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Date: Tue, 16 May 2023 09:16:26 +0800
From: Zhihao Cheng <chengzhihao1@...wei.com>
To: Amir Goldstein <amir73il@...il.com>,
Christian Brauner <brauner@...nel.org>
CC: <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 2023/5/15 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.
>> 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...
Yes, I can hardly reproduce the problem without the time delay in kernel.
>
> Thanks,
> Amir.
> .
>
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