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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Sep 11, 2019 at 06:48:52AM -0700, Andrew Morton wrote:
> 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?

No, as I said in my other mail clone3() is not in any released kernel
yet. clone3() is going to be released in v5.3.

Christian