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Subject[PATCH v2 04/19] x86: implement ifence()
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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@infradead.org>
Suggested-by: Alan Cox <alan.cox@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@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 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")
#endif

+/* prevent speculative execution past this barrier */
+#define ifence() 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..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();
}

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