On Tue 16-04-24 19:27:05, Amir Goldstein wrote: > On Tue, Apr 16, 2024 at 4:22 PM Jan Kara <jack@xxxxxxx> wrote: > > > > On Mon 15-04-24 17:47:45, Amir Goldstein wrote: > > > On Mon, Apr 15, 2024 at 5:03 PM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > On Sat 13-04-24 12:32:32, Amir Goldstein wrote: > > > > > On Sat, Apr 13, 2024 at 11:45 AM Hillf Danton <hdanton@xxxxxxxx> wrote: > > > > > > On Fri, 12 Apr 2024 23:42:19 -0700 Amir Goldstein > > > > > > > On Sat, Apr 13, 2024 at 4:41=E2=80=AFAM Hillf Danton <hdanton@xxxxxxxx> wrote: > > > > > > > > On Thu, 11 Apr 2024 01:11:20 -0700 > > > > > > > > > syzbot found the following issue on: > > > > > > > > > > > > > > > > > > HEAD commit: 6ebf211bb11d Add linux-next specific files for 20240410 > > > > > > > > > git tree: linux-next > > > > > > > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=3D1621af9d180000 > > > > > > > > > > > > > > > > #syz test https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git 6ebf211bb11d > > > > > > > > > > > > > > > > --- x/fs/notify/fsnotify.c > > > > > > > > +++ y/fs/notify/fsnotify.c > > > > > > > > @@ -101,8 +101,8 @@ void fsnotify_sb_delete(struct super_blo > > > > > > > > wait_var_event(fsnotify_sb_watched_objects(sb), > > > > > > > > !atomic_long_read(fsnotify_sb_watched_objects(sb))); > > > > > > > > WARN_ON(fsnotify_sb_has_priority_watchers(sb, FSNOTIFY_PRIO_CONTENT)); > > > > > > > > - WARN_ON(fsnotify_sb_has_priority_watchers(sb, > > > > > > > > - FSNOTIFY_PRIO_PRE_CONTENT)); > > > > > > > > + WARN_ON(fsnotify_sb_has_priority_watchers(sb, FSNOTIFY_PRIO_PRE_CONTENT)); > > > > > > > > + synchronize_srcu(&fsnotify_mark_srcu); > > > > > > > > kfree(sbinfo); > > > > > > > > } > > > > > > > > > > > > > > > > @@ -499,7 +499,7 @@ int fsnotify(__u32 mask, const void *dat > > > > > > > > { > > > > > > > > const struct path *path =3D fsnotify_data_path(data, data_type); > > > > > > > > struct super_block *sb =3D fsnotify_data_sb(data, data_type); > > > > > > > > - struct fsnotify_sb_info *sbinfo =3D fsnotify_sb_info(sb); > > > > > > > > + struct fsnotify_sb_info *sbinfo; > > > > > > > > struct fsnotify_iter_info iter_info = {}; > > > > > > > > struct mount *mnt =3D NULL; > > > > > > > > struct inode *inode2 =3D NULL; > > > > > > > > @@ -529,6 +529,8 @@ int fsnotify(__u32 mask, const void *dat > > > > > > > > inode2_type =3D FSNOTIFY_ITER_TYPE_PARENT; > > > > > > > > } > > > > > > > > > > > > > > > > + iter_info.srcu_idx =3D srcu_read_lock(&fsnotify_mark_srcu); > > > > > > > > + sbinfo =3D fsnotify_sb_info(sb); > > > > > > > > /* > > > > > > > > * Optimization: srcu_read_lock() has a memory barrier which can > > > > > > > > * be expensive. It protects walking the *_fsnotify_marks lists. > > > > > > > > > > > > > > > > > > > > > See comment above. This kills the optimization. > > > > > > > It is not worth letting all the fsnotify hooks suffer the consequence > > > > > > > for the edge case of calling fsnotify hook during fs shutdown. > > > > > > > > > > > > Say nothing before reading your fix. > > > > > > > > > > > > > > Also, fsnotify_sb_info(sb) in fsnotify_sb_has_priority_watchers() > > > > > > > is also not protected and using srcu_read_lock() there completely > > > > > > > nullifies the purpose of fsnotify_sb_info. > > > > > > > > > > > > > > Here is a simplified fix for fsnotify_sb_error() rebased on the > > > > > > > pending mm fixes for this syzbot boot failure: > > > > > > > > > > > > > > #syz test: https://github.com/amir73il/linux fsnotify-fixes > > > > > > > > > > > > Feel