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SubjectRe: [PATCH -next 0/2] fix nfsv4 bugs of opening with O_ACCMODE flag
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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).

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