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 12:47, Jann Horn <jannh@google.com> wrote:
> 
> On Tue, Nov 24, 2020 at 9:43 PM Bae, Chang Seok
> <chang.seok.bae@intel.com> wrote:
>>> 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.
> 
> Where is that overwrite happening? Between the point where your check
> happens, and the point where the old check is, the only calls are to
> fpu__alloc_mathframe() and align_sigframe(), right?
> fpu__alloc_mathframe() just does some size calculations and doesn't
> write anything. align_sigframe() also just does size calculations. Am
> I missing something?

Yeah, you’re right. Right now, I’m thinking your approach is simpler and
providing almost the same function (unless I’m missing here).

Thanks,
Chang