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Subject: Re: [PATCH 1/2] quota: fix null-ptr-deref in ext4_acquire_dquot()
To:     Jan Kara <jack@suse.cz>
References: <20230527014018.47396-1-yebin10@huawei.com>
 <20230527014018.47396-2-yebin10@huawei.com>
 <20230530095726.t2grmww5rzofx5gp@quack3>
CC:     <jack@suse.com>, <linux-kernel@vger.kernel.org>,
        <syzbot+e633c79ceaecbf479854@syzkaller.appspotmail.com>
From:   "yebin (H)" <yebin10@huawei.com>
Message-ID: <647ADA33.5010508@huawei.com>
Date:   Sat, 3 Jun 2023 14:14:11 +0800
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On 2023/5/30 17:57, Jan Kara wrote:
> On Sat 27-05-23 09:40:17, Ye Bin wrote:
>> Syzbot found the following issue:
>> Unable to handle kernel paging request at virtual address dfff800000000005
>> KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
> ...
>> CPU: 0 PID: 6080 Comm: syz-executor747 Not tainted 6.3.0-rc7-syzkaller-g14f8db1c0f9a #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/14/2023
>> pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> pc : ext4_acquire_dquot+0x1d4/0x398 fs/ext4/super.c:6766
>> lr : dquot_to_inode fs/ext4/super.c:6740 [inline]
>> lr : ext4_acquire_dquot+0x1ac/0x398 fs/ext4/super.c:6766
> OK, this is bad...
>
>> Above issue may happens as follows:
>> ProcessA              ProcessB                    ProcessC
>> sys_fsconfig
>>    vfs_fsconfig_locked
>>     reconfigure_super
>>       ext4_remount
>>        dquot_suspend -> suspend all type quota
>>
>>                   sys_fsconfig
>> 		  vfs_fsconfig_locked
>> 		    reconfigure_super
>> 		     ext4_remount
>> 		      dquot_resume
>> 		       ret = dquot_load_quota_sb
>>                          add_dquot_ref
>> 		                           do_open  -> open file O_RDWR
>> 					    vfs_open
>> 					     do_dentry_open
>> 					      get_write_access
>> 					       atomic_inc_unless_negative(&inode->i_writecount)
>>                                                ext4_file_open
>> 					       dquot_file_open
>> 					        dquot_initialize
>> 						  __dquot_initialize
>> 						   dqget
>> 						    if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
>>
>> 			  __dquot_initialize
>> 			   __dquot_initialize
>> 			    dqget
>> 			     if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
>> 	                       ext4_acquire_dquot -> Return error
>> 		       if (ret < 0)
>> 	                 vfs_cleanup_quota_inode
>> 			  dqopt->files[type] = NULL;
> But I don't see how this can happen. The code in dquot_load_quota_sb()
> looks like:
>
>          error = add_dquot_ref(sb, type);
>          if (error)
>                  dquot_disable(sb, type, flags);
>
> So if an error happens in add_dquot_ref(), we'll call dquot_disable().
> dquot_disable() then does:
>
>                  drop_dquot_ref(sb, cnt);
>                  invalidate_dquots(sb, cnt);
>
> and invalidate_dquots() waits for reference count of all dquots to drop to
> 0. Hence if dqget() returned a dquot pointer to ProcessC, then ProcessB
> should wait until ProcessC drops the dquot reference (hence
> ext4_acquire_dquot() is done).
>
> What am I missing?
>
> 								Honza
My reproduction condition is:
mkfs.ext4 -F /dev/sda
tune2fs  -Q usrquota /dev/sda

dquot_disable
...
          if ((flags & DQUOT_USAGE_ENABLED && !(flags & 
DQUOT_LIMITS_ENABLED))
              || (flags & DQUOT_SUSPENDED && flags & (DQUOT_LIMITS_ENABLED |
              DQUOT_USAGE_ENABLED)))
                  return -EINVAL;
...
If without enable DQUOT_LIMITS_ENABLED dquot_disable() will just return 
-EINVAL.
>> 			                              ext4_acquire_dquot
>> 						       -->dquot_to_inode(dquot) is NULL
>>
>> To solve above issue, if quota has been loaded, there's unneed to cleaup quota
>> inode if dquot_load_quota_sb() return failed when do dquot_resume();
>>
>> Reported-by: syzbot+e633c79ceaecbf479854@syzkaller.appspotmail.com
>> Signed-off-by: Ye Bin <yebin10@huawei.com>
>> ---
>>   fs/quota/dquot.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
>> index ffd40dc3e4e9..4b913faa48ec 100644
>> --- a/fs/quota/dquot.c
>> +++ b/fs/quota/dquot.c
>> @@ -2476,7 +2476,7 @@ int dquot_resume(struct super_block *sb, int type)
>>   		flags = dquot_generic_flag(flags, cnt);
>>   		ret = dquot_load_quota_sb(sb, cnt, dqopt->info[cnt].dqi_fmt_id,
>>   					  flags);
>> -		if (ret < 0)
>> +		if (ret < 0 && !sb_has_quota_loaded(sb, cnt))
>>   			vfs_cleanup_quota_inode(sb, cnt);
>>   	}
>>   
>> -- 
>> 2.31.1
>>