Re: [PATCH 14/20] exit/syscall_user_dispatch: Send ordinary signals on failure

[Andy Lutomirski]

On 10/21/21 09:25, Kees Cook wrote:
> On Wed, Oct 20, 2021 at 12:44:00PM -0500, Eric W. Biederman wrote:
> > Use force_fatal_sig instead of calling do_exit directly.  This ensures
> > the ordinary signal handling path gets invoked, core dumps as
> > appropriate get created, and for multi-threaded processes all of the
> > threads are terminated not just a single thread.
> >
> > When asked Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> said [1]:
> > > ebiederm@xxxxxxxxxxxx (Eric W. Biederman) asked:
> > >
> > > > Why does do_syscal_user_dispatch call do_exit(SIGSEGV) and
> > > > do_exit(SIGSYS) instead of force_sig(SIGSEGV) and force_sig(SIGSYS)?
> > > >
> > > > Looking at the code these cases are not expected to happen, so I would
> > > > be surprised if userspace depends on any particular behaviour on the
> > > > failure path so I think we can change this.
> > >
> > > Hi Eric,
> > >
> > > There is not really a good reason, and the use case that originated the
> > > feature doesn't rely on it.
> > >
> > > Unless I'm missing yet another problem and others correct me, I think
> > > it makes sense to change it as you described.
> > >
> > > > Is using do_exit in this way something you copied from seccomp?
> > >
> > > I'm not sure, its been a while, but I think it might be just that.  The
> > > first prototype of SUD was implemented as a seccomp mode.
> >
> > If at some point it becomes interesting we could relax
> > "force_fatal_sig(SIGSEGV)" to instead say
> > "force_sig_fault(SIGSEGV, SEGV_MAPERR, sd->selector)".
> >
> > I avoid doing that in this patch to avoid making it possible
> > to catch currently uncatchable signals.
> >
> > Cc: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > [1] https://lkml.kernel.org/r/87mtr6gdvi.fsf@xxxxxxxxxxxxx
> > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>
> Yeah, looks good. Should be no visible behavior change.
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

I'm confused.  Before this series, this error path would unconditionally 
kill the task (other than the race condition in force_sigsegv(), but at 
least a well-behaved task would get killed).  Now a signal handler might 
be invoked, and it would be invoked after the syscall that triggered the 
fault got processed as a no-op.  If the signal handler never returns, 
that's fine, but if the signal handler *does* return, the process might 
be in an odd state.  For SIGSYS, this behavior is probably fine, but 
having SIGSEGV swallow a syscall seems like a mistake.

Maybe rewind (approximately!) the syscall?  Or actually send SIGSYS?  Or 
actually make the signal uncatchable?

--Andy