Commit e51db735 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

userns: Better restrictions on when proc and sysfs can be mounted 



Rely on the fact that another flavor of the filesystem is already
mounted and do not rely on state in the user namespace.

Verify that the mounted filesystem is not covered in any significant
way.  I would love to verify that the previously mounted filesystem
has no mounts on top but there are at least the directories
/proc/sys/fs/binfmt_misc and /sys/fs/cgroup/ that exist explicitly
for other filesystems to mount on top of.

Refactor the test into a function named fs_fully_visible and call that
function from the mount routines of proc and sysfs.  This makes this
test local to the filesystems involved and the results current of when
the mounts take place, removing a weird threading of the user
namespace, the mount namespace and the filesystems themselves.

Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent 4ce5d2b1

fs/namespace.c

+25 −12
Original line number Diff line number Diff line
@@ -2867,25 +2867,38 @@ bool current_chrooted(void) 
	return chrooted;

}



void update_mnt_policy(struct user_namespace *userns)

bool fs_fully_visible(struct file_system_type *type)

{

	struct mnt_namespace *ns = current->nsproxy->mnt_ns;

	struct mount *mnt;

	bool visible = false;



	down_read(&namespace_sem);

	if (unlikely(!ns))

		return false;



	namespace_lock();

	list_for_each_entry(mnt, &ns->list, mnt_list) {

		switch (mnt->mnt.mnt_sb->s_magic) {

		case SYSFS_MAGIC:

			userns->may_mount_sysfs = true;

			break;

		case PROC_SUPER_MAGIC:

			userns->may_mount_proc = true;

			break;

		struct mount *child;

		if (mnt->mnt.mnt_sb->s_type != type)

			continue;



		/* This mount is not fully visible if there are any child mounts

		 * that cover anything except for empty directories.

		 */

		list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {

			struct inode *inode = child->mnt_mountpoint->d_inode;

			if (!S_ISDIR(inode->i_mode))

				goto next;

			if (inode->i_nlink != 2)

				goto next;

		}

		if (userns->may_mount_sysfs && userns->may_mount_proc)

			break;

		visible = true;

		goto found;

	next:	;

	}

	up_read(&namespace_sem);

found:

	namespace_unlock();

	return visible;

}



static void *mntns_get(struct task_struct *task)

fs/proc/root.c

+5 −2
Original line number Diff line number Diff line
@@ -110,8 +110,11 @@ static struct dentry *proc_mount(struct file_system_type *fs_type, 
		ns = task_active_pid_ns(current);

		options = data;



		if (!current_user_ns()->may_mount_proc ||

		    !ns_capable(ns->user_ns, CAP_SYS_ADMIN))

		if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))

			return ERR_PTR(-EPERM);



		/* Does the mounter have privilege over the pid namespace? */

		if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))

			return ERR_PTR(-EPERM);

	}

fs/sysfs/mount.c

+2 −1
Original line number Diff line number Diff line
@@ -112,7 +112,8 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type, 
	struct super_block *sb;

	int error;



	if (!(flags & MS_KERNMOUNT) && !current_user_ns()->may_mount_sysfs)

	if (!(flags & MS_KERNMOUNT) && !capable(CAP_SYS_ADMIN) &&

	    !fs_fully_visible(fs_type))

		return ERR_PTR(-EPERM);



	info = kzalloc(sizeof(*info), GFP_KERNEL);

include/linux/fs.h

+1 −0
Original line number Diff line number Diff line
@@ -1897,6 +1897,7 @@ extern int vfs_ustat(dev_t, struct kstatfs *); 
extern int freeze_super(struct super_block *super);

extern int thaw_super(struct super_block *super);

extern bool our_mnt(struct vfsmount *mnt);

extern bool fs_fully_visible(struct file_system_type *);



extern int current_umask(void);

include/linux/user_namespace.h

+0 −4
Original line number Diff line number Diff line
@@ -26,8 +26,6 @@ struct user_namespace { 
	kuid_t			owner;

	kgid_t			group;

	unsigned int		proc_inum;

	bool			may_mount_sysfs;

	bool			may_mount_proc;

};



extern struct user_namespace init_user_ns;
@@ -84,6 +82,4 @@ static inline void put_user_ns(struct user_namespace *ns) 


#endif



void update_mnt_policy(struct user_namespace *userns);



#endif /* _LINUX_USER_H */

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