Messages in this thread | | | Subject | Re: [PATCH] x86/retpoline: Also fill return buffer after idle | From | David Woodhouse <> | Date | Tue, 09 Jan 2018 00:00:09 +0000 |
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On Mon, 2018-01-08 at 15:51 -0800, Andi Kleen wrote: > From: Andi Kleen <ak@linux.intel.com> > > This is an extension of the earlier patch to fill the return buffer > on context switch. It uses the assembler macros added earlier. > > When we go into deeper idle states the return buffer could be cleared > in MWAIT, but then another thread which wakes up earlier might > be poisoning the indirect branch predictor. Then when the return > buffer underflows there might an uncontrolled indirect branch. > > To guard against this always fill the return buffer when exiting idle. > > Needed on Skylake and some Broadwells. > > Signed-off-by: Andi Kleen <ak@linux.intel.com> > --- > arch/x86/entry/entry_32.S | 8 ++++++++ > arch/x86/entry/entry_64.S | 8 ++++++++ > arch/x86/include/asm/mwait.h | 11 ++++++++++- > 3 files changed, 26 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S > index 7dee84a3cf83..2687cce8a02e 100644 > --- a/arch/x86/entry/entry_32.S > +++ b/arch/x86/entry/entry_32.S > @@ -1092,3 +1092,11 @@ ENTRY(rewind_stack_do_exit) > call do_exit > 1: jmp 1b > END(rewind_stack_do_exit) > + > +ENTRY(fill_return_buffer) > +#ifdef CONFIG_RETPOLINE > + ALTERNATIVE "ret", "", X86_FEATURE_RETPOLINE > + FILL_RETURN_BUFFER > +#endif > + ret > +END(fill_return_buffer) > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index a33033e2bfe0..92fbec1b0eb5 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -1831,3 +1831,11 @@ ENTRY(rewind_stack_do_exit) > > call do_exit > END(rewind_stack_do_exit) > + > +ENTRY(fill_return_buffer) > +#ifdef CONFIG_RETPOLINE > + ALTERNATIVE "ret", "", X86_FEATURE_RETPOLINE > + FILL_RETURN_BUFFER > +#endif > + ret > +END(fill_return_buffer) > diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h > index 39a2fb29378a..1d9f9269b5e7 100644 > --- a/arch/x86/include/asm/mwait.h > +++ b/arch/x86/include/asm/mwait.h > @@ -87,6 +87,8 @@ static inline void __sti_mwait(unsigned long eax, unsigned long ecx) > :: "a" (eax), "c" (ecx)); > } > > +extern __visible void fill_return_buffer(void); > + > /* > * This uses new MONITOR/MWAIT instructions on P4 processors with PNI, > * which can obviate IPI to trigger checking of need_resched. > @@ -107,8 +109,15 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx) > } > > __monitor((void *)¤t_thread_info()->flags, 0, 0); > - if (!need_resched()) > + if (!need_resched()) { > __mwait(eax, ecx); > + /* > + * idle could have cleared the return buffer, > + * so fill it to prevent uncontrolled > + * speculation. > + */ > + fill_return_buffer(); > + } > } > current_clr_polling(); > }
Probably doesn't matter right there but it's going to end up being used elsewhere with IBRS/IBPB, and the compiler is going to think it needs to save all the call-clobbered registers for that. Do we want to make it use inline asm instead?[unhandled content-type:application/x-pkcs7-signature] | |