diff mbox series

[v2,06/10] ovl: user xattr

Message ID 20201207163255.564116-7-mszeredi@redhat.com (mailing list archive)
State New
Headers show
Series allow unprivileged overlay mounts | expand

Commit Message

Miklos Szeredi Dec. 7, 2020, 4:32 p.m. UTC
Optionally allow using "user.overlay." namespace instead of
"trusted.overlay."

This is necessary for overlayfs to be able to be mounted in an unprivileged
namepsace.

Make the option explicit, since it makes the filesystem format be
incompatible.

Disable redirect_dir and metacopy options, because these would allow
privilege escalation through direct manipulation of the
"user.overlay.redirect" or "user.overlay.metacopy" xattrs.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/overlayfs/inode.c     | 10 ++++++--
 fs/overlayfs/overlayfs.h |  8 +++---
 fs/overlayfs/ovl_entry.h |  1 +
 fs/overlayfs/super.c     | 55 ++++++++++++++++++++++++++++++++++++----
 fs/overlayfs/util.c      |  5 ++--
 5 files changed, 67 insertions(+), 12 deletions(-)

Comments

Amir Goldstein Dec. 8, 2020, 1:10 p.m. UTC | #1
On Mon, Dec 7, 2020 at 6:37 PM Miklos Szeredi <mszeredi@redhat.com> wrote:
>
> Optionally allow using "user.overlay." namespace instead of
> "trusted.overlay."

There are several occurrences of "trusted.overlay" string in code and
Documentation, which is fine. But maybe only adjust the comment for
testing xattr support:

         * Check if upper/work fs supports trusted.overlay.* xattr

>
> This is necessary for overlayfs to be able to be mounted in an unprivileged
> namepsace.
>
> Make the option explicit, since it makes the filesystem format be
> incompatible.
>
> Disable redirect_dir and metacopy options, because these would allow
> privilege escalation through direct manipulation of the
> "user.overlay.redirect" or "user.overlay.metacopy" xattrs.
>
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> ---

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -582,9 +582,10 @@ bool ovl_check_dir_xattr(struct super_block *sb, struct dentry *dentry,
>  #define OVL_XATTR_METACOPY_POSTFIX     "metacopy"
>
>  #define OVL_XATTR_TAB_ENTRY(x) \
> -       [x] = OVL_XATTR_PREFIX x ## _POSTFIX
> +       [x] = { [false] = OVL_XATTR_TRUSTED_PREFIX x ## _POSTFIX, \
> +               [true] = OVL_XATTR_USER_PREFIX x ## _POSTFIX }
>
> -const char *ovl_xattr_table[] = {
> +const char *ovl_xattr_table[][2] = {
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_OPAQUE),
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_REDIRECT),
>         OVL_XATTR_TAB_ENTRY(OVL_XATTR_ORIGIN),
> --

Can you constify this 2D array? I don't even know the syntax for that...

Thanks,
Amir.
Miklos Szeredi Dec. 11, 2020, 2:55 p.m. UTC | #2
On Tue, Dec 8, 2020 at 2:14 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Mon, Dec 7, 2020 at 6:37 PM Miklos Szeredi <mszeredi@redhat.com> wrote:
> >
> > Optionally allow using "user.overlay." namespace instead of
> > "trusted.overlay."
>
> There are several occurrences of "trusted.overlay" string in code and
> Documentation, which is fine. But maybe only adjust the comment for
> testing xattr support:
>
>          * Check if upper/work fs supports trusted.overlay.* xattr


Updated documentation and comments.

Pushed new series to:

git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git#ovl-unpriv-v3

Based on the feedback, I feel it's ready for v5.11, so merged this
into #overlayfs-next as well.

Thanks,
Miklos
diff mbox series

Patch

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index b584dca845ba..8ec3062999a9 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -329,8 +329,14 @@  static const char *ovl_get_link(struct dentry *dentry,
 
 bool ovl_is_private_xattr(struct super_block *sb, const char *name)
 {
-	return strncmp(name, OVL_XATTR_PREFIX,
-		       sizeof(OVL_XATTR_PREFIX) - 1) == 0;
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	if (ofs->config.userxattr)
+		return strncmp(name, OVL_XATTR_USER_PREFIX,
+			       sizeof(OVL_XATTR_USER_PREFIX) - 1) == 0;
+	else
+		return strncmp(name, OVL_XATTR_TRUSTED_PREFIX,
+			       sizeof(OVL_XATTR_TRUSTED_PREFIX) - 1) == 0;
 }
 
 int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index f8880aa2ba0e..46282111d6e6 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -22,7 +22,9 @@  enum ovl_path_type {
 #define OVL_TYPE_MERGE(type)	((type) & __OVL_PATH_MERGE)
 #define OVL_TYPE_ORIGIN(type)	((type) & __OVL_PATH_ORIGIN)
 
-#define OVL_XATTR_PREFIX XATTR_TRUSTED_PREFIX "overlay."
+#define OVL_XATTR_NAMESPACE "overlay."
+#define OVL_XATTR_TRUSTED_PREFIX XATTR_TRUSTED_PREFIX OVL_XATTR_NAMESPACE
+#define OVL_XATTR_USER_PREFIX XATTR_USER_PREFIX OVL_XATTR_NAMESPACE
 
 enum ovl_xattr {
 	OVL_XATTR_OPAQUE,
@@ -113,10 +115,10 @@  struct ovl_fh {
 #define OVL_FH_FID_OFFSET	(OVL_FH_WIRE_OFFSET + \
 				 offsetof(struct ovl_fb, fid))
 
-extern const char *ovl_xattr_table[];
+extern const char *ovl_xattr_table[][2];
 static inline const char *ovl_xattr(struct ovl_fs *ofs, enum ovl_xattr ox)
 {
-	return ovl_xattr_table[ox];
+	return ovl_xattr_table[ox][ofs->config.userxattr];
 }
 
 static inline int ovl_do_rmdir(struct inode *dir, struct dentry *dentry)
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 1b5a2094df8e..d634c7ba3b9c 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -17,6 +17,7 @@  struct ovl_config {
 	bool nfs_export;
 	int xino;
 	bool metacopy;
+	bool userxattr;
 	bool ovl_volatile;
 };
 
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 290983bcfbb3..189380b946be 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -411,6 +411,7 @@  enum {
 	OPT_INDEX_ON,
 	OPT_INDEX_OFF,
 	OPT_NFS_EXPORT_ON,
+	OPT_USERXATTR,
 	OPT_NFS_EXPORT_OFF,
 	OPT_XINO_ON,
 	OPT_XINO_OFF,
@@ -429,6 +430,7 @@  static const match_table_t ovl_tokens = {
 	{OPT_REDIRECT_DIR,		"redirect_dir=%s"},
 	{OPT_INDEX_ON,			"index=on"},
 	{OPT_INDEX_OFF,			"index=off"},
+	{OPT_USERXATTR,			"userxattr"},
 	{OPT_NFS_EXPORT_ON,		"nfs_export=on"},
 	{OPT_NFS_EXPORT_OFF,		"nfs_export=off"},
 	{OPT_XINO_ON,			"xino=on"},
@@ -585,6 +587,10 @@  static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			config->ovl_volatile = true;
 			break;
 
+		case OPT_USERXATTR:
+			config->userxattr = true;
+			break;
+
 		default:
 			pr_err("unrecognized mount option \"%s\" or missing value\n",
 					p);
@@ -688,6 +694,28 @@  static int ovl_parse_opt(char *opt, struct ovl_config *config)
 		}
 	}
 
+
+	/* Resolve userxattr -> !redirect && !metacopy dependency */
+	if (config->userxattr) {
+		if (config->redirect_follow && redirect_opt) {
+			pr_err("conflicting options: userxattr,redirect_dir=%s\n",
+			       config->redirect_mode);
+			return -EINVAL;
+		}
+		if (config->metacopy && metacopy_opt) {
+			pr_err("conflicting options: userxattr,metacopy=on\n");
+			return -EINVAL;
+		}
+		/*
+		 * Silently disable default setting of redirect and metacopy.
+		 * This shall be the default in the future as well: these
+		 * options must be explicitly enabled if used together with
+		 * userxattr.
+		 */
+		config->redirect_dir = config->redirect_follow = false;
+		config->metacopy = false;
+	}
+
 	return 0;
 }
 
@@ -1037,8 +1065,14 @@  ovl_posix_acl_default_xattr_handler = {
 	.set = ovl_posix_acl_xattr_set,
 };
 
-static const struct xattr_handler ovl_own_xattr_handler = {
-	.prefix	= OVL_XATTR_PREFIX,
+static const struct xattr_handler ovl_own_trusted_xattr_handler = {
+	.prefix	= OVL_XATTR_TRUSTED_PREFIX,
+	.get = ovl_own_xattr_get,
+	.set = ovl_own_xattr_set,
+};
+
+static const struct xattr_handler ovl_own_user_xattr_handler = {
+	.prefix	= OVL_XATTR_USER_PREFIX,
 	.get = ovl_own_xattr_get,
 	.set = ovl_own_xattr_set,
 };
@@ -1049,12 +1083,22 @@  static const struct xattr_handler ovl_other_xattr_handler = {
 	.set = ovl_other_xattr_set,
 };
 
-static const struct xattr_handler *ovl_xattr_handlers[] = {
+static const struct xattr_handler *ovl_trusted_xattr_handlers[] = {
+#ifdef CONFIG_FS_POSIX_ACL
+	&ovl_posix_acl_access_xattr_handler,
+	&ovl_posix_acl_default_xattr_handler,
+#endif
+	&ovl_own_trusted_xattr_handler,
+	&ovl_other_xattr_handler,
+	NULL
+};
+
+static const struct xattr_handler *ovl_user_xattr_handlers[] = {
 #ifdef CONFIG_FS_POSIX_ACL
 	&ovl_posix_acl_access_xattr_handler,
 	&ovl_posix_acl_default_xattr_handler,
 #endif
-	&ovl_own_xattr_handler,
+	&ovl_own_user_xattr_handler,
 	&ovl_other_xattr_handler,
 	NULL
 };
@@ -1991,7 +2035,8 @@  static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);
 
 	sb->s_magic = OVERLAYFS_SUPER_MAGIC;
-	sb->s_xattr = ovl_xattr_handlers;
+	sb->s_xattr = ofs->config.userxattr ? ovl_user_xattr_handlers :
+		ovl_trusted_xattr_handlers;
 	sb->s_fs_info = ofs;
 	sb->s_flags |= SB_POSIXACL;
 	sb->s_iflags |= SB_I_SKIP_SYNC;
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 23f475627d07..66eaf4db027f 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -582,9 +582,10 @@  bool ovl_check_dir_xattr(struct super_block *sb, struct dentry *dentry,
 #define OVL_XATTR_METACOPY_POSTFIX	"metacopy"
 
 #define OVL_XATTR_TAB_ENTRY(x) \
-	[x] = OVL_XATTR_PREFIX x ## _POSTFIX
+	[x] = { [false] = OVL_XATTR_TRUSTED_PREFIX x ## _POSTFIX, \
+		[true] = OVL_XATTR_USER_PREFIX x ## _POSTFIX }
 
-const char *ovl_xattr_table[] = {
+const char *ovl_xattr_table[][2] = {
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_OPAQUE),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_REDIRECT),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_ORIGIN),