Re: [PATCH linux-next] mqueue: fix IPC namespace use-after-free

[Giuseppe Scrivano <gscrivan@redhat.com>]

Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Tue, Dec 19, 2017 at 07:40:43PM +0100, Giuseppe Scrivano wrote:
>> Giuseppe Scrivano <gscrivan@redhat.com> writes:
>> 
>> > The only issue I've seen with my version is that if I do:
>> >
>> > # unshare -im /bin/sh
>> > # mount -t mqueue mqueue /dev/mqueue
>> > # touch /dev/mqueue/foo
>> > # umount /dev/mqueue
>> > # mount -t mqueue mqueue /dev/mqueue
>> >
>> > then /dev/mqueue/foo doesn't exist at this point.  Your patch does not
>> > have this problem and /dev/mqueue/foo is again accessible after the
>> > second mount.
>> 
>> although, how much is that of an issue?  Is there any other way to delay
>
> You do realize that with your patch you would end up with worse than that -
> mount/touch/umount and now you've got ->mq_sb non-NULL and pointing to freed
> super_block.  With really unpleasant effects when you quit that ipcns...
>
>> the cost of kern_mount_data()?  Most containers have /dev/mqueue mounted
>> but it is not really going to be used.
>
> _What_ cost?  At mount(2) time you are setting the superblock up anyway, so
> what would you be delaying?  kmem_cache_alloc() for struct mount and assignments
> to its fields?  That's noise; if anything, I would expect the main cost with
> a plenty of containers to be in sget() scanning the list of mqueue superblocks.
> And we can get rid of that, while we are at it - to hell with mount_ns(), with
> that approach we can just use mount_nodev() instead.  The logics in
> mq_internal_mount() will deal with multiple instances - if somebody has already
> triggered creation of internal mount, all subsequent calls in that ipcns will
> end up avoiding kern_mount_data() entirely.  And if you have two callers
> racing - sure, you will get two superblocks.  Not for long, though - the first
> one to get to setting ->mq_mnt (serialized on mq_lock) wins, the second loses
> and prompty destroys his vfsmount and superblock.  I seriously suspect that
> variant below would cut down on the cost a whole lot more - as it is, we have
> the total of O(N^2) spent in the loop inside of sget_userns() when we create
> N ipcns and mount in each of those; this patch should cut that to O(N)...

Thanks for the explanation.  I see what you mean now and how this cost is
inevitable as anyway we need to setup the superblock.

Giuseppe