x86: Introduce barrier_nospec

commit b3d7ad85b80bbc404635dca80f5b129f6242bc7a upstream.

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
[bwh: Backported to 3.2: update rdtsc_barrier() instead of rdtsc_ordered()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 41fda4d..59567d1 100644 (file)
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -478,6 +478,10 @@ 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)
+
 /**
  * read_barrier_depends - Flush all pending reads that subsequents reads
  * depend on.
@@ -563,8 +567,7 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
  */
 static __always_inline void rdtsc_barrier(void)
 {
-       alternative(ASM_NOP3, "mfence", X86_FEATURE_MFENCE_RDTSC);
-       alternative(ASM_NOP3, "lfence", X86_FEATURE_LFENCE_RDTSC);
+       barrier_nospec();
 }
 
 /*