linux-kernel - Re: [PATCH v2 12/19] x86/signal: Land on &frame->retcode when vdso isn't mapped

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Message-ID: <CALCETrWMFDqJrOrq3Z1LpryvysMsE=9b_Zm_aDMuqe4uRL8Wyw@mail.gmail.com>
Date:   Tue, 24 Nov 2020 15:08:49 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Guo Ren <guoren@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will@...nel.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2 12/19] x86/signal: Land on &frame->retcode when vdso
 isn't mapped

On Mon, Nov 23, 2020 at 4:30 PM Dmitry Safonov <dima@...sta.com> wrote:
>
> Since commit 9fbbd4dd17d0 ("x86: Don't require the vDSO for handling
> a.out signals") after processing 32-bit signal if there is no vdso
> mapped frame->retcode is used as a landing.
> Do the same for rt ia32 signals.
> It also makes the ia32 compat signals match the native ia32 case.
>
> This shouldn't be mistaken for encouragement for running binaries with
> executable stack, rather something to do in hopefully very rare
> situation with disabled or unmapped vdso and absent SA_RESTORER.
> For non-executable stack it'll segfault on attempt to land, rather than
> land on a random address where vdso was previously mapped.
> For programs with executable stack it'll just do the same for rt signals
> as for non-rt.
>
> Discouraging users to run with executable stack is done separately in
> commit 47a2ebb7f505 ("execve: warn if process starts with executable
> stack").
>
> Signed-off-by: Dmitry Safonov <dima@...sta.com>

The new code is more readable than the old code, too.

Acked-by: Andy Lutomirski <luto@...nel.org>