namei: prepare for idmapped mounts

The various vfs_*() helpers are called by filesystems or by the vfs
itself to perform core operations such as create, link, mkdir, mknod, rename,
rmdir, tmpfile and unlink. Enable them to handle idmapped mounts. If the
inode is accessed through an idmapped mount map it into the
mount's user namespace and pass it down. Afterwards the checks and
operations are identical to non-idmapped mounts. If the initial user
namespace is passed nothing changes so non-idmapped mounts will see
identical behavior as before.

Link: https://lore.kernel.org/r/20210121131959.646623-15-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

1 files changed, 184 insertions, 43 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 38ab51881247..5c9f6f8e90c4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2874,10 +2874,26 @@ void unlock_rename(struct dentry *p1, struct dentry *p2)
 }
 EXPORT_SYMBOL(unlock_rename);
 
-int vfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
-		bool want_excl)
+/**
+ * vfs_create - create new file
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dir:	inode of @dentry
+ * @dentry:	pointer to dentry of the base directory
+ * @mode:	mode of the new file
+ * @want_excl:	whether the file must not yet exist
+ *
+ * Create a new file.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
+ */
+int vfs_create(struct user_namespace *mnt_userns, struct inode *dir,
+	       struct dentry *dentry, umode_t mode, bool want_excl)
 {
-	int error = may_create(&init_user_ns, dir, dentry);
+	int error = may_create(mnt_userns, dir, dentry);
 	if (error)
 		return error;
 
@@ -3353,7 +3369,23 @@ static int do_open(struct nameidata *nd,
 	return error;
 }
 
-struct dentry *vfs_tmpfile(struct dentry *dentry, umode_t mode, int open_flag)
+/**
+ * vfs_tmpfile - create tmpfile
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dentry:	pointer to dentry of the base directory
+ * @mode:	mode of the new tmpfile
+ * @open_flags:	flags
+ *
+ * Create a temporary file.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
+ */
+struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
+			   struct dentry *dentry, umode_t mode, int open_flag)
 {
 	struct dentry *child = NULL;
 	struct inode *dir = dentry->d_inode;
@@ -3361,7 +3393,7 @@ struct dentry *vfs_tmpfile(struct dentry *dentry, umode_t mode, int open_flag)
 	int error;
 
 	/* we want directory to be writable */
-	error = inode_permission(&init_user_ns, dir, MAY_WRITE | MAY_EXEC);
+	error = inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
 	if (error)
 		goto out_err;
 	error = -EOPNOTSUPP;
@@ -3396,6 +3428,7 @@ static int do_tmpfile(struct nameidata *nd, unsigned flags,
 		const struct open_flags *op,
 		struct file *file)
 {
+	struct user_namespace *mnt_userns;
 	struct dentry *child;
 	struct path path;
 	int error = path_lookupat(nd, flags | LOOKUP_DIRECTORY, &path);
@@ -3404,7 +3437,8 @@ static int do_tmpfile(struct nameidata *nd, unsigned flags,
 	error = mnt_want_write(path.mnt);
 	if (unlikely(error))
 		goto out;
-	child = vfs_tmpfile(path.dentry, op->mode, op->open_flag);
+	mnt_userns = mnt_user_ns(path.mnt);
+	child = vfs_tmpfile(mnt_userns, path.dentry, op->mode, op->open_flag);
 	error = PTR_ERR(child);
 	if (IS_ERR(child))
 		goto out2;
@@ -3616,10 +3650,27 @@ inline struct dentry *user_path_create(int dfd, const char __user *pathname,
 }
 EXPORT_SYMBOL(user_path_create);
 
-int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
+/**
+ * vfs_mknod - create device node or file
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dir:	inode of @dentry
+ * @dentry:	pointer to dentry of the base directory
+ * @mode:	mode of the new device node or file
+ * @dev:	device number of device to create
+ *
+ * Create a device node or file.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
+ */
+int vfs_mknod(struct user_namespace *mnt_userns, struct inode *dir,
+	      struct dentry *dentry, umode_t mode, dev_t dev)
 {
 	bool is_whiteout = S_ISCHR(mode) && dev == WHITEOUT_DEV;
-	int error = may_create(&init_user_ns, dir, dentry);
+	int error = may_create(mnt_userns, dir, dentry);
 
