mnt_idmapping: add vfs{g,u}id_t

Introduces new vfs{g,u}id_t types. Similar to k{g,u}id_t the new types
are just simple wrapper structs around regular {g,u}id_t types.

They allows to establish a type safety boundary between {g,u}ids on
idmapped mounts and {g,u}ids as they are represented in filesystems
themselves.

A vfs{g,u}id_t is always created from a k{g,u}id_t, never directly from
a {g,u}id_t as idmapped mounts remap a given {g,u}id according to the
mount's idmapping. This is expressed in the VFS{G,U}IDT_INIT() macros.

A vfs{g,u}id_t may be used as a k{g,u}id_t via AS_K{G,U}IDT(). This
often happens when we need to check whether a {g,u}id mapped according
to an idmapped mount is identical to a given k{g,u}id_t. For an example,
see vfsgid_in_group_p() which determines whether the value of vfsgid_t
matches the value of any of the caller's groups. Similar logic is
expressed in the k{g,u}id_eq_vfs{g,u}id().

The from_vfs{g,u}id() helpers map a given vfs{g,u}id_t from the mount's
idmapping into the filesystem idmapping. They make it possible to update
a filesystem object such as inode->i_{g,u}id with the correct value.

This makes it harder to accidently write a wrong {g,u}id anwywhere. The
vfs{g,u}id_has_fsmapping() helpers check whether a given vfs{g,u}id_t
can be mapped into the filesystem idmapping.

All new helpers are nops on non-idmapped mounts.

I've done work on this roughly 7 months ago but dropped it to focus on
the testsuite. Linus brought this up independently just last week and
it's time to move this along (see [1]).

[1]: https://lore.kernel.org/lkml/CAHk-=win6+ahs1EwLkcq8apqLi_1wXFWbrPf340zYEhObpz4jA@mail.gmail.com

Link: https://lore.kernel.org/r/20220621141454.2914719-2-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

include/linux/mnt_idmapping.h

@@ -13,6 +13,122 @@ struct user_namespace;
  */
 extern struct user_namespace init_user_ns;
 
+typedef struct {
+	uid_t val;
+} vfsuid_t;
+
+typedef struct {
+	gid_t val;
+} vfsgid_t;
+
+#ifdef CONFIG_MULTIUSER
+static inline uid_t __vfsuid_val(vfsuid_t uid)
+{
+	return uid.val;
+}
+
+static inline gid_t __vfsgid_val(vfsgid_t gid)
+{
+	return gid.val;
+}
+#else
+static inline uid_t __vfsuid_val(vfsuid_t uid)
+{
+	return 0;
+}
+
+static inline gid_t __vfsgid_val(vfsgid_t gid)
+{
+	return 0;
+}
+#endif
+
+static inline bool vfsuid_valid(vfsuid_t uid)
+{
+	return __vfsuid_val(uid) != (uid_t)-1;
+}
+
+static inline bool vfsgid_valid(vfsgid_t gid)
+{
+	return __vfsgid_val(gid) != (gid_t)-1;
+}
+
+static inline bool vfsuid_eq(vfsuid_t left, vfsuid_t right)
+{
+	return __vfsuid_val(left) == __vfsuid_val(right);
+}
+
+static inline bool vfsgid_eq(vfsgid_t left, vfsgid_t right)
+{
+	return __vfsgid_val(left) == __vfsgid_val(right);
+}
+
+/**
+ * vfsuid_eq_kuid - check whether kuid and vfsuid have the same value
+ * @kuid: the kuid to compare
+ * @vfsuid: the vfsuid to compare
+ *
+ * Check whether @kuid and @vfsuid have the same values.
+ *
+ * Return: true if @kuid and @vfsuid have the same value, false if not.
+ */
+static inline bool vfsuid_eq_kuid(vfsuid_t vfsuid, kuid_t kuid)
+{
+	return __vfsuid_val(vfsuid) == __kuid_val(kuid);
+}
+
+/**
+ * vfsgid_eq_kgid - check whether kgid and vfsgid have the same value
+ * @kgid: the kgid to compare
+ * @vfsgid: the vfsgid to compare
+ *
+ * Check whether @kgid and @vfsgid have the same values.
+ *
+ * Return: true if @kgid and @vfsgid have the same value, false if not.
+ */
+static inline bool vfsgid_eq_kgid(kgid_t kgid, vfsgid_t vfsgid)
+{
+	return __vfsgid_val(vfsgid) == __kgid_val(kgid);
+}
+
+/*
+ * vfs{g,u}ids are created from k{g,u}ids.
+ * We don't allow them to be created from regular {u,g}id.
+ */
+#define VFSUIDT_INIT(val) (vfsuid_t){ __kuid_val(val) }
+#define VFSGIDT_INIT(val) (vfsgid_t){ __kgid_val(val) }
+
+#define INVALID_VFSUID VFSUIDT_INIT(INVALID_UID)
+#define INVALID_VFSGID VFSGIDT_INIT(INVALID_GID)
+
+/*
+ * Allow a vfs{g,u}id to be used as a k{g,u}id where we want to compare
+ * whether the mapped value is identical to value of a k{g,u}id.
+ */
+#define AS_KUIDT(val) (kuid_t){ __vfsuid_val(val) }
+#define AS_KGIDT(val) (kgid_t){ __vfsgid_val(val) }
+
+#ifdef CONFIG_MULTIUSER
+/**
+ * vfsgid_in_group_p() - check whether a vfsuid matches the caller's groups
+ * @vfsgid: the mnt gid to match
+ *
+ * This function can be used to determine whether @vfsuid matches any of the
+ * caller's groups.
+ *
+ * Return: 1 if vfsuid matches caller's groups, 0 if not.
+ */
+static inline int vfsgid_in_group_p(vfsgid_t vfsgid)
+{
+	return in_group_p(AS_KGIDT(vfsgid));
+}
+#else
+static inline int vfsgid_in_group_p(vfsgid_t vfsgid)
+{
+	return 1;
+}
+#endif
+
 /**
  * initial_idmapping - check whether this is the initial mapping
  * @ns: idmapping to check
@@ -67,21 +183,29 @@ static inline bool no_idmapping(const struct user_namespace *mnt_userns,
  * If @kuid has no mapping in either @mnt_userns or @fs_userns INVALID_UID is
  * returned.
  */
-static inline kuid_t mapped_kuid_fs(struct user_namespace *mnt_userns,
-				    struct user_namespace *fs_userns,
-				    kuid_t kuid)
+
+static inline vfsuid_t make_vfsuid(struct user_namespace *mnt_userns,
+				   struct user_namespace *fs_userns,
+				   kuid_t kuid)
 {
 	uid_t uid;
 
