[RFC PATCH] capabilities: require CAP_SETFCAP to map uid 0 (v3)

[Serge E. Hallyn]

(Eric - this patch (v3) is a cleaned up version of the previous approach.
v4 is at https://git.kernel.org/pub/scm/linux/kernel/git/sergeh/linux.git/log/?h=2021-04-15/setfcap-nsfscaps-v4
and is the approach you suggested.  I can send it also as a separate patch
if you like)

A process running as uid 0 but without cap_setfcap currently can simply
unshare a new user namespace with uid 0 mapped to 0.  While this task
will not have new capabilities against the parent namespace, there is
a loophole due to the way namespaced file capabilities work.  File
capabilities valid in userns 1 are distinguised from file capabilities
valid in userns 2 by the kuid which underlies uid 0.  Therefore
the restricted root process can unshare a new self-mapping namespace,
add a namespaced file capability onto a file, then use that file
capability in the parent namespace.

To prevent that, do not allow mapping uid 0 if the process which
opened the uid_map file does not have CAP_SETFCAP, which is the capability
for setting file capabilities.

A further wrinkle:  a task can unshare its user namespace, then
open its uid_map file itself, and map (only) its own uid.  In this
case we do not have the credential from before unshare,  which was
potentially more restricted.  So, when creating a user namespace, we
record whether the creator had CAP_SETFCAP.  Then we can use that
during map_write().

With this patch:

1. unprivileged user can still unshare -Ur

ubuntu@caps:~$ unshare -Ur
root@caps:~# logout

2. root user can still unshare -Ur

ubuntu@caps:~$ sudo bash
root@caps:/home/ubuntu# unshare -Ur
root@caps:/home/ubuntu# logout

3. root user without CAP_SETFCAP cannot unshare -Ur:

root@caps:/home/ubuntu# /sbin/capsh --drop=cap_setfcap --
root@caps:/home/ubuntu# /sbin/setcap cap_setfcap=p /sbin/setcap
unable to set CAP_SETFCAP effective capability: Operation not permitted
root@caps:/home/ubuntu# unshare -Ur
unshare: write failed /proc/self/uid_map: Operation not permitted

Signed-off-by: Serge Hallyn <serge@xxxxxxxxxx>

Changelog:
   * fix logic in the case of writing to another task's uid_map
   * rename 'ns' to 'map_ns', and make a file_ns local variable
   * use /* comments */
   * update the CAP_SETFCAP comment in capability.h
   * rename parent_unpriv to parent_can_setfcap (and reverse the
     logic)
   * remove printks
   * clarify (i hope) the code comments
   * update capability.h comment
   * renamed parent_can_setfcap to parent_could_setfcap
   * made the check its own disallowed_0_mapping() fn
   * moved the check into new_idmap_permitted
---
 include/linux/user_namespace.h  |  3 ++
 include/uapi/linux/capability.h |  3 +-
 kernel/user_namespace.c         | 61 +++++++++++++++++++++++++++++++--
 3 files changed, 63 insertions(+), 4 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 64cf8ebdc4ec..f6c5f784be5a 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -63,6 +63,9 @@ struct user_namespace {
 	kgid_t			group;
 	struct ns_common	ns;
 	unsigned long		flags;
+	/* parent_could_setfcap: true if the creator if this ns had CAP_SETFCAP
+	 * in its effective capability set at the child ns creation time. */
+	bool			parent_could_setfcap;
 
 #ifdef CONFIG_KEYS
 	/* List of joinable keyrings in this namespace.  Modification access of
diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
index c6ca33034147..2ddb4226cd23 100644
--- a/include/uapi/linux/capability.h
+++ b/include/uapi/linux/capability.h
@@ -335,7 +335,8 @@ struct vfs_ns_cap_data {
 
 #define CAP_AUDIT_CONTROL    30
 
-/* Set or remove capabilities on files */
+/* Set or remove capabilities on files.
+   Map uid=0 into a child user namespace. */
 
