linux-kernel - Re: [PATCH] fs: f2fs: fix UAF in f2fs_available_free

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Date:   Thu, 4 Nov 2021 15:35:24 +0800
From:   Dongliang Mu <mudongliangabcd@...il.com>
To:     Chao Yu <chao@...nel.org>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs: f2fs: fix UAF in f2fs_available_free_memory

On Thu, Nov 4, 2021 at 3:16 PM Chao Yu <chao@...nel.org> wrote:
>
> On 2021/11/3 22:22, Dongliang Mu wrote:
> > f2fs_fill_super
> > -> f2fs_build_segment_manager
> >     -> create_discard_cmd_control
> >        -> f2fs_start_discard_thread
> >
> > It invokes kthread_run to create a thread and run issue_discard_thread.
> >
> > However, if f2fs_build_node_manager fails, the control flow goes to
> > free_nm and calls f2fs_destroy_node_manager. This function will free
> > sbi->nm_info. However, if issue_discard_thread accesses sbi->nm_info
> > after the deallocation, but before the f2fs_stop_discard_thread, it will
> > cause UAF(Use-after-free).
> >
> > -> f2fs_destroy_segment_manager
> >     -> destroy_discard_cmd_control
> >        -> f2fs_stop_discard_thread
> >
> > Fix this by switching the order of f2fs_build_segment_manager
> > and f2fs_build_node_manager.
> >
> > Signed-off-by: Dongliang Mu <mudongliangabcd@...il.com>
> > ---
> >   fs/f2fs/super.c | 16 ++++++++--------
> >   1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> > index 78ebc306ee2b..1a23b64cfb74 100644
> > --- a/fs/f2fs/super.c
> > +++ b/fs/f2fs/super.c
> > @@ -4135,18 +4135,18 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
> >       }
> >
> >       /* setup f2fs internal modules */
> > -     err = f2fs_build_segment_manager(sbi);
> > -     if (err) {
> > -             f2fs_err(sbi, "Failed to initialize F2FS segment manager (%d)",
> > -                      err);
> > -             goto free_sm;
> > -     }
> >       err = f2fs_build_node_manager(sbi);
> >       if (err) {
> >               f2fs_err(sbi, "Failed to initialize F2FS node manager (%d)",
> >                        err);
> >               goto free_nm;
> >       }
> > +     err = f2fs_build_segment_manager(sbi);
> > +     if (err) {
> > +             f2fs_err(sbi, "Failed to initialize F2FS segment manager (%d)",
> > +                      err);
> > +             goto free_sm;
> > +     }
> >
> >       /* For write statistics */
> >       sbi->sectors_written_start = f2fs_get_sectors_written(sbi);
> > @@ -4351,10 +4351,10 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
> >       sbi->node_inode = NULL;
> >   free_stats:
> >       f2fs_destroy_stats(sbi);
> > -free_nm:
> > -     f2fs_destroy_node_manager(sbi);
> >   free_sm:
> >       f2fs_destroy_segment_manager(sbi);
> > +free_nm:
> > +     f2fs_destroy_node_manager(sbi);
>
> IIRC, above two functions shouldn't not be called reversely due to some
> resource dependency, Jaegeuk, please help to confirm this.
>
> So I suggest to call destroy_discard_cmd_control() before
> f2fs_destroy_node_manager(), is it fine to you?

Maybe f2fs_stop_discard_thread is better than
destroy_discard_cmd_control. It only stops the kthread, leading to
fewer side effects.

How do you think?

>
> Thanks,
>
> >       f2fs_destroy_post_read_wq(sbi);
> >   stop_ckpt_thread:
> >       f2fs_stop_ckpt_thread(sbi);
> >