linux-kernel - Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

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Message-ID: <DB68C825-25F9-48F9-AFAD-4F6C7DCA11F8@intel.com>
Date:   Mon, 12 Apr 2021 22:30:23 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Andy Lutomirski <luto@...nel.org>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "Gross, Jurgen" <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Jann Horn <jannh@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Carlos O'Donell <carlos@...hat.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        libc-alpha <libc-alpha@...rceware.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal
 stack overflow

On Mar 26, 2021, at 03:30, Borislav Petkov <bp@...en8.de> wrote:
> On Thu, Mar 25, 2021 at 09:56:53PM -0700, Andy Lutomirski wrote:
>> We really ought to have a SIGSIGFAIL signal that's sent, double-fault
>> style, when we fail to send a signal.
> 
> Yeap, we should be able to tell userspace that we couldn't send a
> signal, hohumm.

Hi Boris,

Let me clarify some details as preparing to include this in a revision.

So, IIUC, a number needs to be assigned for this new SIGFAIL. At a glance, not
sure which one to pick there in signal.h -- 1-31 fully occupied and the rest
for 33 different real-time signals.

Also, perhaps, force_sig(SIGFAIL) here, instead of return -1 -- to die with
SIGSEGV.

Thanks,
Chang