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From: "Bae, Chang Seok" <chang.seok.bae@intel.com>
To: Jann Horn <jannh@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
	Andy Lutomirski <luto@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"Brown, Len" <len.brown@intel.com>,
	"Hansen, Dave" <dave.hansen@intel.com>,
	"H.J. Lu" <hjl.tools@gmail.com>,
	"Dave Martin" <Dave.Martin@arm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	"Luck, Tony" <tony.luck@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	"libc-alpha@sourceware.org" <libc-alpha@sourceware.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	"kernel list" <linux-kernel@vger.kernel.org>,
	Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>,
	Roland McGrath <roland@redhat.com>,
	"Sang, Oliver" <oliver.sang@intel.com>
Subject: Re: [PATCH v2 3/4] x86/signal: Prevent an alternate stack overflow before a signal delivery
Date: Mon, 8 Feb 2021 20:29:50 +0000	[thread overview]
Message-ID: <81DF502F-9327-4365-AD17-21CFAE94ED0B@intel.com> (raw)
In-Reply-To: <CAG48ez1JK6pMT2UD1v0FwiCQq48FbE5Eb0d3tK=kK4Sg0TG7OQ@mail.gmail.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;

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

  parent reply	other threads:[~2021-02-08 21:28 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-19 19:02 [PATCH v2 0/4] x86: Improve Minimum Alternate Stack Size Chang S. Bae
2020-11-19 19:02 ` [PATCH v2 1/4] x86/signal: Introduce helpers to get the maximum signal frame size Chang S. Bae
2020-11-25 11:17   ` Borislav Petkov
2020-11-30 20:40     ` Bae, Chang Seok
2020-12-01 14:27       ` Borislav Petkov
2020-11-19 19:02 ` [PATCH v2 2/4] x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ Chang S. Bae
2020-11-26 17:44   ` Borislav Petkov
2020-11-27  9:30     ` Michael Kerrisk (man-pages)
2020-11-19 19:02 ` [PATCH v2 3/4] x86/signal: Prevent an alternate stack overflow before a signal delivery Chang S. Bae
2020-11-20 23:04   ` Jann Horn
2020-11-24 18:22     ` Bae, Chang Seok
2020-11-24 18:41       ` Jann Horn
2020-11-24 20:43         ` Bae, Chang Seok
2020-11-24 20:47           ` Jann Horn
2020-11-24 20:55             ` Bae, Chang Seok
2021-02-08 20:29         ` Bae, Chang Seok [this message]
2020-11-19 19:02 ` [PATCH v2 4/4] selftest/x86/signal: Include test cases for validating sigaltstack Chang S. Bae
2020-11-27 17:32   ` Borislav Petkov

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