linux-kernel - Re: [PATCH 2/2] selftests/timens: add a test for vfork+exit

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Message-ID: <CAEWA0a6DHqZOduKhJi7o12RprGt2LGqGOC86TKN1bTXn36u7hw@mail.gmail.com>
Date:   Thu, 13 Oct 2022 10:46:30 -0700
From:   Andrei Vagin <avagin@...gle.com>
To:     Alexey Izbyshev <izbyshev@...ras.ru>
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
        Andrei Vagin <avagin@...il.com>,
        Christian Brauner <brauner@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH 2/2] selftests/timens: add a test for vfork+exit

On Sun, Oct 9, 2022 at 9:10 AM Alexey Izbyshev <izbyshev@...ras.ru> wrote:
>
> On 2022-09-21 03:31, Andrei Vagin wrote:
> > From: Andrei Vagin <avagin@...il.com>

<snip>

> > +     if (pid == 0) {
> > +             char now_str[64];
> > +             char *cargv[] = {"exec", now_str, NULL};
> > +             char *cenv[] = {NULL};
> > +
> > +             // Check that we are still in the source timens.
> > +             if (check("child before exec", &now))
> > +                     return 1;
>
> I know this is just a test, but...
>
> Creating threads in a vfork()-child is quite dangerous (like most other
> things that touch the libc state, which is shared with the parent
> process). Here it works probably only because pthread_create() followed
> by pthread_join() restores everything into more-or-less the original
> state before returning control to the parent, but this is something that
> libcs don't guarantee and that can break at any moment.
>
> Also, returning from a vfork()-child is explicitly forbidden by the
> vfork() contract because the parent would then return to an invalid
> stack frame that could be arbitrarily clobbered by code executed in the
> child after main() returned. Moreover, if I'm not mistaken, on x86 with
> Intel CET-enabled glibc (assuming the support for CET is ever merged
> into the kernel) such return would cause the parent to always trap
> because the shadow stack will become inconsistent with the normal stack.
> Instead, _exit() should be used here...
>

Hi Alexey,

You are right, it isn't a good idea to create threads from the vfork-ed
process. Now, vfork isn't a special case in the kernel code, so I think
we can just remove the check() call from here. I have sent an updated
version of this patch, pls take a look at it.

Thanks,
Andrei