[PATCH 04/24] fsuidgid: add fsid mapping helpers

[Christian Brauner <christian.brauner@ubuntu.com>]

This adds a set of helpers to translate between kfsuid/kfsgid and their
userspace fsuid/fsgid counter parts relative to a given user namespace.

- kuid_t make_kfsuid(struct user_namespace *from, uid_t fsuid)
  Maps a user-namespace fsuid pair into a kfsuid.
  If no fsuid mappings have been written it behaves identical to calling
  make_kuid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- kgid_t make_kfsgid(struct user_namespace *from, gid_t fsgid)
  Maps a user-namespace fsgid pair into a kfsgid.
  If no fsgid mappings have been written it behaves identical to calling
  make_kgid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- uid_t from_kfsuid(struct user_namespace *to, kuid_t fsuid)
  Creates a fsuid from a kfsuid user-namespace pair if possible.
  If no fsuid mappings have been written it behaves identical to calling
  from_kuid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- gid_t from_kfsgid(struct user_namespace *to, kgid_t fsgid)
  Creates a fsgid from a kfsgid user-namespace pair if possible.
  If no fsgid mappings have been written it behaves identical to calling
  make_kgid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- uid_t from_kfsuid_munged(struct user_namespace *to, kuid_t fsuid)
  Always creates a fsuid from a kfsuid user-namespace pair.
  If no fsuid mappings have been written it behaves identical to calling
  from_kuid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- gid_t from_kfsgid_munged(struct user_namespace *to, kgid_t fsgid)
  Always creates a fsgid from a kfsgid user-namespace pair if possible.
  If no fsgid mappings have been written it behaves identical to calling
  make_kgid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- bool kfsuid_has_mapping(struct user_namespace *ns, kuid_t uid)
  Check whether this kfsuid has a mapping in the provided user namespace.
  If no fsuid mappings have been written it behaves identical to calling
  from_kuid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

- bool kfsgid_has_mapping(struct user_namespace *ns, kgid_t gid)
  Check whether this kfsgid has a mapping in the provided user namespace.
  If no fsgid mappings have been written it behaves identical to calling
  make_kgid(). This ensures backwards compatibility for workloads unaware
  or not in need of fsid mappings.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 include/linux/fsuidgid.h |  70 +++++++++++++++
 kernel/user_namespace.c  | 189 ++++++++++++++++++++++++++++++++++++---
 2 files changed, 246 insertions(+), 13 deletions(-)
 create mode 100644 include/linux/fsuidgid.h

diff --git a/include/linux/fsuidgid.h b/include/linux/fsuidgid.h
new file mode 100644
index 000000000000..0ebfdaa796ab
--- /dev/null
+++ b/include/linux/fsuidgid.h
@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_FSUIDGID_H
+#define _LINUX_FSUIDGID_H
+
+#include <linux/uidgid.h>
+
+#ifdef CONFIG_USER_NS_FSID
+
+extern kuid_t make_kfsuid(struct user_namespace *from, uid_t fsuid);
+extern kgid_t make_kfsgid(struct user_namespace *from, gid_t fsgid);
+extern uid_t from_kfsuid(struct user_namespace *to, kuid_t kfsuid);
+extern gid_t from_kfsgid(struct user_namespace *to, kgid_t kfsgid);
+extern uid_t from_kfsuid_munged(struct user_namespace *to, kuid_t kfsuid);
+extern gid_t from_kfsgid_munged(struct user_namespace *to, kgid_t kfsgid);
+
+static inline bool kfsuid_has_mapping(struct user_namespace *ns, kuid_t kfsuid)
+{
+	return from_kfsuid(ns, kfsuid) != (uid_t) -1;
+}
+
+static inline bool kfsgid_has_mapping(struct user_namespace *ns, kgid_t kfsgid)
+{
+	return from_kfsgid(ns, kfsgid) != (gid_t) -1;
+}
+
+#else
+
+static inline kuid_t make_kfsuid(struct user_namespace *from, uid_t fsuid)
+{
+	return make_kuid(from, fsuid);
+}
+
+static inline kgid_t make_kfsgid(struct user_namespace *from, gid_t fsgid)
+{
+	return make_kgid(from, fsgid);
+}
+
+static inline uid_t from_kfsuid(struct user_namespace *to, kuid_t kfsuid)
+{
+	return from_kuid(to, kfsuid);
+}
+
+static inline gid_t from_kfsgid(struct user_namespace *to, kgid_t kfsgid)
+{
+	return from_kgid(to, kfsgid);
+}
+
+static inline uid_t from_kfsuid_munged(struct user_namespace *to, kuid_t kfsuid)
+{
+	return from_kuid_munged(to, kfsuid);
+}
+
+static inline gid_t from_kfsgid_munged(struct user_namespace *to, kgid_t kfsgid)
+{
+	return from_kgid_munged(to, kfsgid);
+}
+
+static inline bool kfsuid_has_mapping(struct user_namespace *ns, kuid_t kfsuid)
+{
+	return kuid_has_mapping(ns, kfsuid);
+}
+
+static inline bool kfsgid_has_mapping(struct user_namespace *ns, kgid_t kfsgid)
+{
+	return kgid_has_mapping(ns, kfsgid);
+}
+
+#endif /* CONFIG_USER_NS_FSID */
+
+#endif /* _LINUX_FSUIDGID_H */
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index cbdf456f95f0..398be02de5c3 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -20,13 +20,14 @@
 #include <linux/fs_struct.h>
 #include <linux/bsearch.h>
 #include <linux/sort.h>
+#include <linux/fsuidgid.h>
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 static DEFINE_MUTEX(userns_state_mutex);
 
