[PATCH v8 02/17] x86: Support indirect thunks from assembly code

[Andrew Cooper <andrew.cooper3@citrix.com>]

Introduce INDIRECT_CALL and INDIRECT_JMP which either degrade to a normal
indirect branch, or dispatch to the __x86_indirect_thunk_* symbols.

Update all the manual indirect branches in to use the new thunks.  The
indirect branches in the early boot and kexec path are left intact as we can't
use the compiled-in thunks at those points.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
v7:
 * Protect the jmp from the trampoline into the high mappings on the AP boot
   path, and the hand-crafted indirect jump in the PV IO emulation stubs.
 * Rebase over compiler changes
 * Spelling fixes
 * Fix clang build
v8:
 * Spelling/grammar fixes
---
 xen/Rules.mk                             |  4 ++--
 xen/arch/x86/Rules.mk                    |  6 +++++
 xen/arch/x86/boot/trampoline.S           | 24 +++++++++++++++++--
 xen/arch/x86/extable.c                   |  4 ++--
 xen/arch/x86/pv/emul-priv-op.c           | 41 ++++++++++++++++++++++----------
 xen/arch/x86/x86_64/entry.S              |  6 +++--
 xen/arch/x86/x86_emulate/x86_emulate.c   |  4 ++--
 xen/common/wait.c                        |  8 ++++---
 xen/include/asm-x86/asm_defns.h          |  8 +++++++
 xen/include/asm-x86/indirect_thunk_asm.h | 41 ++++++++++++++++++++++++++++++++
 10 files changed, 121 insertions(+), 25 deletions(-)
 create mode 100644 xen/include/asm-x86/indirect_thunk_asm.h

diff --git a/xen/Rules.mk b/xen/Rules.mk
index 2659f8a..3cf4075 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -66,8 +66,8 @@ endif
 
 AFLAGS-y                += -D__ASSEMBLY__
 
-# Clang's built-in assembler can't handle .code16/.code32/.code64 yet
-AFLAGS-$(clang)         += -no-integrated-as
+# Clang's built-in assembler can't handle embedded .include's
+CFLAGS-$(clang)         += -no-integrated-as
 
 ALL_OBJS := $(ALL_OBJS-y)
 
diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index abcc4d4..70e9d8f 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -37,3 +37,9 @@ CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register
 CFLAGS += -DCONFIG_INDIRECT_THUNK
 export CONFIG_INDIRECT_THUNK=y
 endif
+
+# Set up the assembler include path properly for older GCC toolchains.  Clang
+# objects to the agument being passed however.
+ifneq ($(clang),y)
+CFLAGS += -Wa,-I$(BASEDIR)/include
+endif
diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index 4d640f3..f70d913 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -153,8 +153,28 @@ trampoline_protmode_entry:
         .code64
 start64:
         /* Jump to high mappings. */
-        movabs  $__high_start,%rax
-        jmpq    *%rax
+        movabs  $__high_start, %rdi
+
+#ifdef CONFIG_INDIRECT_THUNK
+        /*
+         * If booting virtualised, or hot-onlining a CPU, sibling threads can
+         * attempt Branch Target Injection against this jmp.
+         *
+         * We've got no usable stack so can't use a RETPOLINE thunk, and are
+         * further than disp32 from the high mappings so couldn't use
+         * JUMP_THUNK even if it was a non-RETPOLINE thunk.  Furthermore, an
+         * LFENCE isn't necessarily safe to use at this point.
+         *
+         * As this isn't a hotpath, use a fully serialising event to reduce
+         * the speculation window as much as possible.  %ebx needs preserving
+         * for __high_start.
+         */
+        mov     %ebx, %esi
+        cpuid
+        mov     %esi, %ebx
+#endif
+
+        jmpq    *%rdi
 
 #include "wakeup.S"
 
diff --git a/xen/arch/x86/extable.c b/xen/arch/x86/extable.c
index 6fffe05..72f30d9 100644
--- a/xen/arch/x86/extable.c
+++ b/xen/arch/x86/extable.c
@@ -158,7 +158,7 @@ static int __init stub_selftest(void)
         memcpy(ptr, tests[i].opc, ARRAY_SIZE(tests[i].opc));
         unmap_domain_page(ptr);
 
-        asm volatile ( "call *%[stb]\n"
+        asm volatile ( "INDIRECT_CALL %[stb]\n"
                        ".Lret%=:\n\t"
                        ".pushsection .fixup,\"ax\"\n"
                        ".Lfix%=:\n\t"
@@ -167,7 +167,7 @@ static int __init stub_selftest(void)
                        ".popsection\n\t"
                        _ASM_EXTABLE(.Lret%=, .Lfix%=)
                        : [exn] "+m" (res)
-                       : [stb] "rm" (addr), "a" (tests[i].rax));
+                       : [stb] "r" (addr), "a" (tests[i].rax));
         ASSERT(res == tests[i].res.raw);
     }
 
