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Date:   Mon, 15 May 2023 17:16:12 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Zhihao Cheng <chengzhihao1@huawei.com>, miklos@szeredi.hu,
        linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference
 at realinode in rcu-walk mode
Message-ID: <20230515-dulden-symmetrie-3b5d934478d4@brauner>
References: <20230515133629.1974610-1-chengzhihao1@huawei.com>
 <20230515133629.1974610-2-chengzhihao1@huawei.com>
 <20230515-sprachen-zeltlager-6925dfbe19c1@brauner>
 <CAOQ4uxjo3rzdrjmbXr=SgJbrBf_EA-HpXH25LORo_vPY=q0jWQ@mail.gmail.com>
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On Mon, May 15, 2023 at 05:58:55PM +0300, Amir Goldstein wrote:
> On Mon, May 15, 2023 at 4:58 PM Christian Brauner <brauner@kernel.org> wrote:
> >
> > On Mon, May 15, 2023 at 09:36:28PM +0800, Zhihao Cheng wrote:
> > > Following process:
> > >           P1                     P2
> > >  path_lookupat
> > >   link_path_walk
> > >    inode_permission
> > >     ovl_permission
> > >       ovl_i_path_real(inode, &realpath)
> > >         path->dentry = ovl_i_dentry_upper(inode)
> > >                           drop_cache
> > >                          __dentry_kill(ovl_dentry)
> > >                           iput(ovl_inode)
> > >                            ovl_destroy_inode(ovl_inode)
> > >                             dput(oi->__upperdentry)
> > >                              dentry_kill(upperdentry)
> > >                               dentry_unlink_inode
> > >                                upperdentry->d_inode = NULL
> > >       realinode = d_inode(realpath.dentry) // return NULL
> > >       inode_permission(realinode)
> > >        inode->i_sb  // NULL pointer dereference
> > > , will trigger an null pointer dereference at realinode:
> > >   [  335.664979] BUG: kernel NULL pointer dereference,
> > >                  address: 0000000000000002
> > >   [  335.668032] CPU: 0 PID: 2592 Comm: ls Not tainted 6.3.0
> > >   [  335.669956] RIP: 0010:inode_permission+0x33/0x2c0
> > >   [  335.678939] Call Trace:
> > >   [  335.679165]  <TASK>
> > >   [  335.679371]  ovl_permission+0xde/0x320
> > >   [  335.679723]  inode_permission+0x15e/0x2c0
> > >   [  335.680090]  link_path_walk+0x115/0x550
> > >   [  335.680771]  path_lookupat.isra.0+0xb2/0x200
> > >   [  335.681170]  filename_lookup+0xda/0x240
> > >   [  335.681922]  vfs_statx+0xa6/0x1f0
> > >   [  335.682233]  vfs_fstatat+0x7b/0xb0
> > >
> > > Fetch a reproducer in [Link].
> > >
> > > Add a new helper ovl_i_path_realinode() to get realpath and real inode
> > > after non-nullptr checking, use the helper to replace the current realpath
> > > getting logic.
> > >
> > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217405
> > > Fixes: 4b7791b2e958 ("ovl: handle idmappings in ovl_permission()")
> > > Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
> > > Suggested-by: Christian Brauner <brauner@kernel.org>
> > > ---
> > >  fs/overlayfs/inode.c | 31 ++++++++++++++++++++++++-------
> > >  1 file changed, 24 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > > index 541cf3717fc2..cc3ef5a6666a 100644
> > > --- a/fs/overlayfs/inode.c
> > > +++ b/fs/overlayfs/inode.c
> > > @@ -278,6 +278,26 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
> > >       return err;
> > >  }
> > >
> > > +static inline int ovl_i_path_realinode(struct inode *inode,
> > > +                                    struct path *realpath,
> > > +                                    struct inode **realinode, int rcu)
> > > +{
> > > +     /* Careful in RCU walk mode */
> > > +     ovl_i_path_real(inode, realpath);
> > > +     if (!realpath->dentry) {
> > > +             WARN_ON(!rcu);
> > > +             return -ECHILD;
> > > +     }
> > > +
> > > +     *realinode = d_inode(realpath->dentry);
> > > +     if (!*realinode) {
> > > +             WARN_ON(!rcu);
> > > +             return -ECHILD;
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> >
> > If you want to return the inode wouldn't it possibly make more sense to
> > return the inode from the function directly? But not fuzzed. Maybe Amir
> > has a preference. As I said, I'm even fine with the original approach.
> 
> Sorry for not reviewing v1, I was traveling, even though it is hard to use
> this excuse when I was traveling with Christian who did review v1 :)

Well, I did only do it this morning. :)

> 
> >
> > static inline struct inode *ovl_i_path_realinode(struct inode *inode,
> >                                                  struct path *realpath,
> >                                                  int rcu)
> > {
> >         struct inode *realinode;
> >
> >         /* Careful in RCU walk mode */
> >         ovl_i_path_real(inode, realpath);
> >         if (!realpath->dentry) {
> >                 WARN_ON(!rcu);
> >                 return ERR_PTR(-ECHILD);
> >         }
> >
> >         realinode = d_inode(realpath->dentry);
> >         if (!realinode) {
> >                 WARN_ON(!rcu);
> >                 return ERR_PTR(-ECHILD);
> >         }
> >
> >         return realinode;
> > }
> >
> 
> I think this helper is over engineered ;-)

Yes. As mentioned before, I would've been happy even with v1 that didn't
have any helper.

> The idea for a helper that returns inode is good,
> but I see no reason to mix RCU walk in this helper
> and don't even need a new helper (see untested patch below).

Looks good to me too.

> 
> Thanks,
> Amir.
> 
> ---
> -void ovl_i_path_real(struct inode *inode, struct path *path)
> +struct inode *ovl_i_path_real(struct inode *inode, struct path *path)
>  {
>         struct ovl_path *lowerpath = ovl_lowerpath(OVL_I_E(inode));
> 
> @@ -342,6 +342,8 @@ void ovl_i_path_real(struct inode *inode, struct path *path)
>         } else {
>                 path->mnt = ovl_upper_mnt(OVL_FS(inode->i_sb));
>         }
> +
> +       return path->dentry ? d_inode(path->dentry) : NULL;
>  }
> 
> @@ -295,8 +295,8 @@ int ovl_permission(struct mnt_idmap *idmap,
>         int err;
> 
>         /* Careful in RCU walk mode */
> -       ovl_i_path_real(inode, &realpath);
> -       if (!realpath.dentry) {
> +       realinode = ovl_i_path_real(inode, &realpath);
> +       if (!realpath.dentry || !realinode) {
>                 WARN_ON(!(mask & MAY_NOT_BLOCK));
>                 return -ECHILD;
>         }
> @@ -309,7 +309,6 @@ int ovl_permission(struct mnt_idmap *idmap,
> 
>         if (err)
>                 return err;
> 
> -       realinode = d_inode(realpath.dentry);
>         old_cred = ovl_override_creds(inode->i_sb);

Btw, if the reproducer that Zhihao has posted in the bugzilla link:

#!/bin/bash

mkdir -p /root/tmp/merge /root/tmp/upper/dir /root/tmp/lower /root/tmp/work
touch /root/tmp/upper/dir/file
chown freg:freg -R /root/tmp/upper/dir
mount -t overlay none -oupperdir=/root/tmp/upper,lowerdir=/root/tmp/lower,workdir=/root/tmp/work /root/tmp/merge
ls /root/tmp/merge/dir/file &
echo 3 > /proc/sys/vm/drop_caches

is reliable we should add it to xfstests...