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? ;) -- paul moore www.paul-moore.com