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Date:   Wed, 13 Apr 2022 22:05:37 +0800
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Subject: Re: [PATCH -next 0/2] fix nfsv4 bugs of opening with O_ACCMODE flag
From:   "chenxiaosong (A)" <chenxiaosong2@huawei.com>
To:     Lyu Tao <tao.lyu@epfl.ch>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        "anna@kernel.org" <anna@kernel.org>,
        "bjschuma@netapp.com" <bjschuma@netapp.com>
CC:     "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "liuyongqiang13@huawei.com" <liuyongqiang13@huawei.com>,
        "yi.zhang@huawei.com" <yi.zhang@huawei.com>,
        "zhangxiaoxu5@huawei.com" <zhangxiaoxu5@huawei.com>
References: <20220329113208.2466000-1-chenxiaosong2@huawei.com>
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在 2022/4/13 21:42, chenxiaosong (A) 写道:
> 
> 在 2022/4/13 20:07, Lyu Tao 写道:
>>
>> Hi Xiaosong,
>>
>>
>> Thanks for keeping focusing on this bug.
>>
>>
>> I applied this CVE for the NULL dereference bug at 
>> nfs4_valid_open_stateid() and added the following description to this 
>> CVE due to the NFS maintainers replied that to me.
>>
>> "An issue was discovered in fs/nfs/dir.c in the Linux kernel before 
>> 5.16.5. If an application sets the O_DIRECTORY flag, and tries to open 
>> a regular file, nfs_atomic_open() performs a regular lookup. If a 
>> regular file is found, ENOTDIR should occur, but the server instead 
>> returns uninitialized data in the file descriptor.
>>
>>
>> Actually I'm still confused with the root cause of this bug. In the 
>> original PoC, there is no O_DIRECTORY flag but commit ac795161c936 
>> mentioned.
>>
>> Moreover, in your latest commit ab0fc21bc710, it said "After secondly 
>> opening a file with O_ACCMODE|O_DIRECT flags, 
>> nfs4_valid_open_stateid() will dereference NULL nfs4_state when 
>> lseek()." However, the original PoC opens the file only with 
>> O_RDWR|O_CREAT for the first time.
>>
>>
>> Original PoC:
>>
>> fd = openat("./file1", o_RDWR|O_CREAT, 000);
>>
>> open("./file1", 
>> O_ACCMODE|O_CREAT|O_DIRECT|O_LARGEFILE|O_NOFOLLOW|O_NOATIME|O_CLOEXEC|FASYNC|0xb3000008, 
>> 001);
>>
>> lseek(fd, 9, SEEK_HOLE);
>>
>>
>> I'll update this CVE's description after I figure out these.
>>
>>
>> Best Regards,
>>
>> Tao
>>
> 
> Hi Tao:
> 
> Yes, O_ACCEMODE is _not_ necessary when fistly open() file.
> 
> When open() the file secondly, O_ACCEMODE is necessary if we want to 
> reproduce the bug.
> 
> Waiting for your modification of the CVE's description.
> 
> Best Regards.
> .

My reproducer:
   1. mount -t nfs -o vers=4.2 $server_ip:/ /mnt/
   2. fd = open("/mnt/file", O_ACCMODE|O_DIRECT|O_CREAT)
   3. close(fd)
   4. fd = open("/mnt/file", O_ACCMODE|O_DIRECT)
   5. lseek(fd)

When firstly open() file, O_ACCMODE|O_DIRECT is _not_ necessary, we just 
use O_CREAT to create new file.

When secondly open() file, only O_ACCMODE|O_DIRECT is necessary, 
O_CREAT|O_LARGEFILE|O_NOFOLLOW|O_NOATIME|O_CLOEXEC|FASYNC|0xb3000008 in 
your original PoC is not necessary (however, they are harmless).