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Message-Id: <20171110053757.21170-1-mahesh@bandewar.net> Date: Thu, 9 Nov 2017 21:37:57 -0800 From: Mahesh Bandewar <mahesh@...dewar.net> To: LKML <linux-kernel@...r.kernel.org>, Netdev <netdev@...r.kernel.org> Cc: Kernel-hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, Kees Cook <keescook@...omium.org>, Serge Hallyn <serge@...lyn.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Eric Dumazet <edumazet@...gle.com>, David Miller <davem@...emloft.net>, Mahesh Bandewar <mahesh@...dewar.net>, Mahesh Bandewar <maheshb@...gle.com> Subject: [PATCHv2 2/2] userns: control capabilities of some user namespaces From: Mahesh Bandewar <maheshb@...gle.com> With this new notion of "controlled" user-namespaces, the controlled user-namespaces are marked at the time of their creation while the capabilities of processes that belong to them are controlled using the global mask. Init-user-ns is always uncontrolled and a process that has SYS_ADMIN that belongs to uncontrolled user-ns can create another (child) user- namespace that is uncontrolled. Any other process (that either does not have SYS_ADMIN or belongs to a controlled user-ns) can only create a user-ns that is controlled. global-capability-whitelist (controlled_userns_caps_whitelist) is used at the capability check-time and keeps the semantics for the processes that belong to uncontrolled user-ns as it is. Processes that belong to controlled user-ns however are subjected to different checks- (a) if the capability in question is controlled and process belongs to controlled user-ns, then it's always denied. (b) if the capability in question is NOT controlled then fall back to the traditional check. Signed-off-by: Mahesh Bandewar <maheshb@...gle.com> --- v2: Don't recalculate user-ns flags for every setns() call. v1: Initial submission. include/linux/capability.h | 1 + include/linux/user_namespace.h | 20 ++++++++++++++++++++ kernel/capability.c | 5 +++++ kernel/user_namespace.c | 4 ++++ security/commoncap.c | 8 ++++++++ 5 files changed, 38 insertions(+) diff --git a/include/linux/capability.h b/include/linux/capability.h index 7d79a4689625..a1fd9e460379 100644 --- a/include/linux/capability.h +++ b/include/linux/capability.h @@ -251,6 +251,7 @@ extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns); extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps); int proc_douserns_caps_whitelist(struct ctl_table *table, int write, void __user *buff, size_t *lenp, loff_t *ppos); +bool is_capability_controlled(int cap); extern int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size); diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h index 3fe714da7f5a..647f825c7b5f 100644 --- a/include/linux/user_namespace.h +++ b/include/linux/user_namespace.h @@ -23,6 +23,7 @@ struct uid_gid_map { /* 64 bytes -- 1 cache line */ }; #define USERNS_SETGROUPS_ALLOWED 1UL +#define USERNS_CONTROLLED 2UL #define USERNS_INIT_FLAGS USERNS_SETGROUPS_ALLOWED @@ -103,6 +104,16 @@ static inline void put_user_ns(struct user_namespace *ns) __put_user_ns(ns); } +static inline bool is_user_ns_controlled(const struct user_namespace *ns) +{ + return ns->flags & USERNS_CONTROLLED; +} + +static inline void mark_user_ns_controlled(struct user_namespace *ns) +{ + ns->flags |= USERNS_CONTROLLED; +} + struct seq_operations; extern const struct seq_operations proc_uid_seq_operations; extern const struct seq_operations proc_gid_seq_operations; @@ -161,6 +172,15 @@ static inline struct ns_common *ns_get_owner(struct ns_common *ns) { return ERR_PTR(-EPERM); } + +static inline bool is_user_ns_controlled(const struct user_namespace *ns) +{ + return false; +} + +static inline void mark_user_ns_controlled(struct user_namespace *ns) +{ +} #endif #endif /* _LINUX_USER_H */ diff --git a/kernel/capability.c b/kernel/capability.c index 4a859b7d4902..bffe249922de 100644 --- a/kernel/capability.c +++ b/kernel/capability.c @@ -511,6 +511,11 @@ bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns) } /* Controlled-userns capabilities routines */ +bool is_capability_controlled(int cap) +{ + return !cap_raised(controlled_userns_caps_whitelist, cap); +} + #ifdef CONFIG_SYSCTL int proc_douserns_caps_whitelist(struct ctl_table *table, int write, void __user *buff, size_t *lenp, loff_t *ppos) diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c index c490f1e4313b..600c7dcb9ff7 100644 --- a/kernel/user_namespace.c +++ b/kernel/user_namespace.c @@ -139,6 +139,10 @@ int create_user_ns(struct cred *new) goto fail_keyring; set_cred_user_ns(new, ns); + if (!ns_capable(parent_ns, CAP_SYS_ADMIN) || + is_user_ns_controlled(parent_ns)) + mark_user_ns_controlled(ns); + return 0; fail_keyring: #ifdef CONFIG_PERSISTENT_KEYRINGS diff --git a/security/commoncap.c b/security/commoncap.c index fc46f5b85251..89103f16ac37 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -73,6 +73,14 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns, { struct user_namespace *ns = targ_ns; + /* If the capability is controlled and user-ns that process + * belongs-to is 'controlled' then return EPERM and no need + * to check the user-ns hierarchy. + */ + if (is_user_ns_controlled(cred->user_ns) && + is_capability_controlled(cap)) + return -EPERM; + /* See if cred has the capability in the target user namespace * by examining the target user namespace and all of the target * user namespace's parents. -- 2.15.0.448.gf294e3d99a-goog
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