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

[Eugene Syromiatnikov]

On Wed, Sep 11, 2019 at 04:54:47PM +0200, Christian Brauner wrote:
> On Wed, Sep 11, 2019 at 03:32:13PM +0100, Eugene Syromiatnikov wrote:
> > 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
> 
> Great!
> 
> > replicates legacy clone behaviour: userspace can set 0..CSIGNAL
> > as an exit_signal.   Having ability to set exit_signal in NSIG..CSIGNAL
> 
> Hm. The way I see it for clone3() it would make sense to only have <
> NSIG right away. valid_signal() won't let through anything else
> anyway... Since clone3() isn't out yet it doesn't make sense to
> replicate the (buggy) behavior of legacy clone, right?

I was thinking about that and in the end decided to go with CSIGNAL;
the only issue I see here is that it prevents for libc to easily
switch clone() library call implementation to clone3(), in case there
are some applications that rely on this kind of behaviour; if there's
no such userspace users, then switch to valid_signal() check for both
legacy and clone3 should be fine (along with possible switching to u64
for kernel_clone_args's exit_signal, so the check can done in one place),
but I'm agree with your hesitance to do it right now.

> Christian