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

FWIW, here.

Thanks to the report by Oliver via the kernel test robot, I realized that
this needs to be conditional on the SS_AUTODISARM tag like, :

onsigstack = !(current->sas_ss_flags & SS_AUTODISARM);

Thanks,
Chang
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