[PATCH v2 3/4] x86/signal: Prevent an alternate stack overflow before a signal delivery

[Bae, Chang Seok]

> On Nov 20, 2020, at 15:04, Jann Horn <jannh@google.com> wrote:
> 
> On Thu, Nov 19, 2020 at 8:40 PM Chang S. Bae <chang.seok.bae@intel.com> wrote:
>> 
>> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
>> index ee6f1ceaa7a2..cee41d684dc2 100644
>> --- a/arch/x86/kernel/signal.c
>> +++ b/arch/x86/kernel/signal.c
>> @@ -251,8 +251,13 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>> 
>>        /* This is the X/Open sanctioned signal stack switching.  */
>>        if (ka->sa.sa_flags & SA_ONSTACK) {
>> -               if (sas_ss_flags(sp) == 0)
>> +               if (sas_ss_flags(sp) == 0) {
>> +                       /* If the altstack might overflow, die with SIGSEGV: */
>> +                       if (!altstack_size_ok(current))
>> +                               return (void __user *)-1L;
>> +
>>                        sp = current->sas_ss_sp + current->sas_ss_size;
>> +               }
> 
> A couple lines further down, we have this (since commit 14fc9fbc700d):
> 
>        /*
>         * If we are on the alternate signal stack and would overflow it, don't.
>         * Return an always-bogus address instead so we will die with SIGSEGV.
>         */
>        if (onsigstack && !likely(on_sig_stack(sp)))
>                return (void __user *)-1L;
> 
> Is that not working?

onsigstack is set at the beginning here. If a signal hits under normal stack,
this flag is not set. Then it will miss the overflow.

The added check allows to detect the sigaltstack overflow (always).

Thanks,
Chang