Re: [PATCH v2] fork: check exit_signal passed in clone3() call

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 Sep 2019 18:58:52 +0100 Eugene Syromiatnikov <esyr@redhat.com> wrote:

> Previously, higher 32 bits of exit_signal fields were lost when
> copied to the kernel args structure (that uses int as a type for the
> respective field).  Moreover, as Oleg has noted[1], exit_signal is used
> unchecked, so it has to be checked for sanity before use; for the legacy
> syscalls, applying CSIGNAL mask guarantees that it is at least non-negative;
> however, there's no such thing is done in clone3() code path, and that can
> break at least thread_group_leader.
> 
> Checking user-passed exit_signal against ~CSIGNAL mask solves both
> of these problems.
> 
> [1] https://lkml.org/lkml/2019/9/10/467
> 
> * kernel/fork.c (copy_clone_args_from_user): Fail with -EINVAL if
> args.exit_signal has bits set outside CSIGNAL mask.
> (_do_fork): Note that exit_signal is expected to be checked for the
> sanity by the caller.
> 
> Fixes: 7f192e3cd316 ("fork: add clone3")

What are the user-visible runtime effects of this bug?

Relatedly, should this fix be backported into -stable kernels?  If so, why?