From 438c84c2f0c794f75ab55ce65c505b01bfce4480 Mon Sep 17 00:00:00 2001 From: Miklos Szeredi Date: Mon, 11 Dec 2017 11:28:10 +0100 Subject: ovl: don't follow redirects if redirect_dir=off Overlayfs is following redirects even when redirects are disabled. If this is unintentional (probably the majority of cases) then this can be a problem. E.g. upper layer comes from untrusted USB drive, and attacker crafts a redirect to enable read access to otherwise unreadable directories. If "redirect_dir=off", then turn off following as well as creation of redirects. If "redirect_dir=follow", then turn on following, but turn off creation of redirects (which is what "redirect_dir=off" does now). This is a backward incompatible change, so make it dependent on a config option. Reported-by: David Howells Signed-off-by: Miklos Szeredi --- fs/overlayfs/Kconfig | 10 +++++++ fs/overlayfs/namei.c | 16 ++++++++++++ fs/overlayfs/ovl_entry.h | 2 ++ fs/overlayfs/super.c | 68 ++++++++++++++++++++++++++++++++++++------------ 4 files changed, 79 insertions(+), 17 deletions(-) (limited to 'fs/overlayfs') diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig index cbfc196e5dc5..5ac415466861 100644 --- a/fs/overlayfs/Kconfig +++ b/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 diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c index 625ed8066570..2a12dc2e9840 100644 --- a/fs/overlayfs/namei.c +++ b/fs/overlayfs/namei.c @@ -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; diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h index 752bab645879..9d0bc03bf6e4 100644 --- a/fs/overlayfs/ovl_entry.h +++ b/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; }; diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c index 288d20f9a55a..13a8a8617e44 100644 --- a/fs/overlayfs/super.c +++ b/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); @@ -295,6 +303,11 @@ 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 +326,10 @@ 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) - seq_printf(m, ",redirect_dir=%s", - ofs->config.redirect_dir ? "on" : "off"); + if (strcmp(ofs->config.redirect_mode, ovl_redirect_mode_def()) != 0) + seq_printf(m, ",redirect_dir=%s", ofs->config.redirect_mode); if (ofs->config.index != ovl_index_def) - seq_printf(m, ",index=%s", - ofs->config.index ? "on" : "off"); + seq_printf(m, ",index=%s", ofs->config.index ? "on" : "off"); return 0; } @@ -348,8 +359,7 @@ enum { OPT_UPPERDIR, OPT_WORKDIR, OPT_DEFAULT_PERMISSIONS, - OPT_REDIRECT_DIR_ON, - OPT_REDIRECT_DIR_OFF, + OPT_REDIRECT_DIR, OPT_INDEX_ON, OPT_INDEX_OFF, OPT_ERR, @@ -360,8 +370,7 @@ 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_OFF, "redirect_dir=off"}, + {OPT_REDIRECT_DIR, "redirect_dir=%s"}, {OPT_INDEX_ON, "index=on"}, {OPT_INDEX_OFF, "index=off"}, {OPT_ERR, NULL} @@ -390,10 +399,37 @@ 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 +464,11 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config) config->default_permissions = true; break; - case OPT_REDIRECT_DIR_ON: - config->redirect_dir = true; - break; - - case OPT_REDIRECT_DIR_OFF: - config->redirect_dir = false; + case OPT_REDIRECT_DIR: + kfree(config->redirect_mode); + config->redirect_mode = match_strdup(&args[0]); + if (!config->redirect_mode) + return -ENOMEM; break; case OPT_INDEX_ON: @@ -458,7 +493,7 @@ 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 +1195,6 @@ 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) -- cgit v1.2.1