On Fri, Feb 26, 2021 at 5:08 AM Hillf Danton <hdanton@xxxxxxxx> wrote: > On Thu, 25 Feb 2021 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. > > > 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 > > 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 > > 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 > > 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 > > 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.