 	if (error)
 		return error;
@@ -3666,6 +3717,7 @@ static int may_mknod(umode_t mode)
 static long do_mknodat(int dfd, const char __user *filename, umode_t mode,
 		unsigned int dev)
 {
+	struct user_namespace *mnt_userns;
 	struct dentry *dentry;
 	struct path path;
 	int error;
@@ -3684,18 +3736,22 @@ retry:
 	error = security_path_mknod(&path, dentry, mode, dev);
 	if (error)
 		goto out;
+
+	mnt_userns = mnt_user_ns(path.mnt);
 	switch (mode & S_IFMT) {
 		case 0: case S_IFREG:
-			error = vfs_create(path.dentry->d_inode,dentry,mode,true);
+			error = vfs_create(mnt_userns, path.dentry->d_inode,
+					   dentry, mode, true);
 			if (!error)
 				ima_post_path_mknod(dentry);
 			break;
 		case S_IFCHR: case S_IFBLK:
-			error = vfs_mknod(path.dentry->d_inode,dentry,mode,
-					new_decode_dev(dev));
+			error = vfs_mknod(mnt_userns, path.dentry->d_inode,
+					  dentry, mode, new_decode_dev(dev));
 			break;
 		case S_IFIFO: case S_IFSOCK:
-			error = vfs_mknod(path.dentry->d_inode,dentry,mode,0);
+			error = vfs_mknod(mnt_userns, path.dentry->d_inode,
+					  dentry, mode, 0);
 			break;
 	}
 out:
@@ -3718,9 +3774,25 @@ SYSCALL_DEFINE3(mknod, const char __user *, filename, umode_t, mode, unsigned, d
 	return do_mknodat(AT_FDCWD, filename, mode, dev);
 }
 
-int vfs_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
+/**
+ * vfs_mkdir - create directory
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dir:	inode of @dentry
+ * @dentry:	pointer to dentry of the base directory
+ * @mode:	mode of the new directory
+ *
+ * Create a directory.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
+ */
+int vfs_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
+	      struct dentry *dentry, umode_t mode)
 {
-	int error = may_create(&init_user_ns, dir, dentry);
+	int error = may_create(mnt_userns, dir, dentry);
 	unsigned max_links = dir->i_sb->s_max_links;
 
 	if (error)
@@ -3759,8 +3831,11 @@ retry:
 	if (!IS_POSIXACL(path.dentry->d_inode))
 		mode &= ~current_umask();
 	error = security_path_mkdir(&path, dentry, mode);
-	if (!error)
-		error = vfs_mkdir(path.dentry->d_inode, dentry, mode);
+	if (!error) {
+		struct user_namespace *mnt_userns;
+		mnt_userns = mnt_user_ns(path.mnt);
+		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry, mode);
+	}
 	done_path_create(&path, dentry);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
@@ -3779,9 +3854,24 @@ SYSCALL_DEFINE2(mkdir, const char __user *, pathname, umode_t, mode)
 	return do_mkdirat(AT_FDCWD, pathname, mode);
 }
 
-int vfs_rmdir(struct inode *dir, struct dentry *dentry)
+/**
+ * vfs_rmdir - remove directory
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dir:	inode of @dentry
+ * @dentry:	pointer to dentry of the base directory
+ *
+ * Remove a directory.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
+ */
+int vfs_rmdir(struct user_namespace *mnt_userns, struct inode *dir,
+		     struct dentry *dentry)
 {
-	int error = may_delete(&init_user_ns, dir, dentry, 1);
+	int error = may_delete(mnt_userns, dir, dentry, 1);
 
 	if (error)
 		return error;
@@ -3821,6 +3911,7 @@ EXPORT_SYMBOL(vfs_rmdir);
 
 long do_rmdir(int dfd, struct filename *name)
 {
+	struct user_namespace *mnt_userns;
 	int error = 0;
 	struct dentry *dentry;
 	struct path path;
@@ -3861,7 +3952,8 @@ retry:
 	error = security_path_rmdir(&path, dentry);
 	if (error)
 		goto exit3;
-	error = vfs_rmdir(path.dentry->d_inode, dentry);
+	mnt_userns = mnt_user_ns(path.mnt);
+	error = vfs_rmdir(mnt_userns, path.dentry->d_inode, dentry);
 exit3:
 	dput(dentry);
 exit2:
@@ -3884,6 +3976,7 @@ SYSCALL_DEFINE1(rmdir, const char __user *, pathname)
 
