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

[Al Viro]

On Tue, Dec 19, 2017 at 03:49:24PM -0600, Eric W. Biederman wrote:
> > 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)...
> 
> If that is where the cost is, is there any point in delaying creating
> the internal mount at all?

We won't know without the profiles...  Incidentally, is there any point in
using mount_ns() for procfs?  Similar scheme (with ->proc_mnt instead of
->mq_mnt, of course) would live with mount_nodev() just fine, and it's
definitely less costly - we don't bother with the loop in sget_userns()
at all that way.