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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>,
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        "Dave Martin" <Dave.Martin@arm.com>,
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        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
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        Linux API <linux-api@vger.kernel.org>,
        "kernel list" <linux-kernel@vger.kernel.org>,
        Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>,
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Subject: Re: [PATCH v2 3/4] x86/signal: Prevent an alternate stack overflow
 before a signal delivery
Thread-Topic: [PATCH v2 3/4] x86/signal: Prevent an alternate stack overflow
 before a signal delivery
Thread-Index: AQHWvqcjVskyJoXuH0ORuPNb6HHBoanRpbwAgAX6YYA=
Date:   Tue, 24 Nov 2020 18:22:06 +0000
Message-ID: <B2D7D498-D118-447E-93C6-DB03D42CBA4E@intel.com>
References: <20201119190237.626-1-chang.seok.bae@intel.com>
 <20201119190237.626-4-chang.seok.bae@intel.com>
 <CAG48ez1aKtwYMEHfGX6_FuX9fOruwvCqEGYVL8eLdV8bg-wHCQ@mail.gmail.com>
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> On Nov 20, 2020, at 15:04, Jann Horn <jannh@google.com> wrote:
>=20
> On Thu, Nov 19, 2020 at 8:40 PM Chang S. Bae <chang.seok.bae@intel.com> w=
rote:
>>=20
>> 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,
>>=20
>>        /* This is the X/Open sanctioned signal stack switching.  */
>>        if (ka->sa.sa_flags & SA_ONSTACK) {
>> -               if (sas_ss_flags(sp) =3D=3D 0)
>> +               if (sas_ss_flags(sp) =3D=3D 0) {
>> +                       /* If the altstack might overflow, die with SIGS=
EGV: */
>> +                       if (!altstack_size_ok(current))
>> +                               return (void __user *)-1L;
>> +
>>                        sp =3D current->sas_ss_sp + current->sas_ss_size;
>> +               }
>=20
> A couple lines further down, we have this (since commit 14fc9fbc700d):
>=20
>        /*
>         * 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 SIGS=
EGV.
>         */
>        if (onsigstack && !likely(on_sig_stack(sp)))
>                return (void __user *)-1L;
>=20
> Is that not working?

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

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

Thanks,
Chang=