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Date: Tue, 16 Apr 2024 15:22:07 +0200
From: Jan Kara <jack@suse.cz>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Hillf Danton <hdanton@sina.com>,
	syzbot <syzbot+5e3f9b2a67b45f16d4e6@syzkaller.appspotmail.com>,
	linux-fsdevel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
	linux-kernel@vger.kernel.org
Subject: Re: [syzbot] Re: [syzbot] [ext4?] KASAN: slab-use-after-free Read in
 fsnotify
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On Mon 15-04-24 17:47:45, Amir Goldstein wrote:
> On Mon, Apr 15, 2024 at 5:03 PM Jan Kara <jack@suse.cz> wrote:
> >
> > On Sat 13-04-24 12:32:32, Amir Goldstein wrote:
> > > On Sat, Apr 13, 2024 at 11:45 AM Hillf Danton <hdanton@sina.com> 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@sina.com> 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.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR