attr: handle idmapped mounts

When file attributes are changed most filesystems rely on the
setattr_prepare(), setattr_copy(), and notify_change() helpers for
initialization and permission checking. Let them handle idmapped mounts.
If the inode is accessed through an idmapped mount map it into the
mount's user namespace. Afterwards the checks 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.

Helpers that perform checks on the ia_uid and ia_gid fields in struct
iattr assume that ia_uid and ia_gid are intended values and have already
been mapped correctly at the userspace-kernelspace boundary as we
already do today. 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/20210121131959.646623-8-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, 2 insertions, 2 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 1c68c9edba5e..1cb451e131ec 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1087,7 +1087,7 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 	int error;
 
-	error = setattr_prepare(dentry, attr);
+	error = setattr_prepare(&init_user_ns, dentry, attr);
 	if (error)
 		return error;
 
@@ -1141,7 +1141,7 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 		}
 	}
 
-	setattr_copy(inode, attr);
+	setattr_copy(&init_user_ns, inode, attr);
 	if (attr->ia_valid & ATTR_MODE)
 		error = posix_acl_chmod(inode, inode->i_mode);
 	return error;