Re: [BUG] Race between policy reload sidtab conversion and live conversion
From: Ondrej Mosnacek
Date: Fri Feb 26 2021 - 06:21:36 EST
On Fri, Feb 26, 2021 at 5:08 AM Hillf Danton <hdanton@xxxxxxxx> wrote:
>
On 2021年2月25日 20:06:45 -0500 Paul Moore wrote:
>
> On Wed, Feb 24, 2021 at 4:35 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
>
> > After the switch to RCU, we now have:
>
> > 1. Start live conversion of new entries.
>
> > 2. Convert existing entries.
>
> > 3. RCU-assign the new policy pointer to selinux_state.
>
> > [!!! Now actually both old and new sidtab may be referenced by
>
> > readers, since there is no synchronization barrier previously provided
>
> > by the write lock.]
>
> > 4. Wait for synchronize_rcu() to return.
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> > 5. Now only the new sidtab is visible to readers, so the old one can
>
> > be destroyed.
>
> >
>
> > So the race can happen between 3. and 5., if one thread already sees
>
> > the new sidtab and adds a new entry there, and a second thread still
>
> > has the reference to the old sidtab and also tires to add a new entry;
>
> > live-converting to the new sidtab, which it doesn't expect to change
>
> > by itself. Unfortunately I failed to realize this when reviewing the
>
> > patch :/
>
>
>
> It is possible I'm not fully understanding the problem and/or missing
>
> an important detail - it is rather tricky code, and RCU can be very
>
> hard to reason at times - but I think we may be able to solve this
>
> with some lock fixes inside sidtab_context_to_sid(). Let me try to
>
> explain to see if we are on the same page here ...
>
>
>
> The problem is when we have two (or more) threads trying to
>
> add/convert the same context into a sid; the task with new_sidtab is
>
> looking to add a new sidtab entry, while the task with old_sidtab is
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> looking to convert an entry in old_sidtab into a new entry in
>
> new_sidtab. Boom.
>
>
>
> Looking at the code in sidtab_context_to_sid(), when we have two
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> sidtabs that are currently active (old_sidtab->convert pointer is
>
> valid) and a task with old_sidtab attempts to add a new entry to both
>
> sidtabs it first adds it to the old sidtab then it also adds it to the
>
> new sidtab. I believe the problem is that in this case while the task
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> grabs the old_sidtab->lock, it never grabs the new_sidtab->lock which
>
> allows it to race with tasks that already see only new_sidtab. I
>
> think adding code to sidtab_context_to_sid() which grabs the
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> new_sidtab->lock when adding entries to the new_sidtab *should* solve
>
> the problem.
>
>
>
> Did I miss something important? ;)
>
>
If the convert pointer can be derefered without lock, we can opt to
>
convert context after building sidtab with the risk of AB BA deadlock
>
cut. Below is the minimum change I can think of along your direction.
We could fix this a bit more easily by just having a shared spinlock
for both (well, *all*) sidtabs. Yes, we'd need to have it all the way
up in selinux_state and pass it through to sidtab_init(), but IMHO
that's less bad than trying to get it right with two locks.
--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.