From cf39425ff2e6254a8a4cf94ec42d509eedd9f92a Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Sat, 30 Mar 2013 19:57:41 -0700 Subject: [PATCH] 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: "Eric W. Biederman" --- fs/namespace.c | 37 +++++++++++++++++++++++----------- fs/proc/root.c | 7 +++++-- fs/sysfs/mount.c | 3 ++- include/linux/fs.h | 1 + include/linux/user_namespace.h | 4 ---- kernel/user.c | 2 -- kernel/user_namespace.c | 2 -- 7 files changed, 33 insertions(+), 23 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 64627f883bf20..877e4277f4962 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -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) diff --git a/fs/proc/root.c b/fs/proc/root.c index 38bd5d423fcd2..45e5fb7da09bd 100644 --- a/fs/proc/root.c +++ b/fs/proc/root.c @@ -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); } diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c index afd83273e6cea..4a2da3a4b1b11 100644 --- a/fs/sysfs/mount.c +++ b/fs/sysfs/mount.c @@ -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); diff --git a/include/linux/fs.h b/include/linux/fs.h index 981874773e85a..3050c620f062f 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -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); diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h index b6b215f13b453..4ce009324933e 100644 --- a/include/linux/user_namespace.h +++ b/include/linux/user_namespace.h @@ -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 */ diff --git a/kernel/user.c b/kernel/user.c index 69b4c3d48cdee..5bbb91988e692 100644 --- a/kernel/user.c +++ b/kernel/user.c @@ -51,8 +51,6 @@ struct user_namespace init_user_ns = { .owner = GLOBAL_ROOT_UID, .group = GLOBAL_ROOT_GID, .proc_inum = PROC_USER_INIT_INO, - .may_mount_sysfs = true, - .may_mount_proc = true, }; EXPORT_SYMBOL_GPL(init_user_ns); diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c index d8c30db06c5b7..d58ad1e7a7946 100644 --- a/kernel/user_namespace.c +++ b/kernel/user_namespace.c @@ -97,8 +97,6 @@ int create_user_ns(struct cred *new) set_cred_user_ns(new, ns); - update_mnt_policy(ns); - return 0; } -- 2.39.5