fs: Better permission checking for submounts

To support unprivileged users mounting filesystems two permission
checks have to be performed: a test to see if the user allowed to
create a mount in the mount namespace, and a test to see if
the user is allowed to access the specified filesystem.

The automount case is special in that mounting the original filesystem
grants permission to mount the sub-filesystems, to any user who
happens to stumble across the their mountpoint and satisfies the
ordinary filesystem permission checks.

Attempting to handle the automount case by using override_creds
almost works.  It preserves the idea that permission to mount
the original filesystem is permission to mount the sub-filesystem.
Unfortunately using override_creds messes up the filesystems
ordinary permission checks.

Solve this by being explicit that a mount is a submount by introducing
vfs_submount, and using it where appropriate.

vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to let
sget and friends know that a mount is a submount so they can take appropriate
action.

sget and sget_userns are modified to not perform any permission checks
on submounts.

follow_automount is modified to stop using override_creds as that
has proven problemantic.

do_mount is modified to always remove the new MS_SUBMOUNT flag so
that we know userspace will never by able to specify it.

autofs4 is modified to stop using current_real_cred that was put in
there to handle the previous version of submount permission checking.

cifs is modified to pass the mountpoint all of the way down to vfs_submount.

debugfs is modified to pass the mountpoint all of the way down to
trace_automount by adding a new parameter.  To make this change easier
a new typedef debugfs_automount_t is introduced to capture the type of
the debugfs automount function.

Cc: stable@vger.kernel.org
Fixes: 069d5ac9ae0d ("autofs:  Fix automounts by using current_real_cred()->uid")
Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds")
Reviewed-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

13 files changed, 47 insertions, 22 deletions

diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index 81dd075356b9..d4fb0afc0097 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -202,7 +202,7 @@ static struct vfsmount *afs_mntpt_do_automount(struct dentry *mntpt)
 
         /* try and do the mount */
         _debug("--- attempting mount %s -o %s ---", devname, options);
-        mnt = vfs_kern_mount(&afs_fs_type, 0, devname, options);
+        mnt = vfs_submount(mntpt, &afs_fs_type, devname, options);
         _debug("--- mount result %p ---", mnt);
 
         free_page((unsigned long) devname);
diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
index 1278335ce366..79fbd85db4ba 100644
--- a/fs/autofs4/waitq.c
+++ b/fs/autofs4/waitq.c
@@ -436,8 +436,8 @@ int autofs4_wait(struct autofs_sb_info *sbi,
                 memcpy(&wq->name, &qstr, sizeof(struct qstr));
                 wq->dev = autofs4_get_dev(sbi);
                 wq->ino = autofs4_get_ino(sbi);
-                wq->uid = current_real_cred()->uid;
-                wq->gid = current_real_cred()->gid;
+                wq->uid = current_cred()->uid;
+                wq->gid = current_cred()->gid;
                 wq->pid = pid;
                 wq->tgid = tgid;
                 wq->status = -EINTR; /* Status return if interrupted */
diff --git a/fs/cifs/cifs_dfs_ref.c b/fs/cifs/cifs_dfs_ref.c
index ec9dbbcca3b9..9156be545b0f 100644
--- a/fs/cifs/cifs_dfs_ref.c
+++ b/fs/cifs/cifs_dfs_ref.c
@@ -245,7 +245,8 @@ compose_mount_options_err:
  * @fullpath:           full path in UNC format
  * @ref:                server's referral
  */
-static struct vfsmount *cifs_dfs_do_refmount(struct cifs_sb_info *cifs_sb,
+static struct vfsmount *cifs_dfs_do_refmount(struct dentry *mntpt,
+                struct cifs_sb_info *cifs_sb,
                 const char *fullpath, const struct dfs_info3_param *ref)
 {
         struct vfsmount *mnt;
@@ -259,7 +260,7 @@ static struct vfsmount *cifs_dfs_do_refmount(struct cifs_sb_info *cifs_sb,
         if (IS_ERR(mountdata))
                 return (struct vfsmount *)mountdata;
 
-        mnt = vfs_kern_mount(&cifs_fs_type, 0, devname, mountdata);
+        mnt = vfs_submount(mntpt, &cifs_fs_type, devname, mountdata);
         kfree(mountdata);
         kfree(devname);
         return mnt;
@@ -334,7 +335,7 @@ static struct vfsmount *cifs_dfs_do_automount(struct dentry *mntpt)
                         mnt = ERR_PTR(-EINVAL);
                         break;
                 }
-                mnt = cifs_dfs_do_refmount(cifs_sb,
+                mnt = cifs_dfs_do_refmount(mntpt, cifs_sb,
                                 full_path, referrals + i);
                 cifs_dbg(FYI, "%s: cifs_dfs_do_refmount:%s , mnt:%p\n",
                          __func__, referrals[i].node_name, mnt);
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index f17fcf89e18e..1e30f74a9527 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -187,9 +187,9 @@ static const struct super_operations debugfs_super_operations = {
 
 static struct vfsmount *debugfs_automount(struct path *path)
 {
-        struct vfsmount *(*f)(void *);
-        f = (struct vfsmount *(*)(void *))path->dentry->d_fsdata;
-        return f(d_inode(path->dentry)->i_private);
+        debugfs_automount_t f;
+        f = (debugfs_automount_t)path->dentry->d_fsdata;
+        return f(path->dentry, d_inode(path->dentry)->i_private);
 }
 
 static const struct dentry_operations debugfs_dops = {
@@ -504,7 +504,7 @@ EXPORT_SYMBOL_GPL(debugfs_create_dir);
  */
 struct dentry *debugfs_create_automount(const char *name,
                                         struct dentry *parent,
-                                        struct vfsmount *(*f)(void *),
+                                        debugfs_automount_t f,
                                         void *data)
 {
         struct dentry *dentry = start_creating(name, parent);
diff --git a/fs/namei.c b/fs/namei.c
index 6fa3e9138fe4..da689c9c005e 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1100,7 +1100,6 @@ static int follow_automount(struct path *path, struct nameidata *nd,
                             bool *need_mntput)
 {
         struct vfsmount *mnt;
-        const struct cred *old_cred;
         int err;
 
         if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
@@ -1129,9 +1128,7 @@ static int follow_automount(struct path *path, struct nameidata *nd,
         if (nd->total_link_count >= 40)
                 return -ELOOP;
 
-        old_cred = override_creds(&init_cred);
         mnt = path->dentry->d_op->d_automount(path);
-        revert_creds(old_cred);
         if (IS_ERR(mnt)) {
                 /*
                  * The filesystem is allowed to return -EISDIR here to indicate
diff --git a/fs/namespace.c b/fs/namespace.c
index 487ba30bb5c6..089a6b23135a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -989,6 +989,21 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
 }
 EXPORT_SYMBOL_GPL(vfs_kern_mount);
 
+struct vfsmount *
+vfs_submount(const struct dentry *mountpoint, struct file_system_type *type,
+             const char *name, void *data)
+{
+        /* Until it is worked out how to pass the user namespace
+         * through from the parent mount to the submount don't support
+         * unprivileged mounts with submounts.
+         */
+        if (mountpoint->d_sb->s_user_ns != &init_user_ns)
+                return ERR_PTR(-EPERM);
+
+        return vfs_kern_mount(type, MS_SUBMOUNT, name, data);
+}
+EXPORT_SYMBOL_GPL(vfs_submount);
+
 static struct mount *clone_mnt(struct mount *old, struct dentry *root,
                                         int flag)
 {
@@ -2794,7 +2809,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 
         flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
                    MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
-                   MS_STRICTATIME | MS_NOREMOTELOCK);
+                   MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT);
 
         if (flags & MS_REMOUNT)
                 retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index 5551e8ef67fd..e49d831c4e85 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -226,7 +226,7 @@ static struct vfsmount *nfs_do_clone_mount(struct nfs_server *server,
                                            const char *devname,
                                            struct nfs_clone_mount *mountdata)
 {
-        return vfs_kern_mount(&nfs_xdev_fs_type, 0, devname, mountdata);
+        return vfs_submount(mountdata->dentry, &nfs_xdev_fs_type, devname, mountdata);
 }
 
