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

[chenxiaosong (A)]

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.