Re: [RFC PATCH v3 19/24] x86/cet/shstk: Introduce WRUSS instruction

[Andy Lutomirski <luto@kernel.org>]

On Fri, Aug 31, 2018 at 2:49 PM, Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> On Thu, 2018-08-30 at 09:22 -0700, Yu-cheng Yu wrote:
>> On Thu, 2018-08-30 at 08:55 -0700, Andy Lutomirski wrote:
>> >
>> > On Thu, Aug 30, 2018 at 8:39 AM, Jann Horn <jannh@google.com>
>> > wrote:
>> > >
>> > >
>> > > On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@intel.c
>> > > om
>> > > >
>> > > > 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>
>> > > [...]
>> > > >
>> > > >
>> > > > +static inline int write_user_shstk_64(unsigned long addr,
>> > > > unsigned long val)
>> > > > +{
>> > > > +       int err = 0;
>> > > > +
>> > > > +       asm volatile("1: wrussq %1, (%0)\n"
>> > > > +                    "2:\n"
>> > > > +                    _ASM_EXTABLE_HANDLE(1b, 2b,
>> > > > ex_handler_wruss)
>> > > > +                    :
>> > > > +                    : "r" (addr), "r" (val));
>> > > > +
>> > > > +       return err;
>> > > > +}
>> > > What's up with "err"? You set it to zero, and then you return
>> > > it,
>> > > but
>> > > nothing can ever set it to non-zero, right?
>> > >
>> > > >
>> > > >
>> > > > +__visible bool ex_handler_wruss(const struct
>> > > > exception_table_entry *fixup,
>> > > > +                               struct pt_regs *regs, int
>> > > > trapnr)
>> > > > +{
>> > > > +       regs->ip = ex_fixup_addr(fixup);
>> > > > +       regs->ax = -1;
>> > > > +       return true;
>> > > > +}
>> > > And here you just write into regs->ax, but your "asm volatile"
>> > > doesn't
>> > > reserve that register. This looks wrong to me.
>> > >
>> > > I think you probably want to add something like an explicit
>> > > `"+&a"(err)` output to the asm statements.
>> > We require asm goto support these days.  How about using
>> > that?  You
>> > won't even need a special exception handler.
>
> Maybe something like this?  It looks simple now.
>
> static inline int write_user_shstk_64(unsigned long addr, unsigned
> long val)
> {
>         asm_volatile_goto("wrussq %1, (%0)\n"
>                      "jmp %l[ok]\n"
>                      ".section .fixup,\"ax\"n"
>                      "jmp %l[fail]\n"
>                      ".previous\n"
>                      :: "r" (addr), "r" (val)
>                      :: ok, fail);
> ok:
>         return 0;
> fail:
>         return -1;
> }
>

I think you can get rid of 'jmp %l[ok]' and the ok label and just fall
through.  And you don't need an explicit jmp to fail -- just set the
_EX_HANDLER entry to land on the fail label.