 static bool new_idmap_permitted(const struct file *file,
 				struct user_namespace *ns, int cap_setid,
-				struct uid_gid_map *map);
+				struct uid_gid_map *map, bool map_fsid);
 static void free_user_ns(struct work_struct *work);
 
 static struct ucounts *inc_user_namespaces(struct user_namespace *ns, kuid_t uid)
@@ -583,6 +584,166 @@ projid_t from_kprojid_munged(struct user_namespace *targ, kprojid_t kprojid)
 }
 EXPORT_SYMBOL(from_kprojid_munged);
 
+#ifdef CONFIG_USER_NS_FSID
+/**
+ *	make_kfsuid - Map a user-namespace fsuid pair into a kuid.
+ *	@ns:  User namespace that the fsuid is in
+ *	@fsuid: User identifier
+ *
+ *	Maps a user-namespace fsuid pair into a kernel internal kfsuid,
+ *	and returns that kfsuid.
+ *
+ *	When there is no mapping defined for the user-namespace kfsuid
+ *	pair INVALID_UID is returned.  Callers are expected to test
+ *	for and handle INVALID_UID being returned.  INVALID_UID
+ *	may be tested for using uid_valid().
+ */
+kuid_t make_kfsuid(struct user_namespace *ns, uid_t fsuid)
+{
+	unsigned extents = ns->fsuid_map.nr_extents;
+	smp_rmb();
+
+	/* Map the fsuid to a global kernel fsuid */
+	if (extents == 0)
+		return KUIDT_INIT(map_id_down(&ns->uid_map, fsuid));
+
+	return KUIDT_INIT(map_id_down(&ns->fsuid_map, fsuid));
+}
+EXPORT_SYMBOL(make_kfsuid);
+
+/**
+ *	from_kfsuid - Create a fsuid from a kfsuid user-namespace pair.
+ *	@targ: The user namespace we want a fsuid in.
+ *	@kfsuid: The kernel internal fsuid to start with.
+ *
+ *	Map @kfsuid into the user-namespace specified by @targ and
+ *	return the resulting fsuid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	If @kfsuid has no mapping in @targ (uid_t)-1 is returned.
+ */
+uid_t from_kfsuid(struct user_namespace *targ, kuid_t kfsuid)
+{
+	unsigned extents = targ->fsuid_map.nr_extents;
+	smp_rmb();
+
+	/* Map the fsuid from a global kernel fsuid */
+	if (extents == 0)
+		return map_id_up(&targ->uid_map, __kuid_val(kfsuid));
+
+	return map_id_up(&targ->fsuid_map, __kuid_val(kfsuid));
+}
+EXPORT_SYMBOL(from_kfsuid);
+
+/**
+ *	from_kfsuid_munged - Create a fsuid from a kfsuid user-namespace pair.
+ *	@targ: The user namespace we want a fsuid in.
+ *	@kfsuid: The kernel internal fsuid to start with.
+ *
+ *	Map @kfsuid into the user-namespace specified by @targ and
+ *	return the resulting fsuid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	Unlike from_kfsuid from_kfsuid_munged never fails and always
+ *	returns a valid fsuid.  This makes from_kfsuid_munged appropriate
+ *	for use in syscalls like stat and getuid where failing the
+ *	system call and failing to provide a valid fsuid are not an
+ *	options.
+ *
+ *	If @kfsuid has no mapping in @targ overflowuid is returned.
+ */
+uid_t from_kfsuid_munged(struct user_namespace *targ, kuid_t kfsuid)
+{
+	uid_t fsuid;
+	fsuid = from_kfsuid(targ, kfsuid);
+
+	if (fsuid == (uid_t) -1)
+		fsuid = overflowuid;
+	return fsuid;
+}
+EXPORT_SYMBOL(from_kfsuid_munged);
+
+/**
+ *	make_kfsgid - Map a user-namespace fsgid pair into a kfsgid.
