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

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Date:   Tue, 16 May 2023 20:25:26 +0800
From:   Zhihao Cheng <chengzhihao1@...wei.com>
To:     Amir Goldstein <amir73il@...il.com>
CC:     <miklos@...redi.hu>, <brauner@...nel.org>,
        <linux-unionfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] ovl: get_acl: Fix null pointer dereference at
 realinode in rcu-walk mode

在 2023/5/16 19:40, Amir Goldstein 写道:
> On Mon, May 15, 2023 at 4:39 PM Zhihao Cheng <chengzhihao1@...wei.com> wrote:
>>
>> Following process:
>>           P1                     P2
>>   path_openat
>>    link_path_walk
>>     may_lookup
>>      inode_permission(rcu)
>>       ovl_permission
>>        acl_permission_check
>>         check_acl
>>          get_cached_acl_rcu
>>           ovl_get_inode_acl
>>            realinode = ovl_inode_real(ovl_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
>>              ovl_inode_upper
>>               upperdentry = ovl_i_dentry_upper(ovl_inode)
>>               d_inode(upperdentry) // returns NULL
>>            IS_POSIXACL(realinode) // NULL pointer dereference
>> , will trigger an null pointer dereference at realinode:
>>    [  205.472797] BUG: kernel NULL pointer dereference, address:
>>                   0000000000000028
>>    [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
>>                   6.3.0-12064-g2edfa098e750-dirty #1216
>>    [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
>>    [  205.489584] Call Trace:
>>    [  205.489812]  <TASK>
>>    [  205.490014]  ovl_get_inode_acl+0x26/0x30
>>    [  205.490466]  get_cached_acl_rcu+0x61/0xa0
>>    [  205.490908]  generic_permission+0x1bf/0x4e0
>>    [  205.491447]  ovl_permission+0x79/0x1b0
>>    [  205.491917]  inode_permission+0x15e/0x2c0
>>    [  205.492425]  link_path_walk+0x115/0x550
>>    [  205.493311]  path_lookupat.isra.0+0xb2/0x200
>>    [  205.493803]  filename_lookup+0xda/0x240
>>    [  205.495747]  vfs_fstatat+0x7b/0xb0
>>
>> Fetch a reproducer in [Link].
>>
>> Fix it by using helper ovl_i_path_realinode() to get realpath and real
>> inode after non-nullptr checking.
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
>> Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
> 
> Note that this bug is also in 5.15.y, in method ovl_get_acl().
> I hope you will be able to follow up with a simple backport for 5.15 -
> i.e. only need to add a check for NULL realinode at the beginning.
> There was no realpath back then.
> 

Sure. I notice that there is a '[ Upstream commit xxx ]' field in 5.15 
patch, so may I backport it after the fix applied into mainline(6.4)?

> AFAICT, both your patches should apply cleanly to 6.1.y, so should
> be picked up automatically by stable kernel bots.
> 
> Thanks,
> Amir.
> 
>> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com>
>> Suggested-by: Christian Brauner <brauner@...nel.org>
>> ---
>>   fs/overlayfs/inode.c | 14 ++++++--------
>>   1 file changed, 6 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
>> index cc3ef5a6666a..b2021eada8be 100644
>> --- a/fs/overlayfs/inode.c
>> +++ b/fs/overlayfs/inode.c
>> @@ -576,20 +576,18 @@ struct posix_acl *do_ovl_get_acl(struct mnt_idmap *idmap,
>>                                   struct inode *inode, int type,
>>                                   bool rcu, bool noperm)
>>   {
>> -       struct inode *realinode = ovl_inode_real(inode);
>> +       struct inode *realinode;
>>          struct posix_acl *acl;
>>          struct path realpath;
>> +       int err;
>> +
>> +       err = ovl_i_path_realinode(inode, &realpath, &realinode, rcu);
>> +       if (err)
>> +               return ERR_PTR(err);
>>
>>          if (!IS_POSIXACL(realinode))
>>                  return NULL;
>>
>> -       /* Careful in RCU walk mode */
>> -       ovl_i_path_real(inode, &realpath);
>> -       if (!realpath.dentry) {
>> -               WARN_ON(!rcu);
>> -               return ERR_PTR(-ECHILD);
>> -       }
>> -
>>          if (rcu) {
>>                  /*
>>                   * If the layer is idmapped drop out of RCU path walk
>> --
>> 2.39.2
>>
> .
>