[PATCH] fs: Remove implicit nodev for new mounts in non-root userns

[Andy Lutomirski]

Currently, creating a new mount (as opposed to bindmount) in a
non-root userns will implicitly set nodev unless the fs is devpts.
Something like this will be necessary for file systems that allow
the mounter to create device nodes without using mknod (e.g. FUSE
if/when that is allowed), but none of the currently allowed
filesystems do this.

Implicitly adding nodev is problematic, though.  It will make it
unsafe to ever remove the implicit addition, since userspace might
start to rely on it.

This fixes a minor regression in:

    9566d6742852 mnt: Correct permission checks in do_remount

Prior to that commit, MNT_NODEV wasn't enforced for remounts, so
there is existing user code that creates a new mount in a userns
without MS_NODEV and then expects a remount with matching options to
work.  That commit broke code that does this.

Fortunately, since the implicit nodev has no effect on any existing
filesystems, we can still safely remove it.

This replaces the implicit nodev with an explicit nodev requirement:
anyone who mounts a filesystem without FS_USERNS_DEV_MOUNT will get
-EPERM unless they set nodev.  If they set nodev, that setting will
be locked.

As an added benefit, if anything like device namespaces is ever
added, then user code will be able to opt out of nodev by clearing
nodev.

To keep existing code working, this adds FS_USERNS_DEV_MOUNT to all
FS_USERNS_MOUNT filesystems.  All of the current filesystems with
FS_USERNS_MOUNT set are safe.

I confirmed that this is compatible with Sandstorm's revision
1bf0c4847b.  That revision of Sandstorm does not work without this
fix if 9566d6742852 is applied.

Cc: Kenton Varda <kenton@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 fs/namespace.c   | 16 ++++++++++++----
 fs/proc/root.c   |  2 +-
 fs/ramfs/inode.c |  2 +-
 fs/sysfs/mount.c |  2 +-
 ipc/mqueue.c     |  2 +-
 mm/shmem.c       |  4 ++--
 6 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 0acabea58319..835fa9e8307e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2154,12 +2154,20 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
 			put_filesystem(type);
 			return -EPERM;
 		}
-		/* Only in special cases allow devices from mounts
-		 * created outside the initial user namespace.
+
+		/*
+		 * If a filesystem might allow the mounter to put
+		 * device nodes on it without the checks in mknod,
+		 * then require MS_NODEV to mount it.
 		 */
 		if (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {
-			flags |= MS_NODEV;
-			mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
+			if (!(mnt_flags & MNT_NODEV)) {
+				put_filesystem(type);
+				return -EPERM;
+			}
+
+			/* Do not allow nodev to be cleared. */
+			mnt_flags |= MNT_LOCK_NODEV;
 		}
 	}
 
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 094e44d4a6be..2313b280729e 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -159,7 +159,7 @@ 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_MOUNT | FS_USERNS_DEV_MOUNT,
 };
 
 void __init proc_root_init(void)
diff --git a/fs/ramfs/inode.c b/fs/ramfs/inode.c
index d365b1c4eb3c..b95b7302d4cc 100644
--- a/fs/ramfs/inode.c
+++ b/fs/ramfs/inode.c
@@ -261,7 +261,7 @@ static struct file_system_type ramfs_fs_type = {
 	.name		= "ramfs",
 	.mount		= ramfs_mount,
 	.kill_sb	= ramfs_kill_sb,
-	.fs_flags	= FS_USERNS_MOUNT,
+	.fs_flags	= FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
 };
 
 int __init init_ramfs_fs(void)
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 8a49486bf30c..56ba59317e24 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -58,7 +58,7 @@ 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_MOUNT | FS_USERNS_DEV_MOUNT,
 };
 
 int __init sysfs_init(void)
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 4fcf39af1776..56abbc848d4c 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -1394,7 +1394,7 @@ static struct file_system_type mqueue_fs_type = {
 	.name = "mqueue",
 	.mount = mqueue_mount,
 	.kill_sb = kill_litter_super,
-	.fs_flags = FS_USERNS_MOUNT,
+	.fs_flags = FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
 };
 
 int mq_init_ns(struct ipc_namespace *ns)
diff --git a/mm/shmem.c b/mm/shmem.c
index a42add14331c..f4a708a8f9e3 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3149,7 +3149,7 @@ static struct file_system_type shmem_fs_type = {
 	.name		= "tmpfs",
 	.mount		= shmem_mount,
 	.kill_sb	= kill_litter_super,
-	.fs_flags	= FS_USERNS_MOUNT,
+	.fs_flags	= FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
 };
 
 int __init shmem_init(void)
@@ -3208,7 +3208,7 @@ static struct file_system_type shmem_fs_type = {
 	.name		= "tmpfs",
 	.mount		= ramfs_mount,
 	.kill_sb	= kill_litter_super,
-	.fs_flags	= FS_USERNS_MOUNT,
+	.fs_flags	= FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
 };
 
 int __init shmem_init(void)
-- 
1.9.3

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