linux-kernel - Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

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Message-ID: <B8EF2379-0AF6-4D00-B6B8-214CA9073BFC@intel.com>
Date:   Wed, 12 May 2021 18:48:11 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Andy Lutomirski <luto@...nel.org>, "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Dave.Martin@....com" <Dave.Martin@....com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "carlos@...hat.com" <carlos@...hat.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal
 stack overflow

On May 11, 2021, at 11:36, Borislav Petkov <bp@...en8.de> wrote:
> 
> I clumsily tried to register a SIGSEGV handler with
> 
>        act.sa_sigaction = my_sigsegv;
>        sigaction(SIGSEGV, &act, NULL);
> 
> but that doesn't fire - task gets killed. Maybe I'm doing it wrong.

Since the altstack is already overflowed, perhaps set the flag like this -- not
using it to get the handler:

	act.sa_sigaction = my_sigsegv;
+	act.sa_flags = SA_SIGINFO;
	sigaction(SIGSEGV, &act, NULL);

FWIW, I think this is just a workaround for this case; in practice, altstack is
rather a backup for normal stack corruption.

Thanks,
Chang