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 <[email protected]> Suggested-by: Alan Cox <[email protected]> Cc: Tom Lendacky <[email protected]> Cc: Mark Rutland <[email protected]> Cc: Greg KH <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: "H. Peter Anvin" <[email protected]> Cc: [email protected] Signed-off-by: Elena Reshetova <[email protected]> Signed-off-by: Dan Williams <[email protected]> --- 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(); }
