ovl: implement set acl method

The current way of setting and getting posix acls through the generic
xattr interface is error prone and type unsafe. The vfs needs to
interpret and fixup posix acls before storing or reporting it to
userspace. Various hacks exist to make this work. The code is hard to
understand and difficult to maintain in it's current form. Instead of
making this work by hacking posix acls through xattr handlers we are
building a dedicated posix acl api around the get and set inode
operations. This removes a lot of hackiness and makes the codepaths
easier to maintain. A lot of background can be found in [1].

In order to build a type safe posix api around get and set acl we need
all filesystem to implement get and set acl.

Now that we have added get and set acl inode operations that allow easy
access to the dentry we give overlayfs it's own get and set acl inode
operations.

The set acl inode operation is duplicates most of the ovl posix acl
xattr handler. The main difference being that the set acl inode
operation relies on the new posix acl api. Once the vfs has been
switched over the custom posix acl xattr handler will be removed
completely.

Note, until the vfs has been switched to the new posix acl api this
patch is a non-functional change.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Acked-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

fs/overlayfs/dir.c

@@ -1313,6 +1313,7 @@ const struct inode_operations ovl_dir_inode_operations = {
 	.listxattr	= ovl_listxattr,
 	.get_inode_acl	= ovl_get_inode_acl,
 	.get_acl	= ovl_get_acl,
+	.set_acl	= ovl_set_acl,
 	.update_time	= ovl_update_time,
 	.fileattr_get	= ovl_fileattr_get,
 	.fileattr_set	= ovl_fileattr_set,

fs/overlayfs/inode.c

@@ -592,6 +592,98 @@ struct posix_acl *do_ovl_get_acl(struct user_namespace *mnt_userns,
 
 	return acl;
 }
+
+static int ovl_set_or_remove_acl(struct dentry *dentry, struct inode *inode,
+				 struct posix_acl *acl, int type)
+{
+	int err;
+	struct path realpath;
+	const char *acl_name;
+	const struct cred *old_cred;
+	struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
+	struct dentry *upperdentry = ovl_dentry_upper(dentry);
+	struct dentry *realdentry = upperdentry ?: ovl_dentry_lower(dentry);
+
+	err = ovl_want_write(dentry);
+	if (err)
+		return err;
+
+	/*
+	 * If ACL is to be removed from a lower file, check if it exists in
+	 * the first place before copying it up.
+	 */
+	acl_name = posix_acl_xattr_name(type);
+	if (!acl && !upperdentry) {
+		struct posix_acl *real_acl;
+
+		ovl_path_lower(dentry, &realpath);
+		old_cred = ovl_override_creds(dentry->d_sb);
+		real_acl = vfs_get_acl(mnt_user_ns(realpath.mnt), realdentry,
+				       acl_name);
+		revert_creds(old_cred);
+		posix_acl_release(real_acl);
+		if (IS_ERR(real_acl)) {
+			err = PTR_ERR(real_acl);
+			goto out_drop_write;
+		}
+	}
+
+	if (!upperdentry) {
+		err = ovl_copy_up(dentry);
+		if (err)
+			goto out_drop_write;
+
+		realdentry = ovl_dentry_upper(dentry);
+	}
+
+	old_cred = ovl_override_creds(dentry->d_sb);
+	if (acl)
+		err = ovl_do_set_acl(ofs, realdentry, acl_name, acl);
+	else
+		err = ovl_do_remove_acl(ofs, realdentry, acl_name);
+	revert_creds(old_cred);
+
+	/* copy c/mtime */
+	ovl_copyattr(inode);
+
+out_drop_write:
+	ovl_drop_write(dentry);
+	return err;
+}
+
+int ovl_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
+		struct posix_acl *acl, int type)
+{
+	int err;
+	struct inode *inode = d_inode(dentry);
+	struct dentry *workdir = ovl_workdir(dentry);
+	struct inode *realinode = ovl_inode_real(inode);
+
+	if (!IS_POSIXACL(d_inode(workdir)))
+		return -EOPNOTSUPP;
+	if (!realinode->i_op->set_acl)
+		return -EOPNOTSUPP;
+	if (type == ACL_TYPE_DEFAULT && !S_ISDIR(inode->i_mode))
+		return acl ? -EACCES : 0;
+	if (!inode_owner_or_capable(&init_user_ns, inode))
+		return -EPERM;
+
+	/*
+	 * Check if sgid bit needs to be cleared (actual setacl operation will
+	 * be done with mounter's capabilities and so that won't do it for us).
+	 */
+	if (unlikely(inode->i_mode & S_ISGID) && type == ACL_TYPE_ACCESS &&
+	    !in_group_p(inode->i_gid) &&
+	    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID)) {
+		struct iattr iattr = { .ia_valid = ATTR_KILL_SGID };
+
+		err = ovl_setattr(&init_user_ns, dentry, &iattr);
+		if (err)
+			return err;
+	}
+
+	return ovl_set_or_remove_acl(dentry, inode, acl, type);
+}
 #endif
 
