fs: support mapped mounts of mapped filesystems

In previous patches we added new and modified existing helpers to handle
idmapped mounts of filesystems mounted with an idmapping. In this final
patch we convert all relevant places in the vfs to actually pass the
filesystem's idmapping into these helpers.

With this the vfs is in shape to handle idmapped mounts of filesystems
mounted with an idmapping. Note that this is just the generic
infrastructure. Actually adding support for idmapped mounts to a
filesystem mountable with an idmapping is follow-up work.

In this patch we extend the definition of an idmapped mount from a mount
that that has the initial idmapping attached to it to a mount that has
an idmapping attached to it which is not the same as the idmapping the
filesystem was mounted with.

As before we do not allow the initial idmapping to be attached to a
mount. In addition this patch prevents that the idmapping the filesystem
was mounted with can be attached to a mount created based on this
filesystem.

This has multiple reasons and advantages. First, attaching the initial
idmapping or the filesystem's idmapping doesn't make much sense as in
both cases the values of the i_{g,u}id and other places where k{g,u}ids
are used do not change. Second, a user that really wants to do this for
whatever reason can just create a separate dedicated identical idmapping
to attach to the mount. Third, we can continue to use the initial
idmapping as an indicator that a mount is not idmapped allowing us to
continue to keep passing the initial idmapping into the mapping helpers
to tell them that something isn't an idmapped mount even if the
filesystem is mounted with an idmapping.

Link: https://lore.kernel.org/r/20211123114227.3124056-11-brauner@kernel.org (v1)
Link: https://lore.kernel.org/r/20211130121032.3753852-11-brauner@kernel.org (v2)
Link: https://lore.kernel.org/r/20211203111707.3901969-11-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
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 <christian.brauner@ubuntu.com>

1 files changed, 9 insertions, 8 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3d6d514943ab..493b87e3616b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1641,7 +1641,7 @@ static inline void i_gid_write(struct inode *inode, gid_t gid)
 static inline kuid_t i_uid_into_mnt(struct user_namespace *mnt_userns,
 				    const struct inode *inode)
 {
-	return mapped_kuid_fs(mnt_userns, &init_user_ns, inode->i_uid);
+	return mapped_kuid_fs(mnt_userns, i_user_ns(inode), inode->i_uid);
 }
 
 /**
@@ -1655,7 +1655,7 @@ static inline kuid_t i_uid_into_mnt(struct user_namespace *mnt_userns,
 static inline kgid_t i_gid_into_mnt(struct user_namespace *mnt_userns,
 				    const struct inode *inode)
 {
-	return mapped_kgid_fs(mnt_userns, &init_user_ns, inode->i_gid);
+	return mapped_kgid_fs(mnt_userns, i_user_ns(inode), inode->i_gid);
 }
 
 /**
@@ -1669,7 +1669,7 @@ static inline kgid_t i_gid_into_mnt(struct user_namespace *mnt_userns,
 static inline void inode_fsuid_set(struct inode *inode,
 				   struct user_namespace *mnt_userns)
 {
-	inode->i_uid = mapped_fsuid(mnt_userns, &init_user_ns);
+	inode->i_uid = mapped_fsuid(mnt_userns, i_user_ns(inode));
 }
 
 /**
@@ -1683,7 +1683,7 @@ static inline void inode_fsuid_set(struct inode *inode,
 static inline void inode_fsgid_set(struct inode *inode,
 				   struct user_namespace *mnt_userns)
 {
-	inode->i_gid = mapped_fsgid(mnt_userns, &init_user_ns);
+	inode->i_gid = mapped_fsgid(mnt_userns, i_user_ns(inode));
 }
 
 /**
@@ -1704,10 +1704,10 @@ static inline bool fsuidgid_has_mapping(struct super_block *sb,
 	kuid_t kuid;
 	kgid_t kgid;
 
-	kuid = mapped_fsuid(mnt_userns, &init_user_ns);
+	kuid = mapped_fsuid(mnt_userns, fs_userns);
 	if (!uid_valid(kuid))
 		return false;
-	kgid = mapped_fsgid(mnt_userns, &init_user_ns);
+	kgid = mapped_fsgid(mnt_userns, fs_userns);
 	if (!gid_valid(kgid))
 		return false;
 	return kuid_has_mapping(fs_userns, kuid) &&
@@ -2653,13 +2653,14 @@ static inline struct user_namespace *file_mnt_user_ns(struct file *file)
  * is_idmapped_mnt - check whether a mount is mapped
  * @mnt: the mount to check
  *
- * If @mnt has an idmapping attached to it @mnt is mapped.
+ * If @mnt has an idmapping attached different from the
+ * filesystem's idmapping then @mnt is mapped.
  *
  * Return: true if mount is mapped, false if not.
  */
 static inline bool is_idmapped_mnt(const struct vfsmount *mnt)
 {
-	return mnt_user_ns(mnt) != &init_user_ns;
+	return mnt_user_ns(mnt) != mnt->mnt_sb->s_user_ns;
 }
 
 extern long vfs_truncate(const struct path *, loff_t);