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

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