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Message-ID: <20171126064037.GB30279@mail.hallyn.com> Date: Sun, 26 Nov 2017 00:40:37 -0600 From: "Serge E. Hallyn" <serge@...lyn.com> To: Mahesh Bandewar <mahesh@...dewar.net> Cc: 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>, Serge Hallyn <serge@...lyn.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Eric Dumazet <edumazet@...gle.com>, David Miller <davem@...emloft.net>, Mahesh Bandewar <maheshb@...gle.com> Subject: Re: [PATCHv2 2/2] userns: control capabilities of some user namespaces 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. > +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). > +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. > + > /* 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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