On Wed, Jan 13, 2021 at 10:42:12PM -0600, Haitao Huang wrote: > On Mon, 11 Jan 2021 18:08:10 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx> > 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/X9e2jOWz1hfXVpQ5@xxxxxxxxxx > > > > > > 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. > > > > /Jarkko > > I did this experiment just now and find it runs much much slower than both > original code and code with synchronize_srcu_expedited fix in this patch. > Haitao Yeah, but using expedited is not really in the scope of a bug fix. It's a change that needs to be considered separately. And it's more complicated than that. It also needs data to back it up. How we can test and compare? /Jarkko
