[tip:x86/pti] x86/retpoline: Add LFENCE to the retpoline/RSB filling RSB macros

[tip-bot for Tom Lendacky <tipbot@zytor.com>]

Commit-ID:  607d93a2658a0075047e87e47fde5bcc626a1918
Gitweb:     https://git.kernel.org/tip/607d93a2658a0075047e87e47fde5bcc626a1918
Author:     Tom Lendacky <thomas.lendacky@amd.com>
AuthorDate: Sat, 13 Jan 2018 17:27:30 -0600
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 14 Jan 2018 18:22:15 +0100

x86/retpoline: Add LFENCE to the retpoline/RSB filling RSB macros

The PAUSE instruction is currently used in the retpoline and RSB filling
macros as a speculation trap.  The use of PAUSE was originally suggested
because it showed a very, very small difference in the amount of
cycles/time used to execute the retpoline as compared to LFENCE.  On AMD,
the PAUSE instruction is not a serializing instruction, so the pause/jmp
loop will use excess power as it is speculated over waiting for return
to mispredict to the correct target.

The RSB filling macro is applicable to AMD, and, if software is unable to
verify that LFENCE is serializing on AMD (possible when running under a
hypervisor), the generic retpoline support will be used and, so, is also
applicable to AMD.  Keep the current usage of PAUSE for Intel, but add an
LFENCE instruction to the speculation trap for AMD.

The same sequence has been adopted by GCC for the GCC generated retpolines.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@alien8.de>
Acked-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Link: https://lkml.kernel.org/r/20180113232730.31060.36287.stgit@tlendack-t1.amdoffice.net

---
 arch/x86/include/asm/nospec-branch.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 402a11c..7b45d84 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -11,7 +11,7 @@
  * Fill the CPU return stack buffer.
  *
  * Each entry in the RSB, if used for a speculative 'ret', contains an
- * infinite 'pause; jmp' loop to capture speculative execution.
+ * 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
@@ -38,11 +38,13 @@
 	call	772f;				\
 773:	/* speculation trap */			\
 	pause;					\
+	lfence;					\
 	jmp	773b;				\
 772:						\
 	call	774f;				\
 775:	/* speculation trap */			\
 	pause;					\
+	lfence;					\
 	jmp	775b;				\
 774:						\
 	dec	reg;				\
@@ -73,6 +75,7 @@
 	call	.Ldo_rop_\@
 .Lspec_trap_\@:
 	pause
+	lfence
 	jmp	.Lspec_trap_\@
 .Ldo_rop_\@:
 	mov	\reg, (%_ASM_SP)
@@ -165,6 +168,7 @@
 	"       .align 16\n"					\
 	"901:	call   903f;\n"					\
 	"902:	pause;\n"					\
+	"    	lfence;\n"					\
 	"       jmp    902b;\n"					\
 	"       .align 16\n"					\
 	"903:	addl   $4, %%esp;\n"				\