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: Serge Hallyn <serge.hallyn@canonical.com>
Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/fs/namespace.c b/fs/namespace.c
index 8505b5ece5de98bc41a9a00efcb0a11035607508..968d4c5eae03aa18b1326e09371aa5ada795939e 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -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);

diff --git a/fs/pnode.c b/fs/pnode.c
index 3e000a51ac0d09556d184d26422a91d0bc4c0ff9..8b29d2164da6aef6edb88b9872368c849e658530 100644 (file)
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -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);

diff --git a/fs/pnode.h b/fs/pnode.h
index 19b853a3445cb907665b4403484984a0c525af68..a0493d5ebfbf52be2eb07a794df459ab2a32cd6a 100644 (file)
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -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)
 {