch
+     * serialising instruction in the else clause.
+     */
+.L\@_skip_wrmsr:
+    lfence
+    UNLIKELY_END(\@_serialise)
+.endm
+
+.macro SPEC_CTRL_EXIT_TO_XEN_IST
+/*
+ * Requires %rbx=stack_end
+ * Clobbers %rax, %rcx, %rdx
+ */
+    testb $BTI_IST_WRMSR, STACK_CPUINFO_FIELD(bti_ist_info)(%rbx)
+    jz .L\@_skip
+
+    DO_SPEC_CTRL_EXIT_TO_XEN
+
+.L\@_skip:
+.endm
+
 #endif /* __ASSEMBLY__ */
 #endif /* !__X86_SPEC_CTRL_ASM_H__ */