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[PATCH v2 04/19] x86: implement ifence()

The new barrier, 'ifence', ensures that no instructions past the
boundary are speculatively executed.

Previously the kernel only needed this fence in 'rdtsc_ordered', but it
can also be used as a mitigation against Spectre variant1 attacks that
speculative access memory past an array bounds check.

'ifence', via 'ifence_array_ptr', is an opt-in fallback to the default
mitigation provided by '__array_ptr'. It is also proposed for blocking
speculation in the 'get_user' path to bypass 'access_ok' checks. For
now, just provide the common definition for later patches to build upon.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Suggested-by: Alan Cox <alan.cox@xxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
 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 7fb336210e1b..b04f572d6d97 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -24,6 +24,10 @@
 #define wmb()	asm volatile("sfence" ::: "memory")
+/* prevent speculative execution past this barrier */
+#define ifence() alternative_2("", "mfence", X86_FEATURE_MFENCE_RDTSC, \
+				   "lfence", X86_FEATURE_LFENCE_RDTSC)
 #define dma_rmb()	rmb()
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 07962f5f6fba..e426d2a33ff3 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);
+	ifence();
 	return rdtsc();