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index ebd6dc1..2b2a58b 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -73,46 +73,63 @@ void (*pv_post_outb_hook)(unsigned int port, u8 value);
 
 typedef void io_emul_stub_t(struct cpu_user_regs *);
 
+void __x86_indirect_thunk_rcx(void);
+
 static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
                                           unsigned int port, unsigned int bytes)
 {
+    struct stubs *this_stubs = &this_cpu(stubs);
+    unsigned long stub_va = this_stubs->addr + STUB_BUF_SIZE / 2;
     bool use_quirk_stub = false;
 
     if ( !ctxt->io_emul_stub )
-        ctxt->io_emul_stub = map_domain_page(_mfn(this_cpu(stubs.mfn))) +
-                                             (this_cpu(stubs.addr) &
-                                              ~PAGE_MASK) +
-                                             STUB_BUF_SIZE / 2;
+        ctxt->io_emul_stub =
+            map_domain_page(_mfn(this_stubs->mfn)) + (stub_va & ~PAGE_MASK);
 
     /* movq $host_to_guest_gpr_switch,%rcx */
     ctxt->io_emul_stub[0] = 0x48;
     ctxt->io_emul_stub[1] = 0xb9;
     *(void **)&ctxt->io_emul_stub[2] = (void *)host_to_guest_gpr_switch;
+
+#ifdef CONFIG_INDIRECT_THUNK
+    /* callq __x86_indirect_thunk_rcx */
+    ctxt->io_emul_stub[10] = 0xe8;
+    *(int32_t *)&ctxt->io_emul_stub[11] =
+        (unsigned long)__x86_indirect_thunk_rcx - (stub_va + 11 + 4);
+
+#else
     /* callq *%rcx */
     ctxt->io_emul_stub[10] = 0xff;
     ctxt->io_emul_stub[11] = 0xd1;
 
+    /*
+     * 3 bytes of P6_NOPS.
+     * TODO: untangle ideal_nops from init/livepatch Kconfig options.
+     */
+    memcpy(&ctxt->io_emul_stub[12], "\x0f\x1f\x00", 3);
+#endif
+
     if ( unlikely(ioemul_handle_quirk) )
-        use_quirk_stub = ioemul_handle_quirk(opcode, &ctxt->io_emul_stub[12],
+        use_quirk_stub = ioemul_handle_quirk(opcode, &ctxt->io_emul_stub[15],
                                              ctxt->ctxt.regs);
 
     if ( !use_quirk_stub )
     {
         /* data16 or nop */
-        ctxt->io_emul_stub[12] = (bytes != 2) ? 0x90 : 0x66;
+        ctxt->io_emul_stub[15] = (bytes != 2) ? 0x90 : 0x66;
         /* <io-access opcode> */
-        ctxt->io_emul_stub[13] = opcode;
+        ctxt->io_emul_stub[16] = opcode;
         /* imm8 or nop */
-        ctxt->io_emul_stub[14] = !(opcode & 8) ? port : 0x90;
+        ctxt->io_emul_stub[17] = !(opcode & 8) ? port : 0x90;
         /* ret (jumps to guest_to_host_gpr_switch) */
-        ctxt->io_emul_stub[15] = 0xc3;
+        ctxt->io_emul_stub[18] = 0xc3;
     }
 
-    BUILD_BUG_ON(STUB_BUF_SIZE / 2 < MAX(16, /* Default emul stub */
-                                         12 + IOEMUL_QUIRK_STUB_BYTES));
+    BUILD_BUG_ON(STUB_BUF_SIZE / 2 < MAX(19, /* Default emul stub */
+                                         15 + IOEMUL_QUIRK_STUB_BYTES));
 
     /* Handy function-typed pointer to the stub. */
-    return (void *)(this_cpu(stubs.addr) + STUB_BUF_SIZE / 2);
+    return (void *)stub_va;
 }
 
 
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 1dd9ccf..cbd73f6 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -474,7 +474,8 @@ handle_exception_saved:
         movzbl UREGS_entry_vector(%rsp),%eax
         leaq  exception_table(%rip),%rdx
         PERFC_INCR(exceptions, %rax, %rbx)
-        callq *(%rdx,%rax,8)
+        mov   (%rdx, %rax, 8), %rdx
+        INDIRECT_CALL %rdx
         testb $3,UREGS_cs(%rsp)
         jz    restore_all_xen
         leaq  VCPU_trap_bounce(%rbx),%rdx
@@ -615,7 +616,8 @@ handle_ist_exception:
 1:      movq  %rsp,%rdi
         movzbl UREGS_entry_vector(%rsp),%eax
         leaq  exception_table(%rip),%rdx
-        callq *(%rdx,%rax,8)
+        mov   (%rdx, %rax, 8), %rdx
+        INDIRECT_CALL %rdx
         cmpb  $TRAP_nmi,UREGS_entry_vector(%rsp)
         jne   ret_from_intr
 
