Re: [syzbot] Re: [syzbot] [ext4?] KASAN: slab-use-after-free Read in fsnotify

[Hillf Danton]

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.