free to post your patch at lore because not everyone has > > > > > > access to sites like github. > > > > > > > > > > > > > > Jan, > > > > > > > > > > > > > > I think that all the functions called from fs shutdown context > > > > > > > should observe that SB_ACTIVE is cleared but wasn't sure? > > > > > > > > > > > > If you composed fix based on SB_ACTIVE that is cleared in > > > > > > generic_shutdown_super() with &sb->s_umount held for write, > > > > > > I wonder what simpler serialization than srcu you could > > > > > > find/create in fsnotify. > > > > > > > > > > As far as I can tell there is no need for serialisation. > > > > > > > > > > The problem is that fsnotify_sb_error() can be called from the > > > > > context of ->put_super() call from generic_shutdown_super(). > > > > > > > > > > It's true that in the repro the thread calling fsnotify_sb_error() > > > > > in the worker thread running quota deferred work from put_super() > > > > > but I think there are sufficient barriers for this worker thread to > > > > > observer the cleared SB_ACTIVE flag. > > > > > > > > > > Anyway, according to syzbot, repro does not trigger the UAF > > > > > with my last fix. > > > > > > > > > > To be clear, any fsnotify_sb_error() that is a result of a user operation > > > > > would be holding an active reference to sb so cannot race with > > > > > fsnotify_sb_delete(), but I am not sure that same is true for ext4 > > > > > worker threads. > > > > > > > > > > Jan, > > > > > > > > > > You wrote that "In theory these two calls can even run in parallel > > > > > and fsnotify() can be holding fsnotify_sb_info pointer while > > > > > fsnotify_sb_delete() is freeing". > > > > > > > > > > Can you give an example of this case? > > > > > > > > Yeah, basically what Hilf writes: > > > > > > > > Task 1 Task 2 > > > > umount() some delayed work, transaction > > > > commit, whatever is still running > > > > before ext4_put_super() completes > > > > ... ext4_error() > > > > fsnotify_sb_error() > > > > fsnotify() > > > > fetches fsnotify_sb_info > > > > generic_shutdown_super() > > > > fsnotify_sb_delete() > > > > frees fsnotify_sb_info > > > > > > OK, so what do you say about Hillf's fix patch? > > > > > > Maybe it is ok to let go of the optimization in fsnotify(), considering > > > that we now have stronger optimizations in the inline hooks and > > > in __fsnotify_parent()? > > > > > > I think that Hillf's patch is missing setting s_fsnotify_info to NULL? > > > > > > @@ -101,8 +101,8 @@ void fsnotify_sb_delete(struct super_blo > > > wait_var_event(fsnotify_sb_watched_objects(sb), > > > !atomic_long_read(fsnotify_sb_watched_objects(sb))); > > > WARN_ON(fsnotify_sb_has_priority_watchers(sb, FSNOTIFY_PRIO_CONTENT)); > > > + WRITE_ONCE(sb->s_fsnotify_info, NULL); > > > + synchronize_srcu(&fsnotify_mark_srcu); > > > kfree(sbinfo); > > > } > > > > So I had a look into this. Yes, something like this should work. We'll see > > whether synchronize_srcu() won't slow down umount too much. If someone will > > complain, we'll have to find a better solution. > > > > Actually, kfree_rcu(sbinfo) may be enough. > We do not actually access sbinfo during mark iteration and > event handling, we only access it to get to the sb connector. > > Something like the attached patch? Hum, thinking about this some more - what if we just freed sb_info from destroy_super_work()? By then we definitely are not getting fsnotify() calls for the superblock so all the problems are solved. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