 /**
diff --git a/fs/nfs/nfs4namespace.c b/fs/nfs/nfs4namespace.c
index d21104912676..d8b040bd9814 100644
--- a/fs/nfs/nfs4namespace.c
+++ b/fs/nfs/nfs4namespace.c
@@ -279,7 +279,7 @@ static struct vfsmount *try_location(struct nfs_clone_mount *mountdata,
                                 mountdata->hostname,
                                 mountdata->mnt_path);
 
-                mnt = vfs_kern_mount(&nfs4_referral_fs_type, 0, page, mountdata);
+                mnt = vfs_submount(mountdata->dentry, &nfs4_referral_fs_type, page, mountdata);
                 if (!IS_ERR(mnt))
                         break;
         }
diff --git a/fs/super.c b/fs/super.c
index 1709ed029a2c..4185844f7a12 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -469,7 +469,7 @@ struct super_block *sget_userns(struct file_system_type *type,
         struct super_block *old;
         int err;
 
-        if (!(flags & MS_KERNMOUNT) &&
+        if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) &&
             !(type->fs_flags & FS_USERNS_MOUNT) &&
             !capable(CAP_SYS_ADMIN))
                 return ERR_PTR(-EPERM);
@@ -499,7 +499,7 @@ retry:
         }
         if (!s) {
                 spin_unlock(&sb_lock);
-                s = alloc_super(type, flags, user_ns);
+                s = alloc_super(type, (flags & ~MS_SUBMOUNT), user_ns);
                 if (!s)
                         return ERR_PTR(-ENOMEM);
                 goto retry;
@@ -540,8 +540,15 @@ struct super_block *sget(struct file_system_type *type,
 {
         struct user_namespace *user_ns = current_user_ns();
 
+        /* We don't yet pass the user namespace of the parent
+         * mount through to here so always use &init_user_ns
+         * until that changes.
+         */
+        if (flags & MS_SUBMOUNT)
+                user_ns = &init_user_ns;
+
         /* Ensure the requestor has permissions over the target filesystem */
-        if (!(flags & MS_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN))
+        if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
                 return ERR_PTR(-EPERM);
 
         return sget_userns(type, test, set, flags, user_ns, data);
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index 014cc564d1c4..233006be30aa 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -97,9 +97,10 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
 struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
                                       const char *dest);
 
+typedef struct vfsmount *(*debugfs_automount_t)(struct dentry *, void *);
 struct dentry *debugfs_create_automount(const char *name,
                                         struct dentry *parent,
-                                        struct vfsmount *(*f)(void *),
+                                        debugfs_automount_t f,
                                         void *data);
 
 void debugfs_remove(struct dentry *dentry);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index c6f55158d5e5..8e0352af06b7 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -90,6 +90,9 @@ struct file_system_type;
 extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
                                       int flags, const char *name,
                                       void *data);
+extern struct vfsmount *vfs_submount(const struct dentry *mountpoint,
+                                     struct file_system_type *type,
+                                     const char *name, void *data);
 
 extern void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list);
 extern void mark_mounts_for_expiry(struct list_head *mounts);
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 36da93fbf188..048a85e9f017 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -132,6 +132,7 @@ struct inodes_stat_t {
 #define MS_LAZYTIME     (1<<25) /* Update the on-disk [acm]times lazily */
 
 /* These sb flags are internal to the kernel */
+#define MS_SUBMOUNT     (1<<26)
 #define MS_NOREMOTELOCK (1<<27)
 #define MS_NOSEC        (1<<28)
 #define MS_BORN         (1<<29)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d7449783987a..310f0ea0d1a2 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7503,7 +7503,7 @@ init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
         ftrace_init_tracefs(tr, d_tracer);
 }
 
-static struct vfsmount *trace_automount(void *ingore)
+static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore)
 {
         struct vfsmount *mnt;
         struct file_system_type *type;
@@ -7516,7 +7516,7 @@ static struct vfsmount *trace_automount(void *ingore)
         type = get_fs_type("tracefs");
         if (!type)
                 return NULL;
-        mnt = vfs_kern_mount(type, 0, "tracefs", NULL);
+        mnt = vfs_submount(mntpt, type, "tracefs", NULL);
         put_filesystem(type);
         if (IS_ERR(mnt))
                 return NULL;