linux-kernel - [PATCH v6 04/13] x86: introduce barrier

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Date:   Mon, 29 Jan 2018 17:02:33 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     tglx@...utronix.de, mingo@...nel.org
Cc:     linux-arch@...r.kernel.org, Tom Lendacky <thomas.lendacky@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com, gregkh@...uxfoundation.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>, torvalds@...ux-foundation.org,
        alan@...ux.intel.com
Subject: [PATCH v6 04/13] 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.

There are no functional changes in this patch.

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Suggested-by: Andi Kleen <ak@...ux.intel.com>
Suggested-by: Ingo Molnar <mingo@...hat.com>
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Kees Cook <keescook@...omium.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org
Signed-off-by: Dan Williams <dan.j.williams@...el.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 551ccbe07c0f..1e7c955b6303 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 07962f5f6fba..30df295f6d94 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();
 }