Re: [RFC] SIGKILL vs. SIGSEGV on late execve() failures

From: Al Viro
Date: Fri Feb 15 2013 - 21:20:24 EST

On Sat, Feb 16, 2013 at 01:50:24AM +0000, Al Viro wrote:
> On Fri, Feb 15, 2013 at 04:46:43PM -0800, Shentino wrote:
> > On Fri, Feb 15, 2013 at 4:38 PM, Shentino <shentino@xxxxxxxxx> wrote:
> > > On Fri, Feb 15, 2013 at 4:04 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > >> How would you manage to have it masked at that point? setup_new_exec()
> > >> is inevitable after success of flush_old_exec() and it will do
> > >> flush_signal_handlers() for us.
> > >
> > > I wouldn't know for sure but I read somewhere that even if execve
> > > resets handled signals, it didn't also say that ignored signals were
> > > also reset. (Source: execve man page.)
> >
> > Also, apologies for the terminology mix-up. By masked, I mean that
> > the signal was ignored as directed by userspace a-la signal(SIGSEGV,
> > SIG_IGN).
> >
> > Plus I *think* that signal ignore masks are preserved across an exec.
> You are correct. OK, what it means is that we do need that force_sigsegv() -
> either there or in all places in ->load_binary() where we currently have
> send_sig_info(SIGSEGV). I don't think that it's an urgent hole, but yes,
> it is a bug. Nice catch.

Arrgh... OK, I'm a blind idiot. These places in binfmt_elf.c currently use
force_sig(), not send_sig_info(). Currently == since 2006 when somebody
noticed the problem. Their counterparts in binfmt_elf_fdpic.c were *not*
noticed. Anyway, that definitely means we want to do it in a single commit;
the only remaining question is whether we have any problems with somebody
ptracing such execve() and then poking the sucker with ptrace(); that _can_
happen with the current mainline for ELF binaries, so this is not something
new. I'm low on coffee and about to crash, so I might be missing some
horrible problem with it, but in this case I'm fairly sure that such a problem
would be present in current mainline.
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