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

["Bae, Chang Seok" <chang.seok.bae@intel.com>]

> On Nov 24, 2020, at 10:41, Jann Horn <jannh@google.com> wrote:
> 
> On Tue, Nov 24, 2020 at 7:22 PM Bae, Chang Seok
> <chang.seok.bae@intel.com> wrote:
>>> 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).
> 
> Ah, I think I understand what you're trying to do. But wouldn't the
> better approach be to ensure that the existing on_sig_stack() check is
> also used if we just switched to the signal stack? Something like:
> 
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index be0d7d4152ec..2f57842fb4d6 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -237,7 +237,7 @@ get_sigframe(struct k_sigaction *ka, struct
> pt_regs *regs, size_t frame_size,
>        unsigned long math_size = 0;
>        unsigned long sp = regs->sp;
>        unsigned long buf_fx = 0;
> -       int onsigstack = on_sig_stack(sp);
> +       bool onsigstack = on_sig_stack(sp);
>        int ret;
> 
>        /* redzone */
> @@ -246,8 +246,10 @@ 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) {
>                        sp = current->sas_ss_sp + current->sas_ss_size;
> +                       onsigstack = true;
> +               }
>        } else if (IS_ENABLED(CONFIG_X86_32) &&
>                   !onsigstack &&
>                   regs->ss != __USER_DS &&

Yeah, but wouldn't it better to avoid overwriting user data if we can? The old
check raises segfault *after* overwritten.

The old check is still helpful to detect an overflow from the nested signal(s)
under sigaltstack.

Thanks,
Chang