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Commit 132c94e3 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

vfs: Carefully propogate mounts across user namespaces 



As a matter of policy MNT_READONLY should not be changable if the
original mounter had more privileges than creator of the mount
namespace.

Add the flag CL_UNPRIVILEGED to note when we are copying a mount from
a mount namespace that requires more privileges to a mount namespace
that requires fewer privileges.

When the CL_UNPRIVILEGED flag is set cause clone_mnt to set MNT_NO_REMOUNT
if any of the mnt flags that should never be changed are set.

This protects both mount propagation and the initial creation of a less
privileged mount namespace.

Cc: stable@vger.kernel.org
Acked-by: default avatarSerge Hallyn <serge.hallyn@canonical.com>
Reported-by: default avatarAndy Lutomirski <luto@amacapital.net>
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent 90563b19

fs/namespace.c

+5 −1
Original line number Diff line number Diff line
@@ -798,6 +798,10 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root, 
	}



	mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~MNT_WRITE_HOLD;

	/* Don't allow unprivileged users to change mount flags */

	if ((flag & CL_UNPRIVILEGED) && (mnt->mnt.mnt_flags & MNT_READONLY))

		mnt->mnt.mnt_flags |= MNT_LOCK_READONLY;



	atomic_inc(&sb->s_active);

	mnt->mnt.mnt_sb = sb;

	mnt->mnt.mnt_root = dget(root);
@@ -2342,7 +2346,7 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns, 
	/* First pass: copy the tree topology */

	copy_flags = CL_COPY_ALL | CL_EXPIRE;

	if (user_ns != mnt_ns->user_ns)

		copy_flags |= CL_SHARED_TO_SLAVE;

		copy_flags |= CL_SHARED_TO_SLAVE | CL_UNPRIVILEGED;

	new = copy_tree(old, old->mnt.mnt_root, copy_flags);

	if (IS_ERR(new)) {

		up_write(&namespace_sem);

fs/pnode.c

+6 −0
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 
#include <linux/mnt_namespace.h>

#include <linux/mount.h>

#include <linux/fs.h>

#include <linux/nsproxy.h>

#include "internal.h"

#include "pnode.h"
@@ -220,6 +221,7 @@ static struct mount *get_source(struct mount *dest, 
int propagate_mnt(struct mount *dest_mnt, struct dentry *dest_dentry,

		    struct mount *source_mnt, struct list_head *tree_list)

{

	struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;

	struct mount *m, *child;

	int ret = 0;

	struct mount *prev_dest_mnt = dest_mnt;
@@ -237,6 +239,10 @@ int propagate_mnt(struct mount *dest_mnt, struct dentry *dest_dentry, 


		source =  get_source(m, prev_dest_mnt, prev_src_mnt, &type);



		/* Notice when we are propagating across user namespaces */

		if (m->mnt_ns->user_ns != user_ns)

			type |= CL_UNPRIVILEGED;



		child = copy_tree(source, source->mnt.mnt_root, type);

		if (IS_ERR(child)) {

			ret = PTR_ERR(child);

fs/pnode.h

+1 −0
Original line number Diff line number Diff line
@@ -23,6 +23,7 @@ 
#define CL_MAKE_SHARED 		0x08

#define CL_PRIVATE 		0x10

#define CL_SHARED_TO_SLAVE	0x20

#define CL_UNPRIVILEGED		0x40



static inline void set_mnt_shared(struct mount *mnt)

{