+ *	@ns:  User namespace that the fsgid is in
+ *	@fsgid: User identifier
+ *
+ *	Maps a user-namespace fsgid pair into a kernel internal kfsgid,
+ *	and returns that kfsgid.
+ *
+ *	When there is no mapping defined for the user-namespace fsgid
+ *	pair INVALID_GID is returned.  Callers are expected to test
+ *	for and handle INVALID_GID being returned.  INVALID_GID
+ *	may be tested for using gid_valid().
+ */
+kgid_t make_kfsgid(struct user_namespace *ns, gid_t fsgid)
+{
+	unsigned extents = ns->fsgid_map.nr_extents;
+	smp_rmb();
+
+	/* Map the fsgid to a global kernel fsgid */
+	if (extents == 0)
+		return KGIDT_INIT(map_id_down(&ns->gid_map, fsgid));
+
+	return KGIDT_INIT(map_id_down(&ns->fsgid_map, fsgid));
+}
+EXPORT_SYMBOL(make_kfsgid);
+
+/**
+ *	from_kfsgid - Create a fsgid from a kfsgid user-namespace pair.
+ *	@targ: The user namespace we want a fsgid in.
+ *	@kfsgid: The kernel internal fsgid to start with.
+ *
+ *	Map @kfsgid into the user-namespace specified by @targ and
+ *	return the resulting fsgid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	If @kfsgid has no mapping in @targ (gid_t)-1 is returned.
+ */
+gid_t from_kfsgid(struct user_namespace *targ, kgid_t kfsgid)
+{
+	unsigned extents = targ->fsgid_map.nr_extents;
+	smp_rmb();
+
+	/* Map the fsgid from a global kernel fsgid */
+	if (extents == 0)
+		return map_id_up(&targ->gid_map, __kgid_val(kfsgid));
+
+	return map_id_up(&targ->fsgid_map, __kgid_val(kfsgid));
+}
+EXPORT_SYMBOL(from_kfsgid);
+
+/**
+ *	from_kfsgid_munged - Create a fsgid from a kfsgid user-namespace pair.
+ *	@targ: The user namespace we want a fsgid in.
+ *	@kfsgid: The kernel internal fsgid to start with.
+ *
+ *	Map @kfsgid into the user-namespace specified by @targ and
+ *	return the resulting fsgid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	Unlike from_kfsgid from_kfsgid_munged never fails and always
+ *	returns a valid fsgid.  This makes from_kfsgid_munged appropriate
+ *	for use in syscalls like stat and getgid where failing the
+ *	system call and failing to provide a valid fsgid are not options.
+ *
+ *	If @kfsgid has no mapping in @targ overflowgid is returned.
+ */
+gid_t from_kfsgid_munged(struct user_namespace *targ, kgid_t kfsgid)
+{
+	gid_t fsgid;
+	fsgid = from_kfsgid(targ, kfsgid);
+
+	if (fsgid == (gid_t) -1)
+		fsgid = overflowgid;
+	return fsgid;
+}
+EXPORT_SYMBOL(from_kfsgid_munged);
+#endif /* CONFIG_USER_NS_FSID */
 
 static int uid_m_show(struct seq_file *seq, void *v)
 {
@@ -659,7 +820,7 @@ static int fsuid_m_show(struct seq_file *seq, void *v)
 	if ((lower_ns == ns) && lower_ns->parent)
 		lower_ns = lower_ns->parent;
 
-	lower = from_kuid(lower_ns, KUIDT_INIT(extent->lower_first));
+	lower = from_kfsuid(lower_ns, KUIDT_INIT(extent->lower_first));
 
 	seq_printf(seq, "%10u %10u %10u\n",
 		extent->first,
@@ -680,7 +841,7 @@ static int fsgid_m_show(struct seq_file *seq, void *v)
 	if ((lower_ns == ns) && lower_ns->parent)
 		lower_ns = lower_ns->parent;
 
