Re: [PATCH 5/9] Allow ptrace from non-init user namespaces

[Eric W. Biederman]

"Serge E. Hallyn" <serge@xxxxxxxxxx> writes:

> ptrace is allowed to tasks in the same user namespace according to
> the usual rules (i.e. the same rules as for two tasks in the init
> user namespace).  ptrace is also allowed to a user namespace to
> which the current task the has CAP_SYS_PTRACE capability.


I don't see how it can go wrong at the moment but
same_or_ancestore_user_ns is too permissive and potentially inefficient.
Can you please replace it with a simple user namespace equality check.

Eric


> Changelog:
> 	Dec 31: Address feedback by Eric:
> 		. Correct ptrace uid check
> 		. Rename may_ptrace_ns to ptrace_capable
> 		. Also fix the cap_ptrace checks.
> 	Jan  1: Use const cred struct
> 	Jan 11: use task_ns_capable() in place of ptrace_capable().
>
> Signed-off-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> ---
>  include/linux/capability.h     |    2 +
>  include/linux/user_namespace.h |    9 +++++++
>  kernel/ptrace.c                |   27 ++++++++++++----------
>  kernel/user_namespace.c        |   16 +++++++++++++
>  security/commoncap.c           |   48 +++++++++++++++++++++++++++++++++------
>  5 files changed, 82 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index cb3d2d9..bc0f262 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -546,6 +546,8 @@ extern const kernel_cap_t __cap_init_eff_set;
>   */
>  #define has_capability(t, cap) (security_real_capable((t), &init_user_ns, (cap)) == 0)
>  
> +#define has_ns_capability(t, ns, cap) (security_real_capable((t), (ns), (cap)) == 0)
> +
>  /**
>   * has_capability_noaudit - Determine if a task has a superior capability available (unaudited)
>   * @t: The task in question
