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

[Andrew Vagin <avagin@gmail.com>]

On Mon, Jun 17, 2013 at 03:56:14PM -0700, Andrew Morton wrote:
> On Mon, 17 Jun 2013 12:58:00 -0700 ebiederm@xmission.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?

It's a problem for Linux Containers. Because usually the root user in
container should have enough rights to mount something (tmpfs,
bindmounts, etc).  Our target is to make containers completely isolated.

A container is isolated with help of namespaces. The user namespace
creates a new sets of capabilities and users.


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