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Subject[patch 07/10] unprivileged mounts: allow unprivileged mounts
From: Miklos Szeredi <mszeredi@suse.cz>

Define a new fs flag FS_SAFE, which denotes, that unprivileged mounting of
this filesystem may not constitute a security problem.

Since most filesystems haven't been designed with unprivileged mounting in
mind, a thorough audit is needed before setting this flag.

For "safe" filesystems also allow unprivileged forced unmounting.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---
Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c 2007-04-26 13:30:04.000000000 +0200
+++ linux/fs/namespace.c 2007-04-26 13:51:29.000000000 +0200
@@ -724,14 +724,16 @@ static bool is_mount_owner(struct vfsmou
/*
* umount is permitted for
* - sysadmin
- * - mount owner, if not forced umount
+ * - mount owner
+ * o if not forced umount,
+ * o if forced umount, and filesystem is "safe"
*/
static bool permit_umount(struct vfsmount *mnt, int flags)
{
if (capable(CAP_SYS_ADMIN))
return true;

- if (flags & MNT_FORCE)
+ if ((flags & MNT_FORCE) && !(mnt->mnt_sb->s_type->fs_flags & FS_SAFE))
return false;

return is_mount_owner(mnt, current->fsuid);
@@ -787,13 +789,17 @@ asmlinkage long sys_oldumount(char __use
* - mountpoint is not a symlink
* - mountpoint is in a mount owned by the user
*/
-static bool permit_mount(struct nameidata *nd, int *flags)
+static bool permit_mount(struct nameidata *nd, struct file_system_type *type,
+ int *flags)
{
struct inode *inode = nd->dentry->d_inode;

if (capable(CAP_SYS_ADMIN))
return true;

+ if (type && !(type->fs_flags & FS_SAFE))
+ return false;
+
if (S_ISLNK(inode->i_mode))
return false;

@@ -1027,7 +1033,7 @@ static int do_loopback(struct nameidata
struct vfsmount *mnt = NULL;
int err;

- if (!permit_mount(nd, &flags))
+ if (!permit_mount(nd, NULL, &flags))
return -EPERM;
if (!old_name || !*old_name)
return -EINVAL;
@@ -1188,26 +1194,46 @@ out:
* create a new mount for userspace and request it to be added into the
* namespace's tree
*/
-static int do_new_mount(struct nameidata *nd, char *type, int flags,
+static int do_new_mount(struct nameidata *nd, char *fstype, int flags,
int mnt_flags, char *name, void *data)
{
+ int err;
struct vfsmount *mnt;
+ struct file_system_type *type;

- if (!type || !memchr(type, 0, PAGE_SIZE))
+ if (!fstype || !memchr(fstype, 0, PAGE_SIZE))
return -EINVAL;

- /* we need capabilities... */
- if (!capable(CAP_SYS_ADMIN))
- return -EPERM;
-
- mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
- if (IS_ERR(mnt))
+ type = get_fs_type(fstype);
+ if (!type)
+ return -ENODEV;
+
+ err = -EPERM;
+ if (!permit_mount(nd, type, &flags))
+ goto out_put_filesystem;
+
+ if (flags & MS_SETUSER) {
+ err = reserve_user_mount();
+ if (err)
+ goto out_put_filesystem;
+ }
+
+ mnt = vfs_kern_mount(type, flags & ~MS_SETUSER, name, data);
+ put_filesystem(type);
+ if (IS_ERR(mnt)) {
+ if (flags & MS_SETUSER)
+ dec_nr_user_mounts();
return PTR_ERR(mnt);
+ }

if (flags & MS_SETUSER)
- set_mnt_user(mnt);
+ __set_mnt_user(mnt);

return do_add_mount(mnt, nd, mnt_flags, NULL);
+
+ out_put_filesystem:
+ put_filesystem(type);
+ return err;
}

/*
@@ -1237,7 +1263,7 @@ int do_add_mount(struct vfsmount *newmnt
if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
goto unlock;

- /* MNT_USER was set earlier */
+ /* some flags may have been set earlier */
newmnt->mnt_flags |= mnt_flags;
if ((err = graft_tree(newmnt, nd)))
goto unlock;
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h 2007-04-26 13:46:26.000000000 +0200
+++ linux/include/linux/fs.h 2007-04-26 13:48:14.000000000 +0200
@@ -96,6 +96,7 @@ extern int dir_notify_enable;
#define FS_REQUIRES_DEV 1
#define FS_BINARY_MOUNTDATA 2
#define FS_HAS_SUBTYPE 4
+#define FS_SAFE 8 /* Safe to mount by unprivileged users */
#define FS_REVAL_DOT 16384 /* Check the paths ".", ".." for staleness */
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move()
* during rename() internally.
--
-
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