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 820495f..ff0a003 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -867,7 +867,7 @@ static inline int mkec(uint8_t e, int32_t ec, ...)
 #ifdef __XEN__
 # define invoke_stub(pre, post, constraints...) do {                    \
     union stub_exception_token res_ = { .raw = ~0 };                    \
-    asm volatile ( pre "\n\tcall *%[stub]\n\t" post "\n"                \
+    asm volatile ( pre "\n\tINDIRECT_CALL %[stub]\n\t" post "\n"        \
                    ".Lret%=:\n\t"                                       \
                    ".pushsection .fixup,\"ax\"\n"                       \
                    ".Lfix%=:\n\t"                                       \
@@ -876,7 +876,7 @@ static inline int mkec(uint8_t e, int32_t ec, ...)
                    ".popsection\n\t"                                    \
                    _ASM_EXTABLE(.Lret%=, .Lfix%=)                       \
                    : [exn] "+g" (res_), constraints,                    \
-                     [stub] "rm" (stub.func),                           \
+                     [stub] "r" (stub.func),                            \
                      "m" (*(uint8_t(*)[MAX_INST_LEN + 1])stub.ptr) );   \
     if ( unlikely(~res_.raw) )                                          \
     {                                                                   \
diff --git a/xen/common/wait.c b/xen/common/wait.c
index 3d3d9fe..a57bc10 100644
--- a/xen/common/wait.c
+++ b/xen/common/wait.c
@@ -203,12 +203,14 @@ void check_wakeup_from_wait(void)
 
     /*
      * Hand-rolled longjmp().  Returns to the pointer on the top of
-     * wqv->stack, and lands on a `rep movs` instruction.
+     * wqv->stack, and lands on a `rep movs` instruction.  All other GPRs are
+     * restored from the stack, so are available for use here.
      */
     asm volatile (
-        "mov %1,%%"__OP"sp; jmp *(%0)"
+        "mov %1,%%"__OP"sp; INDIRECT_JMP %[ip]"
         : : "S" (wqv->stack), "D" (wqv->esp),
-        "c" ((char *)get_cpu_info() - (char *)wqv->esp)
+          "c" ((char *)get_cpu_info() - (char *)wqv->esp),
+          [ip] "r" (*(unsigned long *)wqv->stack)
         : "memory" );
     unreachable();
 }
diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
index 7e8838e..40b0250 100644
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -13,6 +13,14 @@
 #include <asm/cpufeature.h>
 #include <asm/alternative.h>
 
+#ifdef __ASSEMBLY__
+# include <asm/indirect_thunk_asm.h>
+#else
+asm ( "\t.equ CONFIG_INDIRECT_THUNK, "
+      __stringify(IS_ENABLED(CONFIG_INDIRECT_THUNK)) );
+asm ( "\t.include \"asm/indirect_thunk_asm.h\"" );
+#endif
+
 #ifndef __ASSEMBLY__
 void ret_from_intr(void);
 #endif
diff --git a/xen/include/asm-x86/indirect_thunk_asm.h b/xen/include/asm-x86/indirect_thunk_asm.h
new file mode 100644
index 0000000..96bcc25
--- /dev/null
+++ b/xen/include/asm-x86/indirect_thunk_asm.h
@@ -0,0 +1,41 @@
+/*
+ * Warning!  This file is included at an assembler level for .c files, causing
+ * usual #ifdef'ary to turn into comments.
+ */
+
+.macro INDIRECT_BRANCH insn:req arg:req
+/*
+ * Create an indirect branch.  insn is one of call/jmp, arg is a single
+ * register.
+ *
+ * With no compiler support, this degrades into a plain indirect call/jmp.
+ * With compiler support, dispatch to the correct __x86_indirect_thunk_*
+ */
+    .if CONFIG_INDIRECT_THUNK == 1
+
+        $done = 0
+        .irp reg, ax, cx, dx, bx, bp, si, di, 8, 9, 10, 11, 12, 13, 14, 15
+        .ifeqs "\arg", "%r\reg"
+            \insn __x86_indirect_thunk_r\reg
+            $done = 1
+           .exitm
+        .endif
+        .endr
+
+        .if $done != 1
+            .error "Bad register arg \arg"
+        .endif
+
+    .else
+        \insn *\arg
+    .endif
+.endm
+
+/* Convenience wrappers. */
+.macro INDIRECT_CALL arg:req
+    INDIRECT_BRANCH call \arg
+.endm
+
+.macro INDIRECT_JMP arg:req
+    INDIRECT_BRANCH jmp \arg
+.endm
-- 
2.1.4


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