1 files changed, 273 insertions, 0 deletions
diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c
new file mode 100644
index 000000000000..4905665c47d0
--- /dev/null
+++ b/fs/mnt_idmapping.c
@@ -0,0 +1,273 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Christian Brauner <brauner@kernel.org> */
+
+#include <linux/cred.h>
+#include <linux/fs.h>
+#include <linux/mnt_idmapping.h>
+#include <linux/slab.h>
+#include <linux/user_namespace.h>
+
+#include "internal.h"
+
+struct mnt_idmap {
+	struct user_namespace *owner;
+	refcount_t count;
+};
+
+/*
+ * Carries the initial idmapping of 0:0:4294967295 which is an identity
+ * mapping. This means that {g,u}id 0 is mapped to {g,u}id 0, {g,u}id 1 is
+ * mapped to {g,u}id 1, [...], {g,u}id 1000 to {g,u}id 1000, [...].
+ */
+struct mnt_idmap nop_mnt_idmap = {
+	.owner	= &init_user_ns,
+	.count	= REFCOUNT_INIT(1),
+};
+EXPORT_SYMBOL_GPL(nop_mnt_idmap);
+
+/**
+ * check_fsmapping - check whether an mount idmapping is allowed
+ * @idmap: idmap of the relevent mount
+ * @sb:    super block of the filesystem
+ *
+ * Return: true if @idmap is allowed, false if not.
+ */
+bool check_fsmapping(const struct mnt_idmap *idmap,
+		     const struct super_block *sb)
+{
+	return idmap->owner != sb->s_user_ns;
+}
+
+/**
+ * initial_idmapping - check whether this is the initial mapping
+ * @ns: idmapping to check
+ *
+ * Check whether this is the initial mapping, mapping 0 to 0, 1 to 1,
+ * [...], 1000 to 1000 [...].
+ *
+ * Return: true if this is the initial mapping, false if not.
+ */
+static inline bool initial_idmapping(const struct user_namespace *ns)
+{
+	return ns == &init_user_ns;
+}
+
+/**
+ * no_idmapping - check whether we can skip remapping a kuid/gid
+ * @mnt_userns: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ *
+ * This function can be used to check whether a remapping between two
+ * idmappings is required.
+ * An idmapped mount is a mount that has an idmapping attached to it that
+ * is different from the filsystem's idmapping and the initial idmapping.
+ * If the initial mapping is used or the idmapping of the mount and the
+ * filesystem are identical no remapping is required.
+ *
+ * Return: true if remapping can be skipped, false if not.
+ */
+static inline bool no_idmapping(const struct user_namespace *mnt_userns,
+				const struct user_namespace *fs_userns)
+{
+	return initial_idmapping(mnt_userns) || mnt_userns == fs_userns;
+}
+
+/**
+ * make_vfsuid - map a filesystem kuid according to an idmapping
+ * @idmap: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ * @kuid : kuid to be mapped
+ *
+ * Take a @kuid and remap it from @fs_userns into @idmap. Use this
+ * function when preparing a @kuid to be reported to userspace.
+ *
+ * If no_idmapping() determines that this is not an idmapped mount we can
+ * simply return @kuid unchanged.
+ * If initial_idmapping() tells us that the filesystem is not mounted with an
+ * idmapping we know the value of @kuid won't change when calling
+ * from_kuid() so we can simply retrieve the value via __kuid_val()
+ * directly.
+ *
+ * Return: @kuid mapped according to @idmap.
+ * If @kuid has no mapping in either @idmap or @fs_userns INVALID_UID is
+ * returned.
+ */
+
+vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
+				   struct user_namespace *fs_userns,
+				   kuid_t kuid)
+{
+	uid_t uid;
+	struct user_namespace *mnt_userns = idmap->owner;
+
+	if (no_idmapping(mnt_userns, fs_userns))
+		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_VFSUID;
+	return VFSUIDT_INIT(make_kuid(mnt_userns, uid));
+}
+EXPORT_SYMBOL_GPL(make_vfsuid);
+
+/**
+ * make_vfsgid - map a filesystem kgid according to an idmapping
+ * @idmap: the mount's idmapping
+ * @fs_userns: the filesystem's idmapping
+ * @kgid : kgid to be mapped
+ *
+ * Take a @kgid and remap it from @fs_userns into @idmap. Use this
+ * function when preparing a @kgid to be reported to userspace.
