[lkml]   [2018]   [Jan]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[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 <>
Suggested-by: Alan Cox <>
Cc: Tom Lendacky <>
Cc: Mark Rutland <>
Cc: Greg KH <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
Cc: "H. Peter Anvin" <>
Signed-off-by: Elena Reshetova <>
Signed-off-by: Dan Williams <>
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, \
#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();

 \ /
  Last update: 2018-01-14 23:25    [W:0.174 / U:4.904 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site