 /**
  * vfs_unlink - unlink a filesystem object
+ * @mnt_userns:	user namespace of the mount the inode was found from
  * @dir:	parent directory
  * @dentry:	victim
  * @delegated_inode: returns victim inode, if the inode is delegated.
@@ -3899,11 +3992,18 @@ SYSCALL_DEFINE1(rmdir, const char __user *, pathname)
  * Alternatively, a caller may pass NULL for delegated_inode.  This may
  * be appropriate for callers that expect the underlying filesystem not
  * to be NFS exported.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
  */
-int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
+int vfs_unlink(struct user_namespace *mnt_userns, struct inode *dir,
+	       struct dentry *dentry, struct inode **delegated_inode)
 {
 	struct inode *target = dentry->d_inode;
-	int error = may_delete(&init_user_ns, dir, dentry, 0);
+	int error = may_delete(mnt_userns, dir, dentry, 0);
 
 	if (error)
 		return error;
@@ -3974,6 +4074,8 @@ retry_deleg:
 	dentry = __lookup_hash(&last, path.dentry, lookup_flags);
 	error = PTR_ERR(dentry);
 	if (!IS_ERR(dentry)) {
+		struct user_namespace *mnt_userns;
+
 		/* Why not before? Because we want correct error value */
 		if (last.name[last.len])
 			goto slashes;
@@ -3984,7 +4086,8 @@ retry_deleg:
 		error = security_path_unlink(&path, dentry);
 		if (error)
 			goto exit2;
-		error = vfs_unlink(path.dentry->d_inode, dentry, &delegated_inode);
+		mnt_userns = mnt_user_ns(path.mnt);
+		error = vfs_unlink(mnt_userns, path.dentry->d_inode, dentry, &delegated_inode);
 exit2:
 		dput(dentry);
 	}
@@ -4033,9 +4136,25 @@ SYSCALL_DEFINE1(unlink, const char __user *, pathname)
 	return do_unlinkat(AT_FDCWD, getname(pathname));
 }
 
-int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
+/**
+ * vfs_symlink - create symlink
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dir:	inode of @dentry
+ * @dentry:	pointer to dentry of the base directory
+ * @oldname:	name of the file to link to
+ *
+ * Create a symlink.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
+ */
+int vfs_symlink(struct user_namespace *mnt_userns, struct inode *dir,
+		struct dentry *dentry, const char *oldname)
 {
-	int error = may_create(&init_user_ns, dir, dentry);
+	int error = may_create(mnt_userns, dir, dentry);
 
 	if (error)
 		return error;
@@ -4073,8 +4192,13 @@ retry:
 		goto out_putname;
 
 	error = security_path_symlink(&path, dentry, from->name);
-	if (!error)
-		error = vfs_symlink(path.dentry->d_inode, dentry, from->name);
+	if (!error) {
+		struct user_namespace *mnt_userns;
+
+		mnt_userns = mnt_user_ns(path.mnt);
+		error = vfs_symlink(mnt_userns, path.dentry->d_inode, dentry,
+				    from->name);
+	}
 	done_path_create(&path, dentry);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
@@ -4099,6 +4223,7 @@ SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newn
 /**
  * vfs_link - create a new link
  * @old_dentry:	object to be linked
+ * @mnt_userns:	the user namespace of the mount
  * @dir:	new parent
  * @new_dentry:	where to create the new link
  * @delegated_inode: returns inode needing a delegation break
@@ -4114,8 +4239,16 @@ SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newn
  * Alternatively, a caller may pass NULL for delegated_inode.  This may
  * be appropriate for callers that expect the underlying filesystem not
  * to be NFS exported.
+ *
+ * If the inode has been found through an idmapped mount the user namespace of
+ * the vfsmount must be passed through @mnt_userns. This function will then take
+ * care to map the inode according to @mnt_userns before checking permissions.
+ * On non-idmapped mounts or if permission checking is to be performed on the
+ * raw inode simply passs init_user_ns.
  */
-int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry, struct inode **delegated_inode)
+int vfs_link(struct dentry *old_dentry, struct user_namespace *mnt_userns,
+	     struct inode *dir, struct dentry *new_dentry,
+	     struct inode **delegated_inode)
 {
 	struct inode *inode = old_dentry->d_inode;
 	unsigned max_links = dir->i_sb->s_max_links;
@@ -4124,7 +4257,7 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 	if (!inode)
 		return -ENOENT;
 
-	error = may_create(&init_user_ns, dir, new_dentry);
+	error = may_create(mnt_userns, dir, new_dentry);
 	if (error)
 		return error;
 
