mnt: Refactor fs_fully_visible into mount_too_revealing

Replace the call of fs_fully_visible in do_new_mount from before the
new superblock is allocated with a call of mount_too_revealing after
the superblock is allocated.   This winds up being a much better location
for maintainability of the code.

The first change this enables is the replacement of FS_USERNS_VISIBLE
with SB_I_USERNS_VISIBLE.  Moving the flag from struct filesystem_type
to sb_iflags on the superblock.

Unfortunately mount_too_revealing fundamentally needs to touch
mnt_flags adding several MNT_LOCKED_XXX flags at the appropriate
times.  If the mnt_flags did not need to be touched the code
could be easily moved into the filesystem specific mount code.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

4 files changed, 29 insertions, 16 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 783004af5707..1a69aa786975 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2375,7 +2375,7 @@ unlock:
         return err;
 }
 
-static bool fs_fully_visible(struct file_system_type *fs_type, int *new_mnt_flags);
+static bool mount_too_revealing(struct vfsmount *mnt, int *new_mnt_flags);
 
 /*
  * create a new mount for userspace and request it to be added into the
@@ -2408,12 +2408,6 @@ 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, &mnt_flags)) {
-                                put_filesystem(type);
-                                return -EPERM;
-                        }
-                }
         }
 
         mnt = vfs_kern_mount(type, flags, name, data);
@@ -2425,6 +2419,11 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
         if (IS_ERR(mnt))
                 return PTR_ERR(mnt);
 
+        if (mount_too_revealing(mnt, &mnt_flags)) {
+                mntput(mnt);
+                return -EPERM;
+        }
+
         err = do_add_mount(real_mount(mnt), path, mnt_flags);
         if (err)
                 mntput(mnt);
@@ -3216,22 +3215,19 @@ bool current_chrooted(void)
         return chrooted;
 }
 
-static bool fs_fully_visible(struct file_system_type *type, int *new_mnt_flags)
+static bool mnt_already_visible(struct mnt_namespace *ns, struct vfsmount *new,
+                                int *new_mnt_flags)
 {
-        struct mnt_namespace *ns = current->nsproxy->mnt_ns;
         int new_flags = *new_mnt_flags;
         struct mount *mnt;
         bool visible = false;
 
-        if (unlikely(!ns))
-                return false;
-
         down_read(&namespace_sem);
         list_for_each_entry(mnt, &ns->list, mnt_list) {
                 struct mount *child;
                 int mnt_flags;
 
-                if (mnt->mnt.mnt_sb->s_type != type)
+                if (mnt->mnt.mnt_sb->s_type != new->mnt_sb->s_type)
                         continue;
 
                 /* This mount is not fully visible if it's root directory
@@ -3298,6 +3294,22 @@ found:
         return visible;
 }
 
+static bool mount_too_revealing(struct vfsmount *mnt, int *new_mnt_flags)
+{
+        struct mnt_namespace *ns = current->nsproxy->mnt_ns;
+        unsigned long s_iflags;
+
+        if (ns->user_ns == &init_user_ns)
+                return false;
+
+        /* Can this filesystem be too revealing? */
+        s_iflags = mnt->mnt_sb->s_iflags;
+        if (!(s_iflags & SB_I_USERNS_VISIBLE))
+                return false;
+
+        return !mnt_already_visible(ns, mnt, new_mnt_flags);
+}
+
 static struct ns_common *mntns_get(struct task_struct *task)
 {
         struct ns_common *ns = NULL;
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 42305ddcbaa0..78fa452d65ed 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -462,6 +462,7 @@ int proc_fill_super(struct super_block *s)
         struct inode *root_inode;
         int ret;
 
+        s->s_iflags |= SB_I_USERNS_VISIBLE;
         s->s_flags |= MS_NODIRATIME | MS_NOSUID | MS_NOEXEC;
         s->s_blocksize = 1024;
         s->s_blocksize_bits = 10;
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 55bc7d6c8aac..a1b2860fec62 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -158,7 +158,7 @@ static struct file_system_type proc_fs_type = {
         .name           = "proc",
         .mount          = proc_mount,
         .kill_sb        = proc_kill_sb,
-        .fs_flags       = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
+        .fs_flags       = FS_USERNS_MOUNT,
 };
 
 void __init proc_root_init(void)
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index f3db82071cfb..f31e36994dfb 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -42,7 +42,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
                 kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
         else if (new_sb)
                 /* Userspace would break if executables appear on sysfs */
-                root->d_sb->s_iflags |= SB_I_NOEXEC;
+                root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC;
 
         return root;
 }
@@ -59,7 +59,7 @@ static struct file_system_type sysfs_fs_type = {
         .name           = "sysfs",
         .mount          = sysfs_mount,
         .kill_sb        = sysfs_kill_sb,
-        .fs_flags       = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
+        .fs_flags       = FS_USERNS_MOUNT,
 };
 
 int __init sysfs_init(void)