Date: Thu, 11 Jan 2018 19:39:52 -0800 From: Dan Williams <dan.j.williams@...el.com> To: "Eric W. Biederman" <ebiederm@...ssion.com> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Mark Rutland <mark.rutland@....com>, Tom Lendacky <thomas.lendacky@....com>, linux-arch@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>, Peter Zijlstra <peterz@...radead.org>, Alan Cox <alan.cox@...el.com>, X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, kernel-hardening@...ts.openwall.com, Thomas Gleixner <tglx@...utronix.de>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Elena Reshetova <elena.reshetova@...el.com>, Alan Cox <alan@...ux.intel.com> Subject: Re: [PATCH v2 04/19] x86: implement ifence() On Thu, Jan 11, 2018 at 6:27 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote: > > Dan Williams <dan.j.williams@...el.com> writes: > > > The new barrier, 'ifence', ensures that no instructions past the > > boundary are speculatively executed. > > This needs a much better description. > > If that description was valid we could add ifence in the syscall > entry path and not have any speculative execution to worry about in the > kernel. True, I'll fix that up. > > Perhaps: > 'ifence', ensures that no speculative execution that reaches the 'ifence' > boundary continues past the 'ifence' boundary. > > > 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. > > This part of the description is probably unnecessary. Probably, but having some redundant information in the changelog eases 'git blame' archaeology expeditions in the future. > > Eric > > > > > Suggested-by: Peter Zijlstra <peterz@...radead.org> > > Suggested-by: Alan Cox <alan.cox@...el.com> > > Cc: Tom Lendacky <thomas.lendacky@....com> > > Cc: Mark Rutland <mark.rutland@....com> > > Cc: Greg KH <gregkh@...uxfoundation.org> > > Cc: Thomas Gleixner <tglx@...utronix.de> > > Cc: Ingo Molnar <mingo@...hat.com> > > Cc: "H. Peter Anvin" <hpa@...or.com> > > Cc: x86@...nel.org > > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com> > > Signed-off-by: Dan Williams <dan.j.williams@...el.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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