[PATCH v3.4] capabilities: require CAP_SETFCAP to map uid 0

[Serge E. Hallyn]

cap_setfcap is required to create file capabilities.

Since 8db6c34f1dbc ("Introduce v3 namespaced file capabilities"), a
process running as uid 0 but without cap_setfcap is able to work around
this as follows: unshare a new user namespace which maps parent uid 0
into the child namespace.  While this task will not have new
capabilities against the parent namespace, there is a loophole due to
the way namespaced file capabilities are represented as xattrs.  File
capabilities valid in userns 1 are distinguished 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 parent uid 0 if the process which
opened the uid_map file does not have CAP_SETFCAP, which is the capability
for setting file capabilities.

As 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

Note: an alternative solution would be to allow uid 0 mappings by
processes without CAP_SETFCAP, but to prevent such a namespace from
writing any file capabilities.  This approach can be seen here:
    https://git.kernel.org/pub/scm/linux/kernel/git/sergeh/linux.git/log/?h=2021-04-15/setfcap-nsfscaps-v4

History:

Commit 95ebabde382 ("capabilities: Don't allow writing ambiguous v3 file
capabilities") tried to fix the issue by preventing v3 fscaps to be
written to disk when the root uid would map to the same uid in nested
user namespaces. This led to regressions for various workloads. For
example, see [1]. Ultimately this is a valid use-case we have to support
meaning we had to revert this change in 3b0c2d3eaa83 ("Revert
95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file
capabilities")").

[1]: https://github.com/containers/buildah/issues/3071

Signed-off-by: Serge Hallyn <serge@xxxxxxxxxx>
Reviewed-by: Andrew G. Morgan <morgan@xxxxxxxxxx>
Tested-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
Reviewed-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
Tested-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

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
   * rename disallowed_0_mapping to verify_root_mapping
   * change verify_root_mapping to Christian's suggested flow
   * correct+clarify comments: parent uid 0 mapping to any
     child uid is a problem.
   * remove unused lower_first variable.
---
 include/linux/user_namespace.h  |  3 ++
 include/uapi/linux/capability.h |  3 +-
 kernel/user_namespace.c         | 65 +++++++++++++++++++++++++++++++--
 3 files changed, 67 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..9a4b980d695b 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 +842,60 @@ static int sort_idmaps(struct uid_gid_map *map)
 	return 0;
 }
 
+/**
+ * verify_root_map() - check the uid 0 mapping
+ * @file: idmapping file
+ * @map_ns: user namespace of the target process
+ * @new_map: requested idmap
+ *
+ * If a process requests mapping parent uid 0 into the new ns, verify that the
+ * process writing the map had the CAP_SETFCAP capability as the target process
+ * will be able to write fscaps that are valid in ancestor user namespaces.
+ *
+ * Return: true if the mapping is allowed, false if not.
+ */
+static bool verify_root_map(const struct file *file,
+			    struct user_namespace *map_ns,
+			    struct uid_gid_map *new_map)
+{
+	int idx;
+	const struct user_namespace *file_ns = file->f_cred->user_ns;
+	struct uid_gid_extent *extent0 = NULL;
+
+	for (idx = 0; idx < new_map->nr_extents; idx++) {
+		if (new_map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+			extent0 = &new_map->extent[idx];
+		else
+			extent0 = &new_map->forward[idx];
+		if (extent0->lower_first == 0)
+			break;
+
+		extent0 = NULL;
+	}
+
+	if (!extent0)
+		return true;
+
+	if (map_ns == file_ns) {
+		/* The process unshared its ns and is writing to its own
+		 * /proc/self/uid_map.  User already has full capabilites in
+		 * the new namespace.  Verify that the parent had CAP_SETFCAP
+		 * when it unshared.
+		 * */
+		if (!file_ns->parent_could_setfcap)
+			return false;
+	} else {
+		/* Process p1 is writing to uid_map of p2, who is in a child
+		 * user namespace to p1's.  Verify that the opener of the map
+		 * file has CAP_SETFCAP against the parent of the new map
+		 * namespace */
+		if (!file_ns_capable(file, map_ns->parent, CAP_SETFCAP))
+			return false;
+	}
+
+	return true;
+}
+
 static ssize_t map_write(struct file *file, const char __user *buf,
 			 size_t count, loff_t *ppos,
 			 int cap_setid,
@@ -848,7 +903,7 @@ 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 +950,7 @@ 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 +1020,7 @@ 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 +1141,10 @@ 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 && !verify_root_map(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.
 	 */
-- 
2.25.1