Hangup calling setuid() from vfork() child

Hangup calling setuid() from vfork() child

[Joshua Hudson]

If there is more than one thread and vfork() calls setuid(), musl libc hangs up.

void *thfunction(void*ig) {sleep(1000);returnNULL;}

int main()
{
    pthread_t id;
    pthread_create(&id, NULL, thfunction, NULL);
    if (vfork() == 0) {
        setuid(0); /* hangup */
        _exit(0);
    }
}

Re: Hangup calling setuid() from vfork() child

[Markus Wichmann]

On Mon, Sep 30, 2019 at 08:29:16AM -0700, Joshua Hudson wrote:
> If there is more than one thread and vfork() calls setuid(), musl libc hangs up.
>
> void *thfunction(void*ig) {sleep(1000);returnNULL;}
>
> int main()
> {
>     pthread_t id;
>     pthread_create(&id, NULL, thfunction, NULL);
>     if (vfork() == 0) {
>         setuid(0); /* hangup */
>         _exit(0);
>     }
> }

That is an interesting interaction between threads and vfork().

The child process has only one thread, but it doesn't know that. It also
can't write it down, since it is sharing memory with the parent (it
would overwrite the parent's variables).

POSIX no longer defines vfork(), and therefore does not define any
safety attributes for it. Is it reasonable to define vfork() as unusable
in a multithreaded process? Calling something as intricate as
__synccall() in a vfork() child is going to corrupt memory on a large
scale.

posix_spawn() circumvents the problem by calling the system calls
directly, BTW.

Ciao,
Markus

Re: Hangup calling setuid() from vfork() child

[Rich Felker]

On Mon, Sep 30, 2019 at 08:29:16AM -0700, Joshua Hudson wrote:
> If there is more than one thread and vfork() calls setuid(), musl libc hangs up.
> 
> void *thfunction(void*ig) {sleep(1000);returnNULL;}
> 
> int main()
> {
>     pthread_t id;
>     pthread_create(&id, NULL, thfunction, NULL);
>     if (vfork() == 0) {
>         setuid(0); /* hangup */
>         _exit(0);
>     }
> }

This is expected; the only legal action after vfork is _exit or
execve. In practice you could probably get by with
syscall(SYS_setuid,0) or similar in the child, but this isn't
supported usage and the specification for vfork has always been clear
that you can't do arbitrary stuff in the child. If you need to, you
should be using fork.

Rich

Re: Hangup calling setuid() from vfork() child

[Rich Felker]

On Mon, Sep 30, 2019 at 07:39:28PM +0200, Markus Wichmann wrote:
> On Mon, Sep 30, 2019 at 08:29:16AM -0700, Joshua Hudson wrote:
> > If there is more than one thread and vfork() calls setuid(), musl libc hangs up.
> >
> > void *thfunction(void*ig) {sleep(1000);returnNULL;}
> >
> > int main()
> > {
> >     pthread_t id;
> >     pthread_create(&id, NULL, thfunction, NULL);
> >     if (vfork() == 0) {
> >         setuid(0); /* hangup */
> >         _exit(0);
> >     }
> > }
> 
> That is an interesting interaction between threads and vfork().
> 
> The child process has only one thread, but it doesn't know that. It also
> can't write it down, since it is sharing memory with the parent (it
> would overwrite the parent's variables).
> 
> POSIX no longer defines vfork(), and therefore does not define any
> safety attributes for it. Is it reasonable to define vfork() as unusable
> in a multithreaded process? Calling something as intricate as
> __synccall() in a vfork() child is going to corrupt memory on a large
> scale.

It's simpler than that. The (retired) specification for vfork did not
allow anything but _exit or execve in the child after vfork, so the
issue doesn't arise and it works perfectly fine with threads as long
as you follow the requirement.

> posix_spawn() circumvents the problem by calling the system calls
> directly, BTW.

Yes, posix_spawn should be used if possible. It even has an attribute
to reset ids to the real ones.

Rich

Re: Hangup calling setuid() from vfork() child

[Joshua Hudson]

> Basically, the vfork() child is in an invalid state and this cannot be repaired without damaging the parent.

Works on glibc just fine.

setuid() is on the list of signal-safe functions.

http://man7.org/linux/man-pages/man7/signal-safety.7.html

How about you call getpid() and check if you're on the process you
think you're on before calling __synccall? Somebody else might have
done syscall(SYS_fork).

> So you might want to enable memory overcommit.

I'm tired of paying the page fault penalty in the parent. It has a
majority of system RAM, and most of the pages are CoW long after the
vfork child hits execve.

Re: Hangup calling setuid() from vfork() child

[Joshua Hudson]

>It's simpler than that. The (retired) specification for vfork did not
>allow anything but _exit or execve in the child after vfork, so the
>issue doesn't arise and it works perfectly fine with threads as long
>as you follow the requirement.

I'm reading the man page for vfork and it says what it actually does, that
is overlay the child process on the memory of the calling process.

posix_spawn can't be used in the originating location, and fork() is
hogging too much memory.