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

[Amir Goldstein]

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.