linux-kernel - [PATCH] fuse: Ensure posix acls are translated outside of init_user

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Date:   Fri, 04 May 2018 11:47:28 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alban Crequy <alban@...volk.io>,
        Seth Forshee <seth.forshee@...onical.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Dongsu Park <dongsu@...volk.io>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: [PATCH] fuse: Ensure posix acls are translated outside of init_user_ns


Ensure the translation happens by failing to read or write
posix acls when the filesystem has not indicated it supports
posix acls.

This ensures that modern cached posix acl support is available
and used when dealing with posix acls.  This is important
because only that path has the code to convernt the uids and
gids in posix acls into the user namespace of a fuse filesystem.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---

Miklos after several attempts to handle this better last cycle.  I
figure we should go with the stupid version for now.  I think I know
how to do better but I don't want that to gate forward progress on
fully unprivileged fuse mounts.  Especially as this is the last known
issue to deal with.

 fs/fuse/fuse_i.h |  1 +
 fs/fuse/inode.c  |  7 +++++++
 fs/fuse/xattr.c  | 43 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 51 insertions(+)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index f630951df8dc..5256ad333b05 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -985,6 +985,7 @@ ssize_t fuse_listxattr(struct dentry *entry, char *list, size_t size);
 int fuse_removexattr(struct inode *inode, const char *name);
 extern const struct xattr_handler *fuse_xattr_handlers[];
 extern const struct xattr_handler *fuse_acl_xattr_handlers[];
+extern const struct xattr_handler *fuse_no_acl_xattr_handlers[];
 
 struct posix_acl;
 struct posix_acl *fuse_get_acl(struct inode *inode, int type);
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 1643043d4fe5..22c76cf8c2e3 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1100,6 +1100,13 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 	    file->f_cred->user_ns != sb->s_user_ns)
 		goto err_fput;
 
+	/*
+	 * If we are not in the initial user namespace posix
+	 * acls must be translated.
+	 */
+	if (sb->s_user_ns != &init_user_ns)
+		sb->s_xattr = fuse_no_acl_xattr_handlers;
+
 	fc = kmalloc(sizeof(*fc), GFP_KERNEL);
 	err = -ENOMEM;
 	if (!fc)
diff --git a/fs/fuse/xattr.c b/fs/fuse/xattr.c
index 3caac46b08b0..433717640f78 100644
--- a/fs/fuse/xattr.c
+++ b/fs/fuse/xattr.c
@@ -192,6 +192,26 @@ static int fuse_xattr_set(const struct xattr_handler *handler,
 	return fuse_setxattr(inode, name, value, size, flags);
 }
 
+static bool no_xattr_list(struct dentry *dentry)
+{
+	return false;
+}
+
+static int no_xattr_get(const struct xattr_handler *handler,
+			struct dentry *dentry, struct inode *inode,
+			const char *name, void *value, size_t size)
+{
+	return -EOPNOTSUPP;
+}
+
+static int no_xattr_set(const struct xattr_handler *handler,
+			struct dentry *dentry, struct inode *nodee,
+			const char *name, const void *value,
+			size_t size, int flags)
+{
+	return -EOPNOTSUPP;
+}
+
 static const struct xattr_handler fuse_xattr_handler = {
 	.prefix = "",
 	.get    = fuse_xattr_get,
@@ -209,3 +229,26 @@ const struct xattr_handler *fuse_acl_xattr_handlers[] = {
 	&fuse_xattr_handler,
 	NULL
 };
+
+static const struct xattr_handler fuse_no_acl_access_xattr_handler = {
+	.name  = XATTR_NAME_POSIX_ACL_ACCESS,
+	.flags = ACL_TYPE_ACCESS,
+	.list  = no_xattr_list,
+	.get   = no_xattr_get,
+	.set   = no_xattr_set,
+};
+
+static const struct xattr_handler fuse_no_acl_default_xattr_handler = {
+	.name  = XATTR_NAME_POSIX_ACL_DEFAULT,
+	.flags = ACL_TYPE_ACCESS,
+	.list  = no_xattr_list,
+	.get   = no_xattr_get,
+	.set   = no_xattr_set,
+};
+
+const struct xattr_handler *fuse_no_acl_xattr_handlers[] = {
+	&fuse_no_acl_access_xattr_handler,
+	&fuse_no_acl_default_xattr_handler,
+	&fuse_xattr_handler,
+	NULL
+};
-- 
2.14.1