[PATCH v3 2/7] userns: Simpilify MNT_NODEV handling.

Seth Forshee
2015-09-16T20:04:05Z

From: "Eric W. Biederman" <ebiederm@xmission.com>

- Consolidate the testing if a device node may be opened in a new
function may_open_dev.

- Move the check for allowing access to device nodes on filesystems
not mounted in the initial user namespace from mount time to open
time and include it in may_open_dev.

This set of changes removes the implicit adding of MNT_NODEV which
simplifies the logic in fs/namespace.c and removes a potentially
problematic user visible difference in how normal and unprivileged
mount namespaces work.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/block_dev.c | 2 +-
fs/namei.c | 9 ++++++++-
fs/namespace.c | 18 ++++--------------
include/linux/fs.h | 1 +
4 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 22ea424ee741..26cee058dc02 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1730,7 +1730,7 @@ struct block_device *lookup_bdev(const char *pathname)
if (!S_ISBLK(inode->i_mode))
goto fail;
error = -EACCES;
- if (path.mnt->mnt_flags & MNT_NODEV)
+ if (!may_open_dev(&path))
goto fail;
error = -ENOMEM;
bdev = bd_acquire(inode);
diff --git a/fs/namei.c b/fs/namei.c
index 726d211db484..fcc5751d6395 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2663,6 +2663,13 @@ int vfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
}
EXPORT_SYMBOL(vfs_create);

+bool may_open_dev(const struct path *path)
+{
+ return !(path->mnt->mnt_flags & MNT_NODEV) &&
+ ((path->mnt->mnt_sb->s_user_ns == &init_user_ns) ||
+ (path->mnt->mnt_sb->s_type->fs_flags & FS_USERNS_DEV_MOUNT));
+}
+
static int may_open(struct path *path, int acc_mode, int flag)
{
struct dentry *dentry = path->dentry;
@@ -2685,7 +2692,7 @@ static int may_open(struct path *path, int acc_mode, int flag)
break;
case S_IFBLK:
case S_IFCHR:
- if (path->mnt->mnt_flags & MNT_NODEV)
+ if (!may_open_dev(path))
return -EACCES;
/*FALLTHRU*/
case S_IFIFO:
diff --git a/fs/namespace.c b/fs/namespace.c
index d023a353dc63..da70f7c4ece1 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2177,13 +2177,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
}
if ((mnt->mnt.mnt_flags & MNT_LOCK_NODEV) &&
!(mnt_flags & MNT_NODEV)) {
- /* Was the nodev implicitly added in mount? */
- if ((mnt->mnt_ns->user_ns != &init_user_ns) &&
- !(sb->s_type->fs_flags & FS_USERNS_DEV_MOUNT)) {
- mnt_flags |= MNT_NODEV;
- } else {
- return -EPERM;
- }
+ return -EPERM;
}
if ((mnt->mnt.mnt_flags & MNT_LOCK_NOSUID) &&
!(mnt_flags & MNT_NOSUID)) {
@@ -2396,13 +2390,6 @@ 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 (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {
- flags |= MS_NODEV;
- mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
- }
if (type->fs_flags & FS_USERNS_VISIBLE) {
if (!fs_fully_visible(type, &mnt_flags))
return -EPERM;
@@ -3238,6 +3225,9 @@ static bool fs_fully_visible(struct file_system_type *type, int *new_mnt_flags)
mnt_flags = mnt->mnt.mnt_flags;
if (mnt->mnt.mnt_sb->s_iflags & SB_I_NOEXEC)
mnt_flags &= ~(MNT_LOCK_NOSUID | MNT_LOCK_NOEXEC);
+ if (mnt->mnt.mnt_sb->s_user_ns != &init_user_ns &&
+ !(mnt->mnt.mnt_sb->s_type->fs_flags & FS_USERNS_DEV_MOUNT))
+ mnt_flags &= ~(MNT_LOCK_NODEV);

/* Verify the mount flags are equal to or more permissive
* than the proposed new mount.
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 79c15ab2159d..5ec201e8308c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1537,6 +1537,7 @@ extern void dentry_unhash(struct dentry *dentry);
*/
extern void inode_init_owner(struct inode *inode, const struct inode *dir,
umode_t mode);
+extern bool may_open_dev(const struct path *path);
/*
* VFS FS_IOC_FIEMAP helper definitions.
*/
--
1.9.1