Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

On Thu, 2018-06-14 at 11:30 +1000, Balbir Singh wrote:
> On Thu, 2018-06-07 at 07:37 -0700, Yu-cheng Yu wrote:
> > WRUSS is a new kernel-mode instruction but writes directly
> > to user shadow stack memory.  This is used to construct
> > a return address on the shadow stack for the signal
> > handler.
> > 
> > This instruction can fault if the user shadow stack is
> > invalid shadow stack memory.  In that case, the kernel does
> > fixup.
> > 
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > ---
> >  arch/x86/include/asm/special_insns.h          | 44 +++++++++++++++++++++++++++
> >  arch/x86/lib/x86-opcode-map.txt               |  2 +-
> >  arch/x86/mm/fault.c                           | 13 +++++++-
> >  tools/objtool/arch/x86/lib/x86-opcode-map.txt |  2 +-
> >  4 files changed, 58 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> > index 317fc59b512c..8ce532fcc171 100644
> > --- a/arch/x86/include/asm/special_insns.h
> > +++ b/arch/x86/include/asm/special_insns.h
> > @@ -237,6 +237,50 @@ static inline void clwb(volatile void *__p)
> >  		: [pax] "a" (p));
> >  }
> >  
> > +#ifdef CONFIG_X86_INTEL_CET
> > +
> > +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> > +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> > +{
> > +	int err;
> > +
> > +	asm volatile("1:.byte 0x66, 0x0f, 0x38, 0xf5, 0x37\n"
> 
> It would nice to use something like ASM_WRUSS/Q like ASM_CLAC/ASM_STAC.
> Is the 0x37 spurious? I don't see addr/val being used in the instructions
> either.
> 

Yes, this is being revised.  We are going to require a GCC and binutils
that support CET.  I will put in the WRUSS instruction, no '.byte' any
more.

Yu-cheng