On Tue 27-11-18 17:57:12, PanBian wrote:
> On Tue, Nov 27, 2018 at 10:25:51AM +0100, Jan Kara wrote:
> > On Sun 25-11-18 08:15:23, Pan Bian wrote:
> > > After calling dput(new_dentry), new_dentry is passed to fsnotify_move.
> > > This may result in a use-after-free bug. This patch moves the put
> > > operation late.
> > >
> > > Fixes: da1ce0670c14("vfs: add cross-rename")
> > > Signed-off-by: Pan Bian <bianpan2...@163.com>
> >
> > The code is actually fine AFAICT. One new_dentry reference is passed into
> > vfs_rename() and another one is acquired directly inside vfs_rename(). That
> > is the one dropped early but there's still the original reference passed in
> > protecting new_dentry from freeing. Am I missing something?
>
> I am not quite sure about the actual execution logic. But I guess new_dentry
> reference may be dropped outside vfs_rename in cocurrent executions.
> Otherwise, there is no need to acquire & drop new_dentry reference as it
> is always alive along vfs_rename.
I don't think that's the case. The dget() - dput() pair just looks
superfluous to me in vfs_rename(). Am I missing something Miklos?
Honza
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index 0cab649..8b104d9 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -4498,7 +4498,6 @@ int vfs_rename(struct inode *old_dir, struct dentry
> > > *old_dentry,
> > > unlock_two_nondirectories(source, target);
> > > else if (target)
> > > inode_unlock(target);
> > > - dput(new_dentry);
> > > if (!error) {
> > > fsnotify_move(old_dir, new_dir, old_name.name, is_dir,
> > > !(flags & RENAME_EXCHANGE) ? target : NULL,
> > > old_dentry);
> > > @@ -4507,6 +4506,7 @@ int vfs_rename(struct inode *old_dir, struct dentry
> > > *old_dentry,
> > > new_is_dir, NULL, new_dentry);
> > > }
> > > }
> > > + dput(new_dentry);
> > > release_dentry_name_snapshot(&old_name);
> > >
> > > return error;
> > > --
> > > 2.7.4
> > >
> > >
> > --
> > Jan Kara <j...@suse.com>
> > SUSE Labs, CR
>
--
Jan Kara <j...@suse.com>
SUSE Labs, CR
