[Virtio-fs] [PATCH v3 09/10] virtiofsd: Optionally fill lo_inode.fhandle

[Hanna Reitz]

On 18.08.21 15:32, Vivek Goyal wrote:
> On Tue, Aug 17, 2021 at 08:14:46PM -0400, Vivek Goyal wrote:
>> On Tue, Aug 17, 2021 at 03:45:19PM -0400, Vivek Goyal wrote:
>>> On Tue, Aug 17, 2021 at 10:27:16AM +0200, Hanna Reitz wrote:
>>>> On 16.08.21 21:44, Vivek Goyal wrote:
>>>>> On Wed, Aug 11, 2021 at 08:41:18AM +0200, Hanna Reitz wrote:
>>>>>
>>>>> [..]
>>>>>>>> But given the inotify complications, there’s really a good reason we should
>>>>>>>> use mountinfo.
>>>>>>>>
>>>>>>>>>> It’s a bit tricky because our sandboxing prevents easy access to mountinfo,
>>>>>>>>>> but if that’s the only way...
>>>>>>>>> yes. We already have lo->proc_self_fd. Maybe we need to keep
>>>>>>>>> /proc/self/mountinfo open in lo->proc_self_mountinfo. I am assuming
>>>>>>>>> that any mount table changes will still be visible despite the fact
>>>>>>>>> I have fd open (and don't have to open new fd to notice new mount/unmount
>>>>>>>>> changes).
>>>>>>>> Well, yes, that was my idea.  Unfortunately, I wasn’t quite successful yet;
>>>>>>>> when I tried keeping the fd open, reading from it would just return 0
>>>>>>>> bytes.  Perhaps that’s because we bind-mount /proc/self/fd to /proc so that
>>>>>>>> nothing else in /proc is visible. Perhaps we need to bind-mount
>>>>>>>> /proc/self/mountinfo into /proc/self/fd before that...
>>>>>>> Or perhaps open /proc/self/mountinfo and save fd in lo->proc_mountinfo
>>>>>>> before /proc/self/fd is bind mounted on /proc?
>>>>>> Yes, I tried that, and then reading would just return 0 bytes.
>>>>> Hi Hanna,
>>>>>
>>>>> I tried this simple patch and I can read /proc/self/mountinfo before
>>>>> bind mounting /proc/self/fd and after bind mounting /proc/self/fd. Am
>>>>> I missing something.
>>>> Yes, but I tried reading it in the main loop (where we’d actually need it).
>>>> It looks like the umount2(".", MNT_DETACH) in setup_mounts() breaks it.
>>> Good point. I modified my code and notice too that after umoutn2() it
>>> always reads 0 bytes. I can understand that all the other mount points
>>> could go away but new rootfs mount point of virtiofsd should still be
>>> visible, IIUC. I don't understand why.
>>>
>>> Anyway, I tried re-opening /proc/self/mountinfo file after umount2(".",
>>> MNT_DETACH), and that seems to work and it shows root mount point. I
>>> created a bind mount and it shows that too.
>>>
>>> So looks like quick fix can be that we re-open /proc/self/mountinfo. But
>>> that means we can't bind /proc/self/fd on /proc/. We could bind mount
>>> /proc/self on /proc. Not sure is it safe enough.
>> Or may be I can do this.
>>
>> - Open O_PATH fd for /proc/self
>>    proc_self = open("/proc/self");
>> - Bind mount /proc/self/fd on /proc
>> - pivot_root() and umount() stuff
>> - Openat(proc_self, "mountinfo")
>> - close(proc_self)
>>
>> If this works, then we don't have the security issue and we managed
>> to open mountinfo after pivot_root() and umount(). Will give it a
>> try and see if it works tomorrow.
> Hi Hanna,
>
> This seems to work for me. I think key is to open mountinfo after
> pivot_root() and then it works. If it is opened before pivot_root()
> then it does not work. Not sure why.

Great, your code looks good to me.  I was afraid this was going to be 
really complicated, but that doesn’t look too bad.

Thanks!

Hanna