-	lower = from_kgid(lower_ns, KGIDT_INIT(extent->lower_first));
+	lower = from_kfsgid(lower_ns, KGIDT_INIT(extent->lower_first));
 
 	seq_printf(seq, "%10u %10u %10u\n",
 		extent->first,
@@ -931,7 +1092,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 			 size_t count, loff_t *ppos,
 			 int cap_setid,
 			 struct uid_gid_map *map,
-			 struct uid_gid_map *parent_map)
+			 struct uid_gid_map *parent_map, bool map_fsid)
 {
 	struct seq_file *seq = file->private_data;
 	struct user_namespace *ns = seq->private;
@@ -1051,7 +1212,7 @@ 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, ns, cap_setid, &new_map, map_fsid))
 		goto out;
 
 	ret = -EPERM;
@@ -1129,7 +1290,7 @@ ssize_t proc_uid_map_write(struct file *file, const char __user *buf,
 		return -EPERM;
 
 	return map_write(file, buf, size, ppos, CAP_SETUID,
-			 &ns->uid_map, &ns->parent->uid_map);
+			 &ns->uid_map, &ns->parent->uid_map, false);
 }
 
 ssize_t proc_gid_map_write(struct file *file, const char __user *buf,
@@ -1146,7 +1307,7 @@ ssize_t proc_gid_map_write(struct file *file, const char __user *buf,
 		return -EPERM;
 
 	return map_write(file, buf, size, ppos, CAP_SETGID,
-			 &ns->gid_map, &ns->parent->gid_map);
+			 &ns->gid_map, &ns->parent->gid_map, false);
 }
 
 ssize_t proc_projid_map_write(struct file *file, const char __user *buf,
@@ -1164,7 +1325,7 @@ ssize_t proc_projid_map_write(struct file *file, const char __user *buf,
 
 	/* Anyone can set any valid project id no capability needed */
 	return map_write(file, buf, size, ppos, -1,
-			 &ns->projid_map, &ns->parent->projid_map);
+			 &ns->projid_map, &ns->parent->projid_map, false);
 }
 
 #ifdef CONFIG_USER_NS_FSID
@@ -1182,7 +1343,7 @@ ssize_t proc_fsuid_map_write(struct file *file, const char __user *buf,
 		return -EPERM;
 
 	return map_write(file, buf, size, ppos, CAP_SETUID, &ns->fsuid_map,
-			 &ns->parent->fsuid_map);
+			 &ns->parent->fsuid_map, true);
 }
 
 ssize_t proc_fsgid_map_write(struct file *file, const char __user *buf,
@@ -1199,13 +1360,13 @@ ssize_t proc_fsgid_map_write(struct file *file, const char __user *buf,
 		return -EPERM;
 
 	return map_write(file, buf, size, ppos, CAP_SETGID, &ns->fsgid_map,
-			 &ns->parent->fsgid_map);
+			 &ns->parent->fsgid_map, true);
 }
 #endif
 
 static bool new_idmap_permitted(const struct file *file,
 				struct user_namespace *ns, int cap_setid,
-				struct uid_gid_map *new_map)
+				struct uid_gid_map *new_map, bool map_fsid)
 {
 	const struct cred *cred = file->f_cred;
 	/* Don't allow mappings that would allow anything that wouldn't
@@ -1215,11 +1376,13 @@ static bool new_idmap_permitted(const struct file *file,
 	    uid_eq(ns->owner, cred->euid)) {
 		u32 id = new_map->extent[0].lower_first;
 		if (cap_setid == CAP_SETUID) {
-			kuid_t uid = make_kuid(ns->parent, id);
+			kuid_t uid = map_fsid ? make_kfsuid(ns->parent, id) :
+						make_kuid(ns->parent, id);
 			if (uid_eq(uid, cred->euid))
 				return true;
 		} else if (cap_setid == CAP_SETGID) {
-			kgid_t gid = make_kgid(ns->parent, id);
+			kgid_t gid = map_fsid ? make_kfsgid(ns->parent, id) :
+						make_kgid(ns->parent, id);
 			if (!(ns->flags & USERNS_SETGROUPS_ALLOWED) &&
 			    gid_eq(gid, cred->egid))
 				return true;
-- 
2.25.0