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Date: Tue, 16 May 2023 20:27:38 +0800
From: Zhihao Cheng <chengzhihao1@...wei.com>
To: Amir Goldstein <amir73il@...il.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
在 2023/5/16 19:45, Amir Goldstein 写道:
> 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 for rebasing, so you will take v2 with modified helper
ovl_i_path_real(return realinode)?
>
> Thanks,
> Amir.
> .
>
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