 	if (no_idmapping(mnt_userns, fs_userns))
-		return kuid;
+		return VFSUIDT_INIT(kuid);
 	if (initial_idmapping(fs_userns))
 		uid = __kuid_val(kuid);
 	else
 		uid = from_kuid(fs_userns, kuid);
 	if (uid == (uid_t)-1)
-		return INVALID_UID;
-	return make_kuid(mnt_userns, uid);
+		return INVALID_VFSUID;
+	return VFSUIDT_INIT(make_kuid(mnt_userns, uid));
+}
+
+static inline kuid_t mapped_kuid_fs(struct user_namespace *mnt_userns,
+				    struct user_namespace *fs_userns,
+				    kuid_t kuid)
+{
+	return AS_KUIDT(make_vfsuid(mnt_userns, fs_userns, kuid));
 }
 
 /**
@@ -104,21 +228,56 @@ static inline kuid_t mapped_kuid_fs(struct user_namespace *mnt_userns,
  * If @kgid has no mapping in either @mnt_userns or @fs_userns INVALID_GID is
  * returned.
  */
-static inline kgid_t mapped_kgid_fs(struct user_namespace *mnt_userns,
-				    struct user_namespace *fs_userns,
-				    kgid_t kgid)
+
+static inline vfsgid_t make_vfsgid(struct user_namespace *mnt_userns,
+				   struct user_namespace *fs_userns,
+				   kgid_t kgid)
 {
 	gid_t gid;
 
 	if (no_idmapping(mnt_userns, fs_userns))
-		return kgid;
+		return VFSGIDT_INIT(kgid);
 	if (initial_idmapping(fs_userns))
 		gid = __kgid_val(kgid);
 	else
 		gid = from_kgid(fs_userns, kgid);
 	if (gid == (gid_t)-1)
-		return INVALID_GID;
-	return make_kgid(mnt_userns, gid);
+		return INVALID_VFSGID;
+	return VFSGIDT_INIT(make_kgid(mnt_userns, gid));
+}
+
+static inline kgid_t mapped_kgid_fs(struct user_namespace *mnt_userns,
+				    struct user_namespace *fs_userns,
+				    kgid_t kgid)
+{
+	return AS_KGIDT(make_vfsgid(mnt_userns, fs_userns, kgid));
+}
+
+/**
+ * from_vfsuid - map a vfsuid into the filesystem idmapping
+ * @mnt_userns: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ * @vfsuid : vfsuid to be mapped
+ *
+ * Map @vfsuid into the filesystem idmapping. This function has to be used in
+ * order to e.g. write @vfsuid to inode->i_uid.
+ *
+ * Return: @vfsuid mapped into the filesystem idmapping
+ */
+static inline kuid_t from_vfsuid(struct user_namespace *mnt_userns,
+				 struct user_namespace *fs_userns,
+				 vfsuid_t vfsuid)
+{
+	uid_t uid;
+
+	if (no_idmapping(mnt_userns, fs_userns))
+		return AS_KUIDT(vfsuid);
+	uid = from_kuid(mnt_userns, AS_KUIDT(vfsuid));
+	if (uid == (uid_t)-1)
+		return INVALID_UID;
+	if (initial_idmapping(fs_userns))
+		return KUIDT_INIT(uid);
+	return make_kuid(fs_userns, uid);
 }
 
