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

On 8/24/21 6:15 PM, Greg Kurz wrote:

On Fri, 20 Aug 2021 13:03:23 +0800
JeffleXu [off-list ref] wrote:

quoted

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.

quoted

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.

quoted

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
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