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

[Andy Lutomirski <luto@amacapital.net>]

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.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.

We require asm goto support these days.  How about using that?  You
won't even need a special exception handler.

Also, please change the BUG to WARN in the you-did-it-wrong 32-bit
case.  And return -EFAULT.

--Andy