Add security.jail.mount_allowed sysctl, which allows to mount and

unmount jail-friendly file systems from within a jail.
Precisely it grants PRIV_VFS_MOUNT, PRIV_VFS_UNMOUNT and
PRIV_VFS_MOUNT_NONUSER privileges for a jailed super-user.
It is turned off by default.

A jail-friendly file system is a file system which driver registers
itself with VFCF_JAIL flag via VFS_SET(9) API.
The lsvfs(1) command can be used to see which file systems are
jail-friendly ones.

There currently no jail-friendly file systems, ZFS will be the first one.
In the future we may consider marking file systems like nullfs as
jail-friendly.

Reviewed by:	rwatson

lib/libc/gen/getvfsbyname.3

@@ -81,6 +81,11 @@ data does not represent real files
 aliases some other mounted FS
 .It Dv VFCF_UNICODE
 stores file names as Unicode
+.It Dv VFCF_JAIL
+can be mounted from within a jail if
+.Va security.jail.mount_allowed
+sysctl is set to
+.Dv 1 .
 .El
 .Sh RETURN VALUES
 .Rv -std getvfsbyname
@@ -99,8 +104,10 @@ argument
 specifies a file system that is unknown or not configured in the kernel.
 .El
 .Sh SEE ALSO
+.Xr jail 2 ,
 .Xr mount 2 ,
 .Xr sysctl 3 ,
+.Xr jail 8 ,
 .Xr mount 8 ,
 .Xr sysctl 8
 .Sh HISTORY

share/man/man9/VFS_SET.9

@@ -70,6 +70,11 @@ Pseudo file system, data does not represent on-disk files.
 Loopback file system layer.
 .It Dv VFCF_UNICODE
 File names are stored as Unicode.
+.It Dv VFCF_JAIL
+can be mounted from within a jail if
+.Va security.jail.mount_allowed
+sysctl is set to
+.Dv 1 .
 .El
 .Sh PSEUDOCODE
 .Bd -literal
@@ -96,6 +101,8 @@ static struct vfsops myfs_vfsops = {
 VFS_SET(myfs_vfsops, skelfs, 0);
 .Ed
 .Sh SEE ALSO
+.Xr jail 2 ,
+.Xr jail 8 ,
 .Xr DECLARE_MODULE 9 ,
 .Xr vfsconf 9 ,
 .Xr vfs_modevent 9

sys/kern/kern_jail.c

@@ -72,6 +72,11 @@ SYSCTL_INT(_security_jail, OID_AUTO, chflags_allowed, CTLFLAG_RW,
     &jail_chflags_allowed, 0,
     "Processes in jail can alter system file flags");
 
+int	jail_mount_allowed = 0;
+SYSCTL_INT(_security_jail, OID_AUTO, mount_allowed, CTLFLAG_RW,
+    &jail_mount_allowed, 0,
+    "Processes in jail can mount/unmount jail-friendly file systems");
+
 /* allprison, lastprid, and prisoncount are protected by allprison_mtx. */
 struct	prisonlist allprison;
 struct	mtx allprison_mtx;
@@ -651,6 +656,18 @@ prison_priv_check(struct ucred *cred, int priv)
 		else
 			return (EPERM);
 
+		/*
+		 * Depending on the global setting, allow privilege of
+		 * mounting/unmounting file systems.
+		 */
+	case PRIV_VFS_MOUNT:
+	case PRIV_VFS_UNMOUNT:
+	case PRIV_VFS_MOUNT_NONUSER:
+		if (jail_mount_allowed)
+			return (0);
+		else
+			return (EPERM);
+
 		/*
 		 * Allow jailed root to bind reserved ports.
 		 */

sys/kern/vfs_mount.c

@@ -847,6 +847,8 @@ vfs_domount(
 			vfsp = vfs_byname_kld(fstype, td, &error);
 		if (vfsp == NULL)
 			return (ENODEV);
+		if (jailed(td->td_ucred) && !(vfsp->vfc_flags & VFCF_JAIL))
+			return (EPERM);
 	}
 	/*
 	 * Get vnode to be covered
@@ -863,6 +865,11 @@ vfs_domount(
 			return (EINVAL);
 		}
 		mp = vp->v_mount;
+		vfsp = mp->mnt_vfc;
+		if (jailed(td->td_ucred) && !(vfsp->vfc_flags & VFCF_JAIL)) {
+			vput(vp);
+			return (EPERM);
+		}
 		MNT_ILOCK(mp);
 		flag = mp->mnt_flag;
 		/*

sys/sys/mount.h

@@ -427,6 +427,7 @@ struct ovfsconf {
 #define	VFCF_SYNTHETIC	0x00080000	/* data does not represent real files */
 #define	VFCF_LOOPBACK	0x00100000	/* aliases some other mounted FS */
 #define	VFCF_UNICODE	0x00200000	/* stores file names as Unicode */
+#define	VFCF_JAIL	0x00400000	/* can be mounted from within a jail */
 
 typedef uint32_t fsctlop_t;
 

usr.bin/lsvfs/lsvfs.c

@@ -105,5 +105,10 @@ fmt_flags(int flags)
     strcat(buf, "unicode");
   }
 
+  if(flags & VFCF_JAIL) {
+    if(comma++) strcat(buf, ", ");
+    strcat(buf, "jail");
+  }
+
   return buf;
 }

usr.sbin/jail/jail.8

@@ -546,6 +546,15 @@ or clear system file flags; if non-zero, such users are treated as
 privileged, and may manipulate system file flags subject to the usual
 constraints on
 .Va kern.securelevel .
+.It Va security.jail.mount_allowed
+This MIB entry determines if a privileged user inside a jail will be
+able to mount and unmount file system types marked as jail-friendly.
+The
+.Xr lsvfs 1
+command can be used to find file system types available for mount from within
+a jail.
+This functionality is disabled by default, but can be enabled by setting this
+MIB entry to 1.
 .El
 .Pp
 The read-only sysctl variable
@@ -572,6 +581,7 @@ and
 .Va kern.hostname .
 .Sh SEE ALSO
 .Xr killall 1 ,
+.Xr lsvfs 1 ,
 .Xr newaliases 1 ,
 .Xr pgrep 1 ,
 .Xr pkill 1 ,