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

[Eugene Syromiatnikov]

On Wed, Sep 11, 2019 at 04:16:36PM +0200, Christian Brauner wrote:
> On Wed, Sep 11, 2019 at 03:52:36PM +0200, Christian Brauner wrote:
> > On Wed, Sep 11, 2019 at 06:48:52AM -0700, Andrew Morton wrote:
> > > What are the user-visible runtime effects of this bug?

The userspace can set -1 as an exit_signal, and that will break process
signalling and reaping.

> > > 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.
> 
> Sigh, I spoke to soon... Hm, this is placed in _do_fork(). There's a
> chance that this might be visible in legacy clone if anyone passes in an
> invalid signal greater than NSIG right now somehow, they'd now get
> EINVAL if I'm seeing this right.
> 
> So an alternative might be to only fix this in clone3() only right now
> and get this patch into 5.3 to not release clone3() with this bug from
> legacy clone duplicated.
> And we defer the actual legacy clone fix until after next merge window
> having it stew in linux-next for a couple of rcs. Distros often pull in
> rcs so if anyone notices a regression for legacy clone we'll know about
> it... valid_signal() checks at process exit time when the parent is
> supposed to be notifed will catch faulty signals anyway so it's not that
> big of a deal.

As the patch is written, only copy_clone_args_from_user is touched (which
is used only by clone3 and not legacy clone), and the check added
replicates legacy clone behaviour: userspace can set 0..CSIGNAL
as an exit_signal.   Having ability to set exit_signal in NSIG..CSIGNAL
renge seems to be a bug, but at least it seems to be harmless one
and indeed may be addressed separately in the future.