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Message-ID: <CAF2d9jhiZFiFODNXUY13F+4Pm+59+rZW=CnLPX1VUT3BAJdeeA@mail.gmail.com> Date: Tue, 28 Nov 2017 13:22:12 -0800 From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com> To: "Serge E. Hallyn" <serge@...lyn.com> Cc: Mahesh Bandewar <mahesh@...dewar.net>, LKML <linux-kernel@...r.kernel.org>, Netdev <netdev@...r.kernel.org>, Kernel-hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, Kees Cook <keescook@...omium.org>, "Eric W . Biederman" <ebiederm@...ssion.com>, Eric Dumazet <edumazet@...gle.com>, David Miller <davem@...emloft.net> Subject: Re: [PATCHv2 2/2] userns: control capabilities of some user namespaces On Sat, Nov 25, 2017 at 10:40 PM, Serge E. Hallyn <serge@...lyn.com> wrote: > Quoting Mahesh Bandewar (mahesh@...dewar.net): >> 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> > > Acked-by: Serge Hallyn <serge@...lyn.com> > > Although a few comment addition requests below: > >> --- >> 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); > > Here and at the definition below, please add a comment explaining > that a controlled cap is defined as not being in the sysctl. > will do in v3. >> +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); >> } >> > > Please add a comment explaining that a controlled ns > is one created by a user which did not have CAP_SYS_ADMIN > (or descended from such an ns). > will do in v3. >> +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; > > I'd be curious to see the performance impact on this on a regular > workload (kernel build?) in a controlled ns. > Should it affect? If at all, it should be +ve since, the recursive user-ns hierarchy lookup is avoided with the above check if the capability is controlled. The additional cost otherwise is this check per cap_capable() call. >> + >> /* 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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