 /**
@@ -145,16 +304,53 @@ static inline kuid_t mapped_kuid_user(struct user_namespace *mnt_userns,
 				      struct user_namespace *fs_userns,
 				      kuid_t kuid)
 {
-	uid_t uid;
+	return from_vfsuid(mnt_userns, fs_userns, VFSUIDT_INIT(kuid));
+}
+
+/**
+ * vfsuid_has_fsmapping - check whether a vfsuid maps into the filesystem
+ * @mnt_userns: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ * @vfsuid: vfsuid to be mapped
+ *
+ * Check whether @vfsuid has a mapping in the filesystem idmapping. Use this
+ * function to check whether the filesystem idmapping has a mapping for
+ * @vfsuid.
+ *
+ * Return: true if @vfsuid has a mapping in the filesystem, false if not.
+ */
+static inline bool vfsuid_has_fsmapping(struct user_namespace *mnt_userns,
+					struct user_namespace *fs_userns,
+					vfsuid_t vfsuid)
+{
+	return uid_valid(from_vfsuid(mnt_userns, fs_userns, vfsuid));
+}
+
+/**
+ * from_vfsgid - map a vfsgid into the filesystem idmapping
+ * @mnt_userns: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ * @vfsgid : vfsgid to be mapped
+ *
+ * Map @vfsgid into the filesystem idmapping. This function has to be used in
+ * order to e.g. write @vfsgid to inode->i_gid.
+ *
+ * Return: @vfsgid mapped into the filesystem idmapping
+ */
+static inline kgid_t from_vfsgid(struct user_namespace *mnt_userns,
+				 struct user_namespace *fs_userns,
+				 vfsgid_t vfsgid)
+{
+	gid_t gid;
 
 	if (no_idmapping(mnt_userns, fs_userns))
-		return kuid;
-	uid = from_kuid(mnt_userns, kuid);
-	if (uid == (uid_t)-1)
-		return INVALID_UID;
+		return AS_KGIDT(vfsgid);
+	gid = from_kgid(mnt_userns, AS_KGIDT(vfsgid));
+	if (gid == (gid_t)-1)
+		return INVALID_GID;
 	if (initial_idmapping(fs_userns))
-		return KUIDT_INIT(uid);
-	return make_kuid(fs_userns, uid);
+		return KGIDT_INIT(gid);
+	return make_kgid(fs_userns, gid);
 }
 
 /**
@@ -181,16 +377,26 @@ static inline kgid_t mapped_kgid_user(struct user_namespace *mnt_userns,
 				      struct user_namespace *fs_userns,
 				      kgid_t kgid)
 {
-	gid_t gid;
+	return from_vfsgid(mnt_userns, fs_userns, VFSGIDT_INIT(kgid));
+}
 
-	if (no_idmapping(mnt_userns, fs_userns))
-		return kgid;
-	gid = from_kgid(mnt_userns, kgid);
-	if (gid == (gid_t)-1)
-		return INVALID_GID;
-	if (initial_idmapping(fs_userns))
-		return KGIDT_INIT(gid);
-	return make_kgid(fs_userns, gid);
+/**
+ * vfsgid_has_fsmapping - check whether a vfsgid maps into the filesystem
+ * @mnt_userns: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ * @vfsgid: vfsgid to be mapped
+ *
+ * Check whether @vfsgid has a mapping in the filesystem idmapping. Use this
+ * function to check whether the filesystem idmapping has a mapping for
+ * @vfsgid.
+ *
+ * Return: true if @vfsgid has a mapping in the filesystem, false if not.
+ */
+static inline bool vfsgid_has_fsmapping(struct user_namespace *mnt_userns,
+					struct user_namespace *fs_userns,
+					vfsgid_t vfsgid)
+{
+	return gid_valid(from_vfsgid(mnt_userns, fs_userns, vfsgid));
 }
 
 /**