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? Thanks, Amir.
