[Virtio-fs] [virtiofsd PATCH v4 4/4] virtiofsd: support per-file DAX in FUSE_LOOKUP

[JeffleXu]

On 8/24/21 6:15 PM, Greg Kurz wrote:
> On Fri, 20 Aug 2021 13:03:23 +0800
> JeffleXu <jefflexu at linux.alibaba.com> wrote:
>>
>> Fine. Got it. However the returned fd (opened without O_PATH) is only
>> used for FS_IOC_GETFLAGS/FS_IOC_FSGETXATTR ioctl, while in most cases
>> for special device files, these two ioctls should return -ENOTTY.
>>
> 
> The actual problem is that a FIFO will cause openat() to block until
> the other end of the FIFO is open for writing...

Got it.

> 
>> If it's really a security issue, then lo_inode_open() could be used to
> 
> ... and cause a DoS on virtiofsd. So yes, this is a security issue and
> lo_inode_open() was introduced specifically to handle this.
> 
>> get a temporary fd, i.e., check if it's a special file before opening.
>> After all, FUSE_OPEN also handles in this way. Besides, I can't
>> understand what "race-free way" means.
>>
> 
> "race-free way" means a way that guarantees that file type
> cannot change between the time you check it and the time
> you open it (TOCTOU error). For example, doing a plain stat(),
> checking st_mode and proceeding to open() is wrong : nothing
> prevents the file to be unlinked and replaced by something
> else between stat() and open().
> 
> We avoid that by keeping O_PATH fds around and using
> lo_inode_open() instead of openat().

Thanks for the detailed explanation. Got it.

> 
> In your case, it seems that you should do the checking after
> you have an actual lo_inode for the target file, and pass
> that to lo_should_enable_dax() instead of the parent lo_inode
> and target name.
> 

Yes, that will be more reasonable. Thanks.

-- 
Thanks,
Jeffle