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Message-ID: <CAF2d9jir4WF=YyBVbAM=WAopYukcZz_==0+LtKepv8Qv8UNaEw@mail.gmail.com>
Date: Fri, 10 Nov 2017 10:49:58 +0900
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: [PATCH resend 2/2] userns: control capabilities of some user namespaces
On Fri, Nov 10, 2017 at 2:25 AM, 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>
>> ---
>> include/linux/capability.h | 1 +
>> include/linux/user_namespace.h | 20 ++++++++++++++++++++
>> kernel/capability.c | 5 +++++
>> kernel/user_namespace.c | 3 +++
>> security/commoncap.c | 8 ++++++++
>> 5 files changed, 37 insertions(+)
>>
>> diff --git a/include/linux/capability.h b/include/linux/capability.h
>> index 6c0b9677c03f..b8c6cac18658 100644
>> --- a/include/linux/capability.h
>> +++ b/include/linux/capability.h
>> @@ -250,6 +250,7 @@ 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 c18e01252346..e890fe81b47e 100644
>> --- a/include/linux/user_namespace.h
>> +++ b/include/linux/user_namespace.h
>> @@ -22,6 +22,7 @@ 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
>>
>> @@ -102,6 +103,16 @@ 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;
>> @@ -160,6 +171,15 @@ 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 62dbe3350c1b..40a38cc4ff43 100644
>> --- a/kernel/capability.c
>> +++ b/kernel/capability.c
>> @@ -510,6 +510,11 @@ 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..f393ea5108f0 100644
>> --- a/kernel/user_namespace.c
>> +++ b/kernel/user_namespace.c
>> @@ -53,6 +53,9 @@ static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
>> cred->cap_effective = CAP_FULL_SET;
>> cred->cap_ambient = CAP_EMPTY_SET;
>> cred->cap_bset = CAP_FULL_SET;
>> + if (!ns_capable(user_ns->parent, CAP_SYS_ADMIN) ||
>> + is_user_ns_controlled(user_ns->parent))
>> + mark_user_ns_controlled(user_ns);
>
> Hm, why do this here, rather than at create_user_ns()? It
> shouldn't be recalculated when someone does setns() should it?
>
You are absolutely right! It doesn't make sense to recalculate for
every setns() call. It's a side effect of couple of iterations /
approaches that I tried before finalizing this one. I'll move this
block to create_user_ns() after the set_cred_user_ns() call so that
this wont be triggered in setns() path.
Thanks,
--mahesh..
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