mnt: Refactor the logic for mounting sysfs and proc in a user namespace

commit 1b852bceb0d111e510d1a15826ecc4a19358d512 upstream.

Fresh mounts of proc and sysfs are a very special case that works very
much like a bind mount.  Unfortunately the current structure can not
preserve the MNT_LOCK... mount flags.  Therefore refactor the logic
into a form that can be modified to preserve those lock bits.

Add a new filesystem flag FS_USERNS_VISIBLE that requires some mount
of the filesystem be fully visible in the current mount namespace,
before the filesystem may be mounted.

Move the logic for calling fs_fully_visible from proc and sysfs into
fs/namespace.c where it has greater access to mount namespace state.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/fs/namespace.c b/fs/namespace.c
index 6dfb6fc342867b6e0f0f3c82bc7b510e821e58b9..8d2384d12e32b7d2b2e65eba1b5a86433cc6699d 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2172,6 +2172,8 @@ unlock:
        return err;
 }
 
+static bool fs_fully_visible(struct file_system_type *fs_type);
+
 /*
  * create a new mount for userspace and request it to be added into the
  * namespace's tree
@@ -2203,6 +2205,10 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
                        flags |= MS_NODEV;
                        mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
                }
+               if (type->fs_flags & FS_USERNS_VISIBLE) {
+                       if (!fs_fully_visible(type))
+                               return -EPERM;
+               }
        }
 
        mnt = vfs_kern_mount(type, flags, name, data);
@@ -3023,7 +3029,7 @@ bool current_chrooted(void)
        return chrooted;
 }
 
-bool fs_fully_visible(struct file_system_type *type)
+static bool fs_fully_visible(struct file_system_type *type)
 {
        struct mnt_namespace *ns = current->nsproxy->mnt_ns;
        struct mount *mnt;

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 5dbadecb234da01be9cc7eb5705e71363da7dfea..7fd8c1f1b1d925e93ad966f432040df830cb6b3d 100644 (file)
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -112,9 +112,6 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
                ns = task_active_pid_ns(current);
                options = data;
 
-               if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
-                       return ERR_PTR(-EPERM);
-
                /* Does the mounter have privilege over the pid namespace? */
                if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
                        return ERR_PTR(-EPERM);
@@ -157,7 +154,7 @@ static struct file_system_type proc_fs_type = {
        .name           = "proc",
        .mount          = proc_mount,
        .kill_sb        = proc_kill_sb,
-       .fs_flags       = FS_USERNS_MOUNT,
+       .fs_flags       = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
 };
 
 void __init proc_root_init(void)

diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 8a49486bf30c9859a5474f3cc5055bc7b9917de4..1c6ac6fcee9fb15c869ef80fc5947ba9117d77ea 100644 (file)
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -31,9 +31,6 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
        bool new_sb;
 
        if (!(flags & MS_KERNMOUNT)) {
-               if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
-                       return ERR_PTR(-EPERM);
-
                if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
                        return ERR_PTR(-EPERM);
        }
@@ -58,7 +55,7 @@ static struct file_system_type sysfs_fs_type = {
        .name           = "sysfs",
        .mount          = sysfs_mount,
        .kill_sb        = sysfs_kill_sb,
-       .fs_flags       = FS_USERNS_MOUNT,
+       .fs_flags       = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
 };
 
 int __init sysfs_init(void)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e11d60cc867bd9010edc8d2425cce03b228d968f..abbd77a27f3d86d4bf6e84379079fb74cbac9d77 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1755,6 +1755,7 @@ struct file_system_type {
 #define FS_HAS_SUBTYPE         4
 #define FS_USERNS_MOUNT                8       /* Can be mounted by userns root */
 #define FS_USERNS_DEV_MOUNT    16 /* A userns mount does not imply MNT_NODEV */
+#define FS_USERNS_VISIBLE      32      /* FS must already be visible */
 #define FS_RENAME_DOES_D_MOVE  32768   /* FS will handle d_move() during rename() internally. */
        struct dentry *(*mount) (struct file_system_type *, int,
                       const char *, void *);
@@ -1841,7 +1842,6 @@ extern int vfs_ustat(dev_t, struct kstatfs *);
 extern int freeze_super(struct super_block *super);
 extern int thaw_super(struct super_block *super);
 extern bool our_mnt(struct vfsmount *mnt);
-extern bool fs_fully_visible(struct file_system_type *);
 
 extern int current_umask(void);