[tip:x86/pti] x86: Introduce barrier_nospec

[tip-bot for Dan Williams <tipbot@zytor.com>]

Commit-ID:  b3d7ad85b80bbc404635dca80f5b129f6242bc7a
Gitweb:     https://git.kernel.org/tip/b3d7ad85b80bbc404635dca80f5b129f6242bc7a
Author:     Dan Williams <dan.j.williams@intel.com>
AuthorDate: Mon, 29 Jan 2018 17:02:33 -0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 30 Jan 2018 21:54:29 +0100

x86: Introduce barrier_nospec

Rename the open coded form of this instruction sequence from
rdtsc_ordered() into a generic barrier primitive, barrier_nospec().

One of the mitigations for Spectre variant1 vulnerabilities is to fence
speculative execution after successfully validating a bounds check. I.e.
force the result of a bounds check to resolve in the instruction pipeline
to ensure speculative execution honors that result before potentially
operating on out-of-bounds data.

No functional changes.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andi Kleen <ak@linux.intel.com>
Suggested-by: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: kernel-hardening@lists.openwall.com
Cc: gregkh@linuxfoundation.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: alan@linux.intel.com
Link: https://lkml.kernel.org/r/151727415361.33451.9049453007262764675.stgit@dwillia2-desk3.amr.corp.intel.com

---
 arch/x86/include/asm/barrier.h | 4 ++++
 arch/x86/include/asm/msr.h     | 3 +--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 173b38f..30d4061 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -48,6 +48,10 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 /* Override the default implementation from linux/nospec.h. */
 #define array_index_mask_nospec array_index_mask_nospec
 
+/* Prevent speculative execution past this barrier. */
+#define barrier_nospec() alternative_2("", "mfence", X86_FEATURE_MFENCE_RDTSC, \
+					   "lfence", X86_FEATURE_LFENCE_RDTSC)
+
 #ifdef CONFIG_X86_PPRO_FENCE
 #define dma_rmb()	rmb()
 #else
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 07962f5..30df295 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -214,8 +214,7 @@ static __always_inline unsigned long long rdtsc_ordered(void)
 	 * that some other imaginary CPU is updating continuously with a
 	 * time stamp.
 	 */
-	alternative_2("", "mfence", X86_FEATURE_MFENCE_RDTSC,
-			  "lfence", X86_FEATURE_LFENCE_RDTSC);
+	barrier_nospec();
 	return rdtsc();
 }