linux-kernel - Re: [PATCH] [RFC] mnt: restrict a number of "struct mnt"

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Date:	Mon, 17 Jun 2013 15:56:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	Andrey Vagin <avagin@...nvz.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Ingo Molnar <mingo@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] [RFC] mnt: restrict a number of "struct mnt"

On Mon, 17 Jun 2013 12:58:00 -0700 ebiederm@...ssion.com (Eric W. Biederman) wrote:

> > I found that a few processes can eat all host memory and nobody can kill them.
> > $ mount -t tmpfs xxx /mnt
> > $ mount --make-shared /mnt
> > $ for i in `seq 30`; do mount --bind /mnt `mktemp -d /mnt/test.XXXXXX` & done
> >
> > All this processes are unkillable, because they took i_mutex and waits
> > namespace_lock.
> >
> > ...
> > 21715 pts/0 ______D __________0:00 __________________\_ mount --bind /mnt /mnt/test.ht6jzO
> > 21716 pts/0 ______D __________0:00 __________________\_ mount --bind /mnt /mnt/test.97K4mI
> > 21717 pts/0 ______R __________0:01 __________________\_ mount --bind /mnt /mnt/test.gO2CD9
> > ...
> >
> > Each of this process doubles a number of mounts, so at the end we will
> > have about 2^32 mounts and the size of struct mnt is 256 bytes, so we
> > need about 1TB of RAM.
> >
> > Another problem is that ___umount___ of a big tree is very hard operation
> > and it requires a lot of time.
> > E.g.:
> > 16411
> > umount("/tmp/xxx", MNT_DETACH) __________________= 0 <7.852066> (7.8 sec)
> > 32795
> > umount("/tmp/xxx", MNT_DETACH) __________________= 0 <34.485501> ( 34 sec)
> >
> > For all this time sys_umoun takes namespace_sem and vfsmount_lock...
> >
> > Due to all this reasons I suggest to restrict a number of mounts.
> > Probably we can optimize this code in a future, but now this restriction
> > can help.
> 
> So for anyone seriously worried about this kind of thing in general we
> already have the memory control group, which is quite capable of
> limiting this kind of thing, and it limits all memory allocations not
> just mount.

What is the exposure here?  By what means can a non-CAP_SYS_ADMIN user
run sys_mount() under the namespace system?

IOW, what does the sysadmin have to do to permit this?  Is that a
typical thing to do, or did the sysadmin make a mistake?
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