linux-kernel - Re: [PATCH 2/3] kill MMF_DUMPABLE and MMF_DUMP

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Date:	Mon, 18 Nov 2013 20:16:00 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"security@...nel.org" <security@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vasily Kulikov <segoon@...nwall.com>,
	Petr Matousek <pmatouse@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Alex Kelly <alex.page.kelly@...il.com>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 2/3] kill MMF_DUMPABLE and MMF_DUMP_SECURELY

On 11/18, Kees Cook wrote:
>
> On Sat, Nov 16, 2013 at 11:01 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> > @@ -1629,24 +1628,13 @@ void set_dumpable(struct mm_struct *mm, int value)
> >
> >         do {
> >                 old = ACCESS_ONCE(mm->flags);
> > -               new = old & ~MMF_DUMPABLE_MASK;
> > -
> > -               switch (value) {
> > -               case SUID_DUMP_ROOT:
> > -                       new |= (1 << MMF_DUMP_SECURELY);
> > -               case SUID_DUMP_USER:
> > -                       new |= (1<< MMF_DUMPABLE);
> > -               }
> > -
> > +               new = (old & ~MMF_DUMPABLE_MASK) | value;
>
> Just to make this safe against insane callers, perhaps mask the value as well?

Well yes, before this patch set_dumpable() silently ignored the wrong
value, perhaps you are right but see below.

>     new = (old & ~MMF_DUMPABLE_MASK) | (value & MMF_DUMPABLE_MASK);
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^

this doesn't really help, with this patch "mm->flags & MMF_DUMPABLE_MASK"
has a room for yet another SUID_DUMP == 4 we do not have yet.

And I don't really like the "silently ignore" logic, so perhaps

		if (WARN_ON(value > SUID_DUMP_ROOT))
			return;

at the start makes more sense?

Or perhaps we do not really need the additional check? suid_dumpable
is always sane, other callers can't use the wrong value.

But I am fine either way, please tell me what do you prefer.

Oleg.

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