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

["chenxiaosong (A)" <chenxiaosong2@huawei.com>]

I will give some detailed code process.

firstly open():

```c
open
   do_sys_open
     do_sys_openat2
       do_filp_open
         path_openat
           open_last_lookups
             lookup_open
               atomic_open
                 nfs_atomic_open
                   create_nfs_open_context
                     flags_to_mode() = FMODE_READ|FMODE_WRITE
                     alloc_nfs_open_context
                       ctx->mode = f_mode // FMODE_READ|FMODE_WRITE
                   // NFS_PROTO(dir)->open_context
                   nfs4_atomic_open
                     nfs4_do_open
                       _nfs4_do_open
                         fmode = _nfs4_ctx_to_openmode(ctx) = 3
                           ret = ctx->mode & (FMODE_READ|FMODE_WRITE) // 
ctx->mode = 3
                         nfs4_opendata_alloc
                           nfs4_map_atomic_open_share
                             return NFS4_SHARE_ACCESS_BOTH
```

secondly open():
```c
open
   do_sys_open
     do_sys_openat2
       do_filp_open
         path_openat
           open_last_lookups
             lookup_open
               return dentry // if (dentry->d_inode) {
           do_open
             vfs_open
               do_dentry_open
                 // f->f_op->open
                 nfs4_file_open
                   if ((openflags & O_ACCMODE) == 3)
                   nfs_open // without sunrpc request
                     alloc_nfs_open_context
                       ctx->state = NULL; // this is point
```

lseek() after secondly open():
```c
lseek
   ksys_lseek
     vfs_llseek
       // file->f_op->llseek
       nfs4_file_llseek
         nfs42_proc_llseek
           _nfs42_proc_llseek(lock)
             nfs4_set_rw_stateid(ctx=lock->open_context)
               nfs4_select_rw_stateid(state=ctx->state)
                 nfs4_valid_open_stateid(state)
                   state->flags // dereference NULL state
```

在 2022/4/13 22:05, chenxiaosong (A) 写道:
> 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).