 #define CAP_SETFCAP	     31
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index af612945a4d0..8c75028a9aae 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -106,6 +106,7 @@ int create_user_ns(struct cred *new)
 	if (!ns)
 		goto fail_dec;
 
+	ns->parent_could_setfcap = cap_raised(new->cap_effective, CAP_SETFCAP);
 	ret = ns_alloc_inum(&ns->ns);
 	if (ret)
 		goto fail_free;
@@ -841,6 +842,56 @@ static int sort_idmaps(struct uid_gid_map *map)
 	return 0;
 }
 
+/*
+ * If mapping uid 0, then file capabilities created by the new namespace will
+ * be effective in the parent namespace.  Adding file capabilities requires
+ * CAP_SETFCAP, which the child namespace will have, so creating such a
+ * mapping requires CAP_SETFCAP in the parent namespace.
+ */
+static bool disallowed_0_mapping(const struct file *file,
+				 struct user_namespace *map_ns,
+				 struct uid_gid_map *new_map)
+{
+	int idx;
+	bool zeromapping = false;
+	const struct user_namespace *file_ns = file->f_cred->user_ns;
+
+	for (idx = 0; idx < new_map->nr_extents; idx++) {
+		struct uid_gid_extent *e;
+		u32 lower_first;
+
+		if (new_map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+			e = &new_map->extent[idx];
+		else
+			e = &new_map->forward[idx];
+		if (e->lower_first == 0) {
+			zeromapping = true;
+			break;
+		}
+	}
+
+	if (!zeromapping)
+		return false;
+
+	if (map_ns == file_ns) {
+		/* The user unshared first and is writing to
+		 * /proc/self/uid_map.  User already has full
+		 * capabilites in the new namespace, so verify
+		 * that the parent has CAP_SETFCAP. */
+		if (!file_ns->parent_could_setfcap)
+			return true;
+	} else {
+		/* Process p1 is writing to uid_map of p2, who
+		 * is in a child user namespace to p1's.  So
+		 * we verify that p1 has CAP_SETFCAP to its
+		 * own namespace */
+		if (!file_ns_capable(file, map_ns->parent, CAP_SETFCAP))
+			return true;
+	}
+
+	return false;
+}
+
 static ssize_t map_write(struct file *file, const char __user *buf,
 			 size_t count, loff_t *ppos,
 			 int cap_setid,
@@ -848,7 +899,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 			 struct uid_gid_map *parent_map)
 {
 	struct seq_file *seq = file->private_data;
-	struct user_namespace *ns = seq->private;
+	struct user_namespace *map_ns = seq->private;
 	struct uid_gid_map new_map;
 	unsigned idx;
 	struct uid_gid_extent extent;
@@ -895,7 +946,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	/*
 	 * Adjusting namespace settings requires capabilities on the target.
 	 */
-	if (cap_valid(cap_setid) && !file_ns_capable(file, ns, CAP_SYS_ADMIN))
+	if (cap_valid(cap_setid) && !file_ns_capable(file, map_ns, CAP_SYS_ADMIN))
 		goto out;
 
 	/* Parse the user data */
@@ -965,7 +1016,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 
 	ret = -EPERM;
 	/* Validate the user is allowed to use user id's mapped to. */
-	if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
+	if (!new_idmap_permitted(file, map_ns, cap_setid, &new_map))
 		goto out;
 
 	ret = -EPERM;
@@ -1086,6 +1137,10 @@ static bool new_idmap_permitted(const struct file *file,
 				struct uid_gid_map *new_map)
 {
 	const struct cred *cred = file->f_cred;
+
+	if (cap_setid == CAP_SETUID && disallowed_0_mapping(file, ns, new_map))
+		return false;
+
 	/* Don't allow mappings that would allow anything that wouldn't
 	 * be allowed without the establishment of unprivileged mappings.
 	 */
-- 
2.17.1