@@ -4141,7 +4274,7 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 	 * be writen back improperly if their true value is unknown to
 	 * the vfs.
 	 */
-	if (HAS_UNMAPPED_ID(&init_user_ns, inode))
+	if (HAS_UNMAPPED_ID(mnt_userns, inode))
 		return -EPERM;
 	if (!dir->i_op->link)
 		return -EPERM;
@@ -4188,6 +4321,7 @@ EXPORT_SYMBOL(vfs_link);
 static int do_linkat(int olddfd, const char __user *oldname, int newdfd,
 	      const char __user *newname, int flags)
 {
+	struct user_namespace *mnt_userns;
 	struct dentry *new_dentry;
 	struct path old_path, new_path;
 	struct inode *delegated_inode = NULL;
@@ -4229,7 +4363,9 @@ retry:
 	error = security_path_link(old_path.dentry, &new_path, new_dentry);
 	if (error)
 		goto out_dput;
-	error = vfs_link(old_path.dentry, new_path.dentry->d_inode, new_dentry, &delegated_inode);
+	mnt_userns = mnt_user_ns(new_path.mnt);
+	error = vfs_link(old_path.dentry, mnt_userns, new_path.dentry->d_inode,
+			 new_dentry, &delegated_inode);
 out_dput:
 	done_path_create(&new_path, new_dentry);
 	if (delegated_inode) {
@@ -4263,12 +4399,14 @@ SYSCALL_DEFINE2(link, const char __user *, oldname, const char __user *, newname
 
 /**
  * vfs_rename - rename a filesystem object
- * @old_dir:	parent of source
- * @old_dentry:	source
- * @new_dir:	parent of destination
- * @new_dentry:	destination
- * @delegated_inode: returns an inode needing a delegation break
- * @flags:	rename flags
+ * @old_mnt_userns:	old user namespace of the mount the inode was found from
+ * @old_dir:		parent of source
+ * @old_dentry:		source
+ * @new_mnt_userns:	new user namespace of the mount the inode was found from
+ * @new_dir:		parent of destination
+ * @new_dentry:		destination
+ * @delegated_inode:	returns an inode needing a delegation break
+ * @flags:		rename flags
  *
  * The caller must hold multiple mutexes--see lock_rename()).
  *
@@ -4314,7 +4452,6 @@ SYSCALL_DEFINE2(link, const char __user *, oldname, const char __user *, newname
 int vfs_rename(struct renamedata *rd)
 {
 	int error;
-	struct user_namespace *mnt_userns = &init_user_ns;
 	struct inode *old_dir = rd->old_dir, *new_dir = rd->new_dir;
 	struct dentry *old_dentry = rd->old_dentry;
 	struct dentry *new_dentry = rd->new_dentry;
@@ -4330,19 +4467,21 @@ int vfs_rename(struct renamedata *rd)
 	if (source == target)
 		return 0;
 
-	error = may_delete(mnt_userns, old_dir, old_dentry, is_dir);
+	error = may_delete(rd->old_mnt_userns, old_dir, old_dentry, is_dir);
 	if (error)
 		return error;
 
 	if (!target) {
-		error = may_create(mnt_userns, new_dir, new_dentry);
+		error = may_create(rd->new_mnt_userns, new_dir, new_dentry);
 	} else {
 		new_is_dir = d_is_dir(new_dentry);
 
 		if (!(flags & RENAME_EXCHANGE))
-			error = may_delete(mnt_userns, new_dir, new_dentry, is_dir);
+			error = may_delete(rd->new_mnt_userns, new_dir,
+					   new_dentry, is_dir);
 		else
-			error = may_delete(mnt_userns, new_dir, new_dentry, new_is_dir);
+			error = may_delete(rd->new_mnt_userns, new_dir,
+					   new_dentry, new_is_dir);
 	}
 	if (error)
 		return error;
@@ -4356,13 +4495,13 @@ int vfs_rename(struct renamedata *rd)
 	 */
 	if (new_dir != old_dir) {
 		if (is_dir) {
-			error = inode_permission(&init_user_ns, source,
+			error = inode_permission(rd->old_mnt_userns, source,
 						 MAY_WRITE);
 			if (error)
 				return error;
 		}
 		if ((flags & RENAME_EXCHANGE) && new_is_dir) {
-			error = inode_permission(&init_user_ns, target,
+			error = inode_permission(rd->new_mnt_userns, target,
 						 MAY_WRITE);
 			if (error)
 				return error;
@@ -4552,8 +4691,10 @@ retry_deleg:
 
 	rd.old_dir	   = old_path.dentry->d_inode;
 	rd.old_dentry	   = old_dentry;
+	rd.old_mnt_userns  = mnt_user_ns(old_path.mnt);
 	rd.new_dir	   = new_path.dentry->d_inode;
 	rd.new_dentry	   = new_dentry;
+	rd.new_mnt_userns  = mnt_user_ns(new_path.mnt);
 	rd.delegated_inode = &delegated_inode;
 	rd.flags	   = flags;
 	error = vfs_rename(&rd);