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Message-ID: <CAOQ4uxi9bhs0xKO-zCj-QiFzd-xZEp8DtG222vG-R6U5HBHcVg@mail.gmail.com>
Date: Tue, 16 May 2023 14:45:39 +0300
From: Amir Goldstein <amir73il@...il.com>
To: Zhihao Cheng <chengzhihao1@...wei.com>
Cc: Christian Brauner <brauner@...nel.org>, miklos@...redi.hu,
linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference
at realinode in rcu-walk mode
On Tue, May 16, 2023 at 4:16 AM Zhihao Cheng <chengzhihao1@...wei.com> wrote:
>
> 在 2023/5/15 23:36, Amir Goldstein 写道:
> > On Mon, May 15, 2023 at 6:16 PM Christian Brauner <brauner@...nel.org> 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@...nel.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@...wei.com>
> >>>>> Suggested-by: Christian Brauner <brauner@...nel.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);
> >>
>
> 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@gmail.com/
After Miklos takes your fixes, I will rebase.
Thanks,
Amir.
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