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

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Message-ID: <724cf9e4b07b7d25135f3f1427f1c9fc@ispras.ru>
Date:   Fri, 14 Oct 2022 01:15:09 +0300
From:   Alexey Izbyshev <izbyshev@...ras.ru>
To:     Andrei Vagin <avagin@...gle.com>
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 2022-10-13 20:46, Andrei Vagin wrote:
> 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.
> 
Hi, Andrei,

While I think you could just skip check_in_thread() in the vfork()-child 
instead of removing check() completely (the rest of the code in check() 
is unlikely to mess up the libc state), I agree that the test is still 
able to catch problems unconditionally affecting all CLONE_VM tasks 
thanks to check_in_thread() in the parent, so I don't see much point in 
holding it up further. Your v2 patch looks good enough to me, thanks!

Alexey