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

[Yu-cheng Yu]

On Thu, 2018-06-07 at 09:40 -0700, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> >  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;
> > +
> 
> Please add a comment indicating what exact opcode this is.

I will fix it.

> 
> Peterz, isn't there some fancy better way we're supposed to handle the
> error return these days?
> 
> > +       asm volatile("1:.byte 0x66, 0x0f, 0x38, 0xf5, 0x37\n"
> > +                    "xor %[err],%[err]\n"
> > +                    "2:\n"
> > +                    ".section .fixup,\"ax\"\n"
> > +                    "3: mov $-1,%[err]; jmp 2b\n"
> > +                    ".previous\n"
> > +                    _ASM_EXTABLE(1b, 3b)
> > +               : [err] "=a" (err)
> > +               : [val] "S" (val), [addr] "D" (addr)
> > +               : "memory");
> > +       return err;
> > +}
> > +#else
> > +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> > +{
> > +       return 0;
> 
> BUG()?  Or just omit the ifdef?  It seems unhelpful to have a stub
> function that does nothing.

I will fix it.

> 
> > +}
> > +#endif
> > +
> > +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> > +{
> > +       int err;
> > +
> 
> Comment here too, please.

OK.

> 
> > +       asm volatile("1:.byte 0x66, 0x48, 0x0f, 0x38, 0xf5, 0x37\n"
> > +                    "xor %[err],%[err]\n"
> > +                    "2:\n"
> > +                    ".section .fixup,\"ax\"\n"
> > +                    "3: mov $-1,%[err]; jmp 2b\n"
> > +                    ".previous\n"
> > +                    _ASM_EXTABLE(1b, 3b)
> > +               : [err] "=a" (err)
> > +               : [val] "S" (val), [addr] "D" (addr)
> > +               : "memory");
> > +       return err;
> > +}
> > +#endif /* CONFIG_X86_INTEL_CET */
> > +
> >  #define nop() asm volatile ("nop")
> >
> >