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>

diff --git a/fs/namespace.c b/fs/namespace.c
index 783004a..1a69aa7 100644 (file)
--- 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 42305dd..78fa452 100644 (file)
--- 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 55bc7d6..a1b2860 100644 (file)
--- 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 f3db820..f31e369 100644 (file)
--- 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)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index dd28814..71988dd 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1328,6 +1328,9 @@ struct mm_struct;
 #define SB_I_CGROUPWB  0x00000001      /* cgroup-aware writeback enabled */
 #define SB_I_NOEXEC    0x00000002      /* Ignore executables on this fs */
 
+/* sb->s_iflags to limit user namespace mounts */
+#define SB_I_USERNS_VISIBLE            0x00000010 /* fstype already mounted */
+
 /* Possible states of 'frozen' field */
 enum {
        SB_UNFROZEN = 0,                /* FS is unfrozen */
@@ -2011,7 +2014,6 @@ 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 *);