Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference at realinode in rcu-walk mode

[Amir Goldstein <amir73il@xxxxxxxxx>]

On Mon, May 15, 2023 at 4:58 PM Christian Brauner <brauner@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> >  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 :)

>
> 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 ;-)
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).

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);