+ *
+ * If no_idmapping() determines that this is not an idmapped mount we can
+ * simply return @kgid unchanged.
+ * If initial_idmapping() tells us that the filesystem is not mounted with an
+ * idmapping we know the value of @kgid won't change when calling
+ * from_kgid() so we can simply retrieve the value via __kgid_val()
+ * directly.
+ *
+ * Return: @kgid mapped according to @idmap.
+ * If @kgid has no mapping in either @idmap or @fs_userns INVALID_GID is
+ * returned.
+ */
+vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
+		     struct user_namespace *fs_userns, kgid_t kgid)
+{
+	gid_t gid;
+	struct user_namespace *mnt_userns = idmap->owner;
+
+	if (no_idmapping(mnt_userns, fs_userns))
+		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_VFSGID;
+	return VFSGIDT_INIT(make_kgid(mnt_userns, gid));
+}
+EXPORT_SYMBOL_GPL(make_vfsgid);
+
+/**
+ * from_vfsuid - map a vfsuid into the filesystem idmapping
+ * @idmap: 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
+ */
+kuid_t from_vfsuid(struct mnt_idmap *idmap,
+		   struct user_namespace *fs_userns, vfsuid_t vfsuid)
+{
+	uid_t uid;
+	struct user_namespace *mnt_userns = idmap->owner;
+
+	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);
+}
+EXPORT_SYMBOL_GPL(from_vfsuid);
+
+/**
+ * from_vfsgid - map a vfsgid into the filesystem idmapping
+ * @idmap: 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
+ */
+kgid_t from_vfsgid(struct mnt_idmap *idmap,
+		   struct user_namespace *fs_userns, vfsgid_t vfsgid)
+{
+	gid_t gid;
+	struct user_namespace *mnt_userns = idmap->owner;
+
+	if (no_idmapping(mnt_userns, fs_userns))
+		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 KGIDT_INIT(gid);
+	return make_kgid(fs_userns, gid);
+}
+EXPORT_SYMBOL_GPL(from_vfsgid);
+
+#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.
+ */
+int vfsgid_in_group_p(vfsgid_t vfsgid)
+{
+	return in_group_p(AS_KGIDT(vfsgid));
+}
+#else
+int vfsgid_in_group_p(vfsgid_t vfsgid)
+{
+	return 1;
+}
+#endif
+EXPORT_SYMBOL_GPL(vfsgid_in_group_p);
+
+struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns)
+{
+	struct mnt_idmap *idmap;
+
+	idmap = kzalloc(sizeof(struct mnt_idmap), GFP_KERNEL_ACCOUNT);
+	if (!idmap)
+		return ERR_PTR(-ENOMEM);
+
+	idmap->owner = get_user_ns(mnt_userns);
+	refcount_set(&idmap->count, 1);
+	return idmap;
+}
+
+/**
+ * mnt_idmap_get - get a reference to an idmapping
+ * @idmap: the idmap to bump the reference on
+ *
+ * If @idmap is not the @nop_mnt_idmap bump the reference count.
+ *
+ * Return: @idmap with reference count bumped if @not_mnt_idmap isn't passed.
+ */
+struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap)
+{
+	if (idmap != &nop_mnt_idmap)
+		refcount_inc(&idmap->count);
+
+	return idmap;
+}
+
+/**
+ * mnt_idmap_put - put a reference to an idmapping
+ * @idmap: the idmap to put the reference on
+ *
+ * If this is a non-initial idmapping, put the reference count when a mount is
+ * released and free it if we're the last user.
+ */
+void mnt_idmap_put(struct mnt_idmap *idmap)
+{
+	if (idmap != &nop_mnt_idmap && refcount_dec_and_test(&idmap->count)) {
+		put_user_ns(idmap->owner);
+		kfree(idmap);
+	}
+}