 int ovl_update_time(struct inode *inode, struct timespec64 *ts, int flags)
@@ -768,6 +860,7 @@ static const struct inode_operations ovl_file_inode_operations = {
 	.listxattr	= ovl_listxattr,
 	.get_inode_acl	= ovl_get_inode_acl,
 	.get_acl	= ovl_get_acl,
+	.set_acl	= ovl_set_acl,
 	.update_time	= ovl_update_time,
 	.fiemap		= ovl_fiemap,
 	.fileattr_get	= ovl_fileattr_get,
@@ -789,6 +882,7 @@ static const struct inode_operations ovl_special_inode_operations = {
 	.listxattr	= ovl_listxattr,
 	.get_inode_acl	= ovl_get_inode_acl,
 	.get_acl	= ovl_get_acl,
+	.set_acl	= ovl_set_acl,
 	.update_time	= ovl_update_time,
 };
 

fs/overlayfs/overlayfs.h

@@ -8,6 +8,8 @@
 #include <linux/uuid.h>
 #include <linux/fs.h>
 #include <linux/namei.h>
+#include <linux/posix_acl.h>
+#include <linux/posix_acl_xattr.h>
 #include "ovl_entry.h"
 
 #undef pr_fmt
@@ -278,6 +280,18 @@ static inline int ovl_removexattr(struct ovl_fs *ofs, struct dentry *dentry,
 	return ovl_do_removexattr(ofs, dentry, ovl_xattr(ofs, ox));
 }
 
+static inline int ovl_do_set_acl(struct ovl_fs *ofs, struct dentry *dentry,
+				 const char *acl_name, struct posix_acl *acl)
+{
+	return vfs_set_acl(ovl_upper_mnt_userns(ofs), dentry, acl_name, acl);
+}
+
+static inline int ovl_do_remove_acl(struct ovl_fs *ofs, struct dentry *dentry,
+				    const char *acl_name)
+{
+	return vfs_remove_acl(ovl_upper_mnt_userns(ofs), dentry, acl_name);
+}
+
 static inline int ovl_do_rename(struct ovl_fs *ofs, struct inode *olddir,
 				struct dentry *olddentry, struct inode *newdir,
 				struct dentry *newdentry, unsigned int flags)
@@ -607,9 +621,12 @@ static inline struct posix_acl *ovl_get_acl(struct user_namespace *mnt_userns,
 {
 	return do_ovl_get_acl(mnt_userns, d_inode(dentry), type, false, false);
 }
+int ovl_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
+		struct posix_acl *acl, int type);
 #else
 #define ovl_get_inode_acl	NULL
 #define ovl_get_acl		NULL
+#define ovl_set_acl		NULL
 #endif
 
 int ovl_update_time(struct inode *inode, struct timespec64 *ts, int flags);