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

From: Amir Goldstein
Date: Mon May 15 2023 - 11:36:50 EST

On Mon, May 15, 2023 at 6:16 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> 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@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 :)
>
> 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...

If only it was that easy to trigger this race.
Look at the debug kernel patch named 'diff' in bugzilla...

Thanks,
Amir.