Patch "x86/retpoline: Simplify vmexit_fill_RSB()" has been added to the 4.9-stable tree

Patch "x86/retpoline: Simplify vmexit_fill_RSB()" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    x86/retpoline: Simplify vmexit_fill_RSB()

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-retpoline-simplify-vmexit_fill_rsb.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From foo@baz Thu Feb  8 03:30:27 CET 2018
From: Borislav Petkov <bp@alien8.de>
Date: Sat, 27 Jan 2018 16:24:33 +0000
Subject: x86/retpoline: Simplify vmexit_fill_RSB()

From: Borislav Petkov <bp@alien8.de>

(cherry picked from commit 1dde7415e99933bb7293d6b2843752cbdb43ec11)

Simplify it to call an asm-function instead of pasting 41 insn bytes at
every call site. Also, add alignment to the macro as suggested here:

  https://support.google.com/faqs/answer/7625886

[dwmw2: Clean up comments, let it clobber %ebx and just tell the compiler]

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: ak@linux.intel.com
Cc: dave.hansen@intel.com
Cc: karahmed@amazon.de
Cc: arjan@linux.intel.com
Cc: torvalds@linux-foundation.org
Cc: peterz@infradead.org
Cc: bp@alien8.de
Cc: pbonzini@redhat.com
Cc: tim.c.chen@linux.intel.com
Cc: gregkh@linux-foundation.org
Link: https://lkml.kernel.org/r/1517070274-12128-3-git-send-email-dwmw@amazon.co.uk
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/entry/entry_32.S             |    3 -
 arch/x86/entry/entry_64.S             |    3 -
 arch/x86/include/asm/asm-prototypes.h |    3 +
 arch/x86/include/asm/nospec-branch.h  |   70 +++-------------------------------
 arch/x86/lib/Makefile                 |    1 
 arch/x86/lib/retpoline.S              |   56 +++++++++++++++++++++++++++
 6 files changed, 71 insertions(+), 65 deletions(-)

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -237,7 +237,8 @@ ENTRY(__switch_to_asm)
 	 * exist, overwrite the RSB with entries which capture
 	 * speculative execution to prevent attack.
 	 */
-	FILL_RETURN_BUFFER %ebx, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
+	/* Clobbers %ebx */
+	FILL_RETURN_BUFFER RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
 #endif
 
 	/* restore callee-saved registers */
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -435,7 +435,8 @@ ENTRY(__switch_to_asm)
 	 * exist, overwrite the RSB with entries which capture
 	 * speculative execution to prevent attack.
 	 */
-	FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
+	/* Clobbers %rbx */
+	FILL_RETURN_BUFFER RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
 #endif
 
 	/* restore callee-saved registers */
--- a/arch/x86/include/asm/asm-prototypes.h
+++ b/arch/x86/include/asm/asm-prototypes.h
@@ -37,4 +37,7 @@ INDIRECT_THUNK(dx)
 INDIRECT_THUNK(si)
 INDIRECT_THUNK(di)
 INDIRECT_THUNK(bp)
+asmlinkage void __fill_rsb(void);
+asmlinkage void __clear_rsb(void);
+
 #endif /* CONFIG_RETPOLINE */
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -7,50 +7,6 @@
 #include <asm/alternative-asm.h>
 #include <asm/cpufeatures.h>
 
-/*
- * Fill the CPU return stack buffer.
- *
- * Each entry in the RSB, if used for a speculative 'ret', contains an
- * infinite 'pause; lfence; jmp' loop to capture speculative execution.
- *
- * This is required in various cases for retpoline and IBRS-based
- * mitigations for the Spectre variant 2 vulnerability. Sometimes to
- * eliminate potentially bogus entries from the RSB, and sometimes
- * purely to ensure that it doesn't get empty, which on some CPUs would
- * allow predictions from other (unwanted!) sources to be used.
- *
- * We define a CPP macro such that it can be used from both .S files and
- * inline assembly. It's possible to do a .macro and then include that
- * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
- */
-
-#define RSB_CLEAR_LOOPS		32	/* To forcibly overwrite all entries */
-#define RSB_FILL_LOOPS		16	/* To avoid underflow */
-
-/*
- * Google experimented with loop-unrolling and this turned out to be
- * the optimal version — two calls, each with their own speculation
- * trap should their return address end up getting used, in a loop.
- */
-#define __FILL_RETURN_BUFFER(reg, nr, sp)	\
-	mov	$(nr/2), reg;			\
-771:						\
-	call	772f;				\
-773:	/* speculation trap */			\
-	pause;					\
-	lfence;					\
-	jmp	773b;				\
-772:						\
-	call	774f;				\
-775:	/* speculation trap */			\
-	pause;					\
-	lfence;					\
-	jmp	775b;				\
-774:						\
-	dec	reg;				\
-	jnz	771b;				\
-	add	$(BITS_PER_LONG/8) * nr, sp;
-
 #ifdef __ASSEMBLY__
 
 /*
@@ -121,17 +77,10 @@
 #endif
 .endm
 
- /*
-  * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
-  * monstrosity above, manually.
-  */
-.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
+/* This clobbers the BX register */
+.macro FILL_RETURN_BUFFER nr:req ftr:req
 #ifdef CONFIG_RETPOLINE
-	ANNOTATE_NOSPEC_ALTERNATIVE
-	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
-		__stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))	\
-		\ftr
-.Lskip_rsb_\@:
+	ALTERNATIVE "", "call __clear_rsb", \ftr
 #endif
 .endm
 
@@ -206,15 +155,10 @@ extern char __indirect_thunk_end[];
 static inline void vmexit_fill_RSB(void)
 {
 #ifdef CONFIG_RETPOLINE
-	unsigned long loops;
-
-	asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
-		      ALTERNATIVE("jmp 910f",
-				  __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
-				  X86_FEATURE_RETPOLINE)
-		      "910:"
-		      : "=r" (loops), ASM_CALL_CONSTRAINT
-		      : : "memory" );
+	alternative_input("",
+			  "call __fill_rsb",
+			  X86_FEATURE_RETPOLINE,
+			  ASM_NO_INPUT_CLOBBER(_ASM_BX, "memory"));
 #endif
 }
 
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -26,6 +26,7 @@ lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) +=
 lib-$(CONFIG_INSTRUCTION_DECODER) += insn.o inat.o
 lib-$(CONFIG_RANDOMIZE_BASE) += kaslr.o
 lib-$(CONFIG_RETPOLINE) += retpoline.o
+OBJECT_FILES_NON_STANDARD_retpoline.o :=y
 
 obj-y += msr.o msr-reg.o msr-reg-export.o hweight.o
 
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -7,6 +7,7 @@
 #include <asm/alternative-asm.h>
 #include <asm/export.h>
 #include <asm/nospec-branch.h>
+#include <asm/bitsperlong.h>
 
 .macro THUNK reg
 	.section .text.__x86.indirect_thunk
@@ -46,3 +47,58 @@ GENERATE_THUNK(r13)
 GENERATE_THUNK(r14)
 GENERATE_THUNK(r15)
 #endif
+
+/*
+ * Fill the CPU return stack buffer.
+ *
+ * Each entry in the RSB, if used for a speculative 'ret', contains an
+ * infinite 'pause; lfence; jmp' loop to capture speculative execution.
+ *
+ * This is required in various cases for retpoline and IBRS-based
+ * mitigations for the Spectre variant 2 vulnerability. Sometimes to
+ * eliminate potentially bogus entries from the RSB, and sometimes
+ * purely to ensure that it doesn't get empty, which on some CPUs would
+ * allow predictions from other (unwanted!) sources to be used.
+ *
+ * Google experimented with loop-unrolling and this turned out to be
+ * the optimal version - two calls, each with their own speculation
+ * trap should their return address end up getting used, in a loop.
+ */
+.macro STUFF_RSB nr:req sp:req
+	mov	$(\nr / 2), %_ASM_BX
+	.align 16
+771:
+	call	772f
+773:						/* speculation trap */
+	pause
+	lfence
+	jmp	773b
+	.align 16
+772:
+	call	774f
+775:						/* speculation trap */
+	pause
+	lfence
+	jmp	775b
+	.align 16
+774:
+	dec	%_ASM_BX
+	jnz	771b
+	add	$((BITS_PER_LONG/8) * \nr), \sp
+.endm
+
+#define RSB_FILL_LOOPS		16	/* To avoid underflow */
+
+ENTRY(__fill_rsb)
+	STUFF_RSB RSB_FILL_LOOPS, %_ASM_SP
+	ret
+END(__fill_rsb)
+EXPORT_SYMBOL_GPL(__fill_rsb)
+
+#define RSB_CLEAR_LOOPS		32	/* To forcibly overwrite all entries */
+
+ENTRY(__clear_rsb)
+	STUFF_RSB RSB_CLEAR_LOOPS, %_ASM_SP
+	ret
+END(__clear_rsb)
+EXPORT_SYMBOL_GPL(__clear_rsb)


Patches currently in stable-queue which might be from bp@alien8.de are

queue-4.9/x86-entry-64-push-extra-regs-right-away.patch
queue-4.9/x86-cpufeatures-add-intel-feature-bits-for-speculation-control.patch
queue-4.9/x86-retpoline-simplify-vmexit_fill_rsb.patch
queue-4.9/x86-cpufeatures-clean-up-spectre-v2-related-cpuid-flags.patch
queue-4.9/x86-asm-move-status-from-thread_struct-to-thread_info.patch
queue-4.9/x86-cpufeatures-add-cpuid_7_edx-cpuid-leaf.patch
queue-4.9/x86-microcode-amd-do-not-load-when-running-on-a-hypervisor.patch
queue-4.9/x86-entry-64-remove-the-syscall64-fast-path.patch
queue-4.9/x86-cpufeature-blacklist-spec_ctrl-pred_cmd-on-early-spectre-v2-microcodes.patch
queue-4.9/x86-nospec-fix-header-guards-names.patch
queue-4.9/x86-retpoline-avoid-retpolines-for-built-in-__init-functions.patch
queue-4.9/x86-cpu-bugs-make-retpoline-module-warning-conditional.patch
queue-4.9/x86-spectre-check-config_retpoline-in-command-line-parser.patch
queue-4.9/x86-alternative-print-unadorned-pointers.patch
queue-4.9/x86-cpuid-fix-up-virtual-ibrs-ibpb-stibp-feature-bits-on-intel.patch
queue-4.9/x86-cpufeatures-add-amd-feature-bits-for-speculation-control.patch
queue-4.9/x86-spectre-simplify-spectre_v2-command-line-parsing.patch
queue-4.9/x86-msr-add-definitions-for-new-speculation-control-msrs.patch
queue-4.9/x86-speculation-add-basic-ibpb-indirect-branch-prediction-barrier-support.patch
queue-4.9/x86-speculation-fix-typo-ibrs_att-which-should-be-ibrs_all.patch
queue-4.9/x86-bugs-drop-one-mitigation-from-dmesg.patch
queue-4.9/x86-pti-do-not-enable-pti-on-cpus-which-are-not-vulnerable-to-meltdown.patch