On Wed, Jan 13, 2021 at 09:46:02AM -0800, Paul E. McKenney wrote:
> On Wed, Jan 13, 2021 at 07:18:23PM +0200, Jarkko Sakkinen wrote:
> > On Tue, Jan 12, 2021 at 07:35:50PM +0100, Borislav Petkov wrote:
> > > + paulmck.
> > > 
> > > On Tue, Jan 12, 2021 at 02:08:10AM +0200, Jarkko Sakkinen wrote:
> > > > On Tue, Jan 05, 2021 at 03:57:49PM +0100, Borislav Petkov wrote:
> > > > > On Wed, Dec 16, 2020 at 03:49:20PM +0200, Jarkko Sakkinen wrote:
> > > > > > Add synchronize_srcu_expedited() to sgx_encl_release() to catch a 
> > > > > > grace
> > > > > > period initiated by sgx_mmu_notifier_release().
> > > > > > 
> > > > > > A trivial example of a failing sequence with tasks A and B:
> > > > > > 
> > > > > > 1. A: -> sgx_release()
> > > > > > 2. B: -> sgx_mmu_notifier_release()
> > > > > > 3. B: -> list_del_rcu()
> > > > > > 3. A: -> sgx_encl_release()
> > > > > > 4. A: -> cleanup_srcu_struct()
> > > > > > 
> > > > > > The loop in sgx_release() observes an empty list because B has 
> > > > > > removed its
> > > > > > entry in the middle, and calls cleanup_srcu_struct() before B has a 
> > > > > > chance
> > > > > > to calls synchronize_srcu().
> > > > > 
> > > > > Leading to what? NULL ptr?
> > > > > 
> > > > > https://lkml.kernel.org/r/x9e2jowz1hfxv...@google.com
> > > > > 
> > > > > already suggested that you should explain the bug better and add the
> > > > > splat but I'm still missing that explanation.
> > > > 
> > > > OK, I'll try to explain it how I understand the issue.
> > > > 
> > > > Consider this loop in the VFS release hook (sgx_release):
> > > > 
> > > >         /*
> > > >          * Drain the remaining mm_list entries. At this point the list 
> > > > contains
> > > >          * entries for processes, which have closed the enclave file 
> > > > but have
> > > >          * not exited yet. The processes, which have exited, are gone 
> > > > from the
> > > >          * list by sgx_mmu_notifier_release().
> > > >          */
> > > >         for ( ; ; )  {
> > > >                 spin_lock(&encl->mm_lock);
> > > > 
> > > >                 if (list_empty(&encl->mm_list)) {
> > > >                         encl_mm = NULL;
> > > >                 } else {
> > > >                         encl_mm = list_first_entry(&encl->mm_list,
> > > >                                                    struct sgx_encl_mm, 
> > > > list);
> > > >                         list_del_rcu(&encl_mm->list);
> > > >                 }
> > > > 
> > > >                 spin_unlock(&encl->mm_lock);
> > > > 
> > > >                 /* The enclave is no longer mapped by any mm. */
> > > >                 if (!encl_mm)
> > > >                         break;
> > > > 
> > > >                 synchronize_srcu(&encl->srcu);
> > > >                 mmu_notifier_unregister(&encl_mm->mmu_notifier, 
> > > > encl_mm->mm);
> > > >                 kfree(encl_mm);
> > > >         }
> > > > 
> > > > 
> > > > At this point all processes have closed the enclave file, but that 
> > > > doesn't
> > > > mean that they all have exited yet.
> > > > 
> > > > Now, let's imagine that there is exactly one entry in the encl->mm_list.
> > > > and sgx_release() execution gets scheduled right after returning from
> > > > synchronize_srcu().
> > > > 
> > > > With some bad luck, some process comes and removes that last entry befoe
> > > > sgx_release() acquires mm_lock. The loop in sgx_release() just leaves
> > > > 
> > > >                 /* The enclave is no longer mapped by any mm. */
> > > >                 if (!encl_mm)
> > > >                         break;
> > > > 
> > > > No synchronize_srcu().
> > > > 
> > > > After writing this, I think that the placement for synchronize_srcu()
> > > > in this patch is not best possible. It should be rather that the
> > > > above loop would also call synchronize_srcu() when leaving.
> > > > 
> > > > I.e. the code change would result:
> > > > 
> > > >         for ( ; ; )  {
> > > >                 spin_lock(&encl->mm_lock);
> > > > 
> > > >                 if (list_empty(&encl->mm_list)) {
> > > >                         encl_mm = NULL;
> > > >                 } else {
> > > >                         encl_mm = list_first_entry(&encl->mm_list,
> > > >                                                    struct sgx_encl_mm, 
> > > > list);
> > > >                         list_del_rcu(&encl_mm->list);
> > > >                 }
> > > > 
> > > >                 spin_unlock(&encl->mm_lock);
> > > > 
> > > >                 /* 
> > > >                  * synchronize_srcu() is mandatory *even* when the list 
> > > > was
> > > >                  * empty, in order make sure that grace periods stays in
> > > >                  * sync even when another task took away the last entry
> > > >                  * (i.e. exiting process when it deletes its mm_list).
> > > >                  */
> > > >                 synchronize_srcu(&encl->srcu);
> > > > 
> > > >                 /* The enclave is no longer mapped by any mm. */
> > > >                 if (!encl_mm)
> > > >                         break;
> > > > 
> > > >                 mmu_notifier_unregister(&encl_mm->mmu_notifier, 
> > > > encl_mm->mm);
> > > >                 kfree(encl_mm);
> > > >         }
> > > > 
> > > > What do you think? Does this start to make more sense now?
> > > > I don't have logs for this but the bug can be also reasoned.
> > > 
> > > It does. Now you need to write it up in a detailed form so that it is
> > > clear to readers months/years from now what exactly can happen. You can
> > > use a two-column format like
> > > 
> > >   CPU A                           CPU B
> > > 
> > > Bla
> > >                                   Blu
> > > 
> > > This happens now here
> > >                                   But this needs to happen there
> > > 
> > > and so on.
> > > 
> > > Also, from reading up a bit on this, Documentation/RCU/checklist.rst says
> > > 
> > > "Use of the expedited primitives should be restricted to rare
> > > configuration-change operations that would not normally be undertaken
> > > while a real-time workload is running."
> > > 
> > > so why are you using synchronize_srcu_expedited()? Grepping the tree
> > > reveals only a couple of call sites only... but I've almost no clue of
> > > RCU so lemme CC Paul.
> > 
> > It spun out of this discussion:
> > 
> > https://lore.kernel.org/linux-sgx/20201215213517.ga34...@kernel.org/raw
> > 
> > My reasoning was that this is not a common case. The main loop
> > that uses synchronize_srcu().
> 
> It seems to me that loading and unloading SGX enclaves qualifies as a
> configuration operation, so use of synchronize_srcu_expedited() should be
> just fine in that case.  This of course implies that SGX enclaves should
> not be loaded or unloaded while an aggressive real-time application
> is running.  Which might well be the case for other reasons.
> 
> So I believe synchronize_srcu_expedited() should be fine in this case.
> 
>                                                       Thanx, Paul

Thank you for explaining this in detail.

I'll leave it out of the bug fix, and reconsider as a separate patch. I
think it should be fine to use it here but is really out-of-scope for the
change.

/Jarkko

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