Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference at realinode in rcu-walk mode
From: Amir Goldstein
Date: Tue May 16 2023 - 08:35:26 EST
On Tue, May 16, 2023 at 3:27 PM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
>
> 在 2023/5/16 19:45, Amir Goldstein 写道:
> > On Tue, May 16, 2023 at 4:16 AM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
> >>
> >> 在 2023/5/15 23:36, Amir Goldstein 写道:
> >>> 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);
> >>>>
> >>
> >> Thanks for reviewings and helpful discussion from Amir and Christian.
> >>
> >
> > You're welcome.
> >
> > Note to self: there is be a trivial conflict with the change to
> > ovl_i_path_real()
> > return type and this patch from my ovl-lazy-lowerdata series:
> > https://lore.kernel.org/linux-unionfs/20230427130539.2798797-7-amir73il@xxxxxxxxx/
> >
> > After Miklos takes your fixes, I will rebase.
>
> Thanks for rebasing, so you will take v2 with modified helper
> ovl_i_path_real(return realinode)?
>
Please post v3 with the modified helper for Miklos to take.
Thanks,
Amir.