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

[Jann Horn <jannh@google.com>]

On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@intel.com> 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.

> @@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>                 error_code |= X86_PF_USER;
>                 flags |= FAULT_FLAG_USER;
>         } else {
> +               /*
> +                * WRUSS is a kernel instrcution and but writes

Nits: typo ("instrcution"), weird grammar ("and but writes")