Merge branch 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs

Pull overlayfs fixes from Miklos Szeredi:

 - fix incomplete syncing of filesystem

 - fix regression in readdir on ovl over 9p

 - only follow redirects when needed

 - misc fixes and cleanups

* 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs:
  ovl: fix overlay: warning prefix
  ovl: Use PTR_ERR_OR_ZERO()
  ovl: Sync upper dirty data when syncing overlayfs
  ovl: update ctx->pos on impure dir iteration
  ovl: Pass ovl_get_nlink() parameters in right order
  ovl: don't follow redirects if redirect_dir=off

Documentation/filesystems/overlayfs.txt

@@ -156,6 +156,40 @@ handle it in two different ways:
    root of the overlay.  Finally the directory is moved to the new
    location.
 
+There are several ways to tune the "redirect_dir" feature.
+
+Kernel config options:
+
+- OVERLAY_FS_REDIRECT_DIR:
+    If this is enabled, then redirect_dir is turned on by  default.
+- OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW:
+    If this is enabled, then redirects are always followed by default. Enabling
+    this results in a less secure configuration.  Enable this option only when
+    worried about backward compatibility with kernels that have the redirect_dir
+    feature and follow redirects even if turned off.
+
+Module options (can also be changed through /sys/module/overlay/parameters/*):
+
+- "redirect_dir=BOOL":
+    See OVERLAY_FS_REDIRECT_DIR kernel config option above.
+- "redirect_always_follow=BOOL":
+    See OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW kernel config option above.
+- "redirect_max=NUM":
+    The maximum number of bytes in an absolute redirect (default is 256).
+
+Mount options:
+
+- "redirect_dir=on":
+    Redirects are enabled.
+- "redirect_dir=follow":
+    Redirects are not created, but followed.
+- "redirect_dir=off":
+    Redirects are not created and only followed if "redirect_always_follow"
+    feature is enabled in the kernel/module config.
+- "redirect_dir=nofollow":
+    Redirects are not created and not followed (equivalent to "redirect_dir=off"
+    if "redirect_always_follow" feature is not enabled).
+
 Non-directories
 ---------------
 

fs/overlayfs/Kconfig

@@ -24,6 +24,16 @@ config OVERLAY_FS_REDIRECT_DIR
 	  an overlay which has redirects on a kernel that doesn't support this
 	  feature will have unexpected results.
 
+config OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW
+	bool "Overlayfs: follow redirects even if redirects are turned off"
+	default y
+	depends on OVERLAY_FS
+	help
+	  Disable this to get a possibly more secure configuration, but that
+	  might not be backward compatible with previous kernels.
+
+	  For more information, see Documentation/filesystems/overlayfs.txt
+
 config OVERLAY_FS_INDEX
 	bool "Overlayfs: turn on inodes index feature by default"
 	depends on OVERLAY_FS

fs/overlayfs/dir.c

@@ -887,7 +887,8 @@ static int ovl_set_redirect(struct dentry *dentry, bool samedir)
 		spin_unlock(&dentry->d_lock);
 	} else {
 		kfree(redirect);
-		pr_warn_ratelimited("overlay: failed to set redirect (%i)\n", err);
+		pr_warn_ratelimited("overlayfs: failed to set redirect (%i)\n",
+				    err);
 		/* Fall back to userspace copy-up */
 		err = -EXDEV;
 	}

fs/overlayfs/namei.c

@@ -435,7 +435,7 @@ int ovl_verify_index(struct dentry *index, struct ovl_path *lower,
 
 	/* Check if index is orphan and don't warn before cleaning it */
 	if (d_inode(index)->i_nlink == 1 &&
-	    ovl_get_nlink(index, origin.dentry, 0) == 0)
+	    ovl_get_nlink(origin.dentry, index, 0) == 0)
 		err = -ENOENT;
 
 	dput(origin.dentry);
@@ -681,6 +681,22 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		if (d.stop)
 			break;
 
+		/*
+		 * Following redirects can have security consequences: it's like
+		 * a symlink into the lower layer without the permission checks.
+		 * This is only a problem if the upper layer is untrusted (e.g
+		 * comes from an USB drive).  This can allow a non-readable file
+		 * or directory to become readable.
+		 *
+		 * Only following redirects when redirects are enabled disables
+		 * this attack vector when not necessary.
+		 */
+		err = -EPERM;
+		if (d.redirect && !ofs->config.redirect_follow) {
+			pr_warn_ratelimited("overlay: refusing to follow redirect for (%pd2)\n", dentry);
+			goto out_put;
+		}
+
 		if (d.redirect && d.redirect[0] == '/' && poe != roe) {
 			poe = roe;
 

fs/overlayfs/overlayfs.h

@@ -180,7 +180,7 @@ static inline int ovl_do_whiteout(struct inode *dir, struct dentry *dentry)
 static inline struct dentry *ovl_do_tmpfile(struct dentry *dentry, umode_t mode)
 {
 	struct dentry *ret = vfs_tmpfile(dentry, mode, 0);
-	int err = IS_ERR(ret) ? PTR_ERR(ret) : 0;
+	int err = PTR_ERR_OR_ZERO(ret);
 
 	pr_debug("tmpfile(%pd2, 0%o) = %i\n", dentry, mode, err);
 	return ret;

fs/overlayfs/ovl_entry.h

@@ -14,6 +14,8 @@ struct ovl_config {
 	char *workdir;
 	bool default_permissions;
 	bool redirect_dir;
+	bool redirect_follow;
+	const char *redirect_mode;
 	bool index;
 };
 

fs/overlayfs/readdir.c

@@ -499,7 +499,7 @@ out:
 	return err;
 
 fail:
-	pr_warn_ratelimited("overlay: failed to look up (%s) for ino (%i)\n",
+	pr_warn_ratelimited("overlayfs: failed to look up (%s) for ino (%i)\n",
 			    p->name, err);
 	goto out;
 }
@@ -663,7 +663,10 @@ static int ovl_iterate_real(struct file *file, struct dir_context *ctx)
 			return PTR_ERR(rdt.cache);
 	}
 
-	return iterate_dir(od->realfile, &rdt.ctx);
+	err = iterate_dir(od->realfile, &rdt.ctx);
+	ctx->pos = rdt.ctx.pos;
+
+	return err;
 }
 
 

fs/overlayfs/super.c

@@ -33,6 +33,13 @@ module_param_named(redirect_dir, ovl_redirect_dir_def, bool, 0644);
 MODULE_PARM_DESC(ovl_redirect_dir_def,
 		 "Default to on or off for the redirect_dir feature");
 
+static bool ovl_redirect_always_follow =
+	IS_ENABLED(CONFIG_OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW);
+module_param_named(redirect_always_follow, ovl_redirect_always_follow,
+		   bool, 0644);
+MODULE_PARM_DESC(ovl_redirect_always_follow,
+		 "Follow redirects even if redirect_dir feature is turned off");
+
 static bool ovl_index_def = IS_ENABLED(CONFIG_OVERLAY_FS_INDEX);
 module_param_named(index, ovl_index_def, bool, 0644);
 MODULE_PARM_DESC(ovl_index_def,
@@ -232,6 +239,7 @@ static void ovl_free_fs(struct ovl_fs *ofs)
 	kfree(ofs->config.lowerdir);
 	kfree(ofs->config.upperdir);
 	kfree(ofs->config.workdir);
+	kfree(ofs->config.redirect_mode);
 	if (ofs->creator_cred)
 		put_cred(ofs->creator_cred);
 	kfree(ofs);
@@ -244,6 +252,7 @@ static void ovl_put_super(struct super_block *sb)
 	ovl_free_fs(ofs);
 }
 
+/* Sync real dirty inodes in upper filesystem (if it exists) */
 static int ovl_sync_fs(struct super_block *sb, int wait)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
@@ -252,14 +261,24 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
 
 	if (!ofs->upper_mnt)
 		return 0;
-	upper_sb = ofs->upper_mnt->mnt_sb;
+
-	if (!upper_sb->s_op->sync_fs)
+	/*
+	 * If this is a sync(2) call or an emergency sync, all the super blocks
+	 * will be iterated, including upper_sb, so no need to do anything.
+	 *
+	 * If this is a syncfs(2) call, then we do need to call
+	 * sync_filesystem() on upper_sb, but enough if we do it when being
+	 * called with wait == 1.
+	 */
+	if (!wait)
 		return 0;
 
-	/* real inodes have already been synced by sync_filesystem(ovl_sb) */
+	upper_sb = ofs->upper_mnt->mnt_sb;
+
 	down_read(&upper_sb->s_umount);
-	ret = upper_sb->s_op->sync_fs(upper_sb, wait);
+	ret = sync_filesystem(upper_sb);
 	up_read(&upper_sb->s_umount);
+
 	return ret;
 }
 
@@ -295,6 +314,11 @@ static bool ovl_force_readonly(struct ovl_fs *ofs)
 	return (!ofs->upper_mnt || !ofs->workdir);
 }
 
+static const char *ovl_redirect_mode_def(void)
+{
+	return ovl_redirect_dir_def ? "on" : "off";
+}
+
 /**
  * ovl_show_options
  *
@@ -313,12 +337,10 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	}
 	if (ofs->config.default_permissions)
 		seq_puts(m, ",default_permissions");
-	if (ofs->config.redirect_dir != ovl_redirect_dir_def)
+	if (strcmp(ofs->config.redirect_mode, ovl_redirect_mode_def()) != 0)
-		seq_printf(m, ",redirect_dir=%s",
+		seq_printf(m, ",redirect_dir=%s", ofs->config.redirect_mode);
-			   ofs->config.redirect_dir ? "on" : "off");
 	if (ofs->config.index != ovl_index_def)
-		seq_printf(m, ",index=%s",
+		seq_printf(m, ",index=%s", ofs->config.index ? "on" : "off");
-			   ofs->config.index ? "on" : "off");
 	return 0;
 }
 
@@ -348,8 +370,7 @@ enum {
 	OPT_UPPERDIR,
 	OPT_WORKDIR,
 	OPT_DEFAULT_PERMISSIONS,
-	OPT_REDIRECT_DIR_ON,
+	OPT_REDIRECT_DIR,
-	OPT_REDIRECT_DIR_OFF,
 	OPT_INDEX_ON,
 	OPT_INDEX_OFF,
 	OPT_ERR,
@@ -360,8 +381,7 @@ static const match_table_t ovl_tokens = {
 	{OPT_UPPERDIR,			"upperdir=%s"},
 	{OPT_WORKDIR,			"workdir=%s"},
 	{OPT_DEFAULT_PERMISSIONS,	"default_permissions"},
-	{OPT_REDIRECT_DIR_ON,		"redirect_dir=on"},
+	{OPT_REDIRECT_DIR,		"redirect_dir=%s"},
-	{OPT_REDIRECT_DIR_OFF,		"redirect_dir=off"},
 	{OPT_INDEX_ON,			"index=on"},
 	{OPT_INDEX_OFF,			"index=off"},
 	{OPT_ERR,			NULL}
@@ -390,10 +410,37 @@ static char *ovl_next_opt(char **s)
 	return sbegin;
 }
 
+static int ovl_parse_redirect_mode(struct ovl_config *config, const char *mode)
+{
+	if (strcmp(mode, "on") == 0) {
+		config->redirect_dir = true;
+		/*
+		 * Does not make sense to have redirect creation without
+		 * redirect following.
+		 */
+		config->redirect_follow = true;
+	} else if (strcmp(mode, "follow") == 0) {
+		config->redirect_follow = true;
+	} else if (strcmp(mode, "off") == 0) {
+		if (ovl_redirect_always_follow)
+			config->redirect_follow = true;
+	} else if (strcmp(mode, "nofollow") != 0) {
+		pr_err("overlayfs: bad mount option \"redirect_dir=%s\"\n",
+		       mode);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int ovl_parse_opt(char *opt, struct ovl_config *config)
 {
 	char *p;
 
+	config->redirect_mode = kstrdup(ovl_redirect_mode_def(), GFP_KERNEL);
+	if (!config->redirect_mode)
+		return -ENOMEM;
+
 	while ((p = ovl_next_opt(&opt)) != NULL) {
 		int token;
 		substring_t args[MAX_OPT_ARGS];
@@ -428,12 +475,11 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			config->default_permissions = true;
 			break;
 
-		case OPT_REDIRECT_DIR_ON:
+		case OPT_REDIRECT_DIR:
-			config->redirect_dir = true;
+			kfree(config->redirect_mode);
-			break;
+			config->redirect_mode = match_strdup(&args[0]);
-
+			if (!config->redirect_mode)
-		case OPT_REDIRECT_DIR_OFF:
+				return -ENOMEM;
-			config->redirect_dir = false;
 			break;
 
 		case OPT_INDEX_ON:
@@ -458,7 +504,7 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 		config->workdir = NULL;
 	}
 
-	return 0;
+	return ovl_parse_redirect_mode(config, config->redirect_mode);
 }
 
 #define OVL_WORKDIR_NAME "work"
@@ -1160,7 +1206,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (!cred)
 		goto out_err;
 
-	ofs->config.redirect_dir = ovl_redirect_dir_def;
 	ofs->config.index = ovl_index_def;
 	err = ovl_parse_opt((char *) data, &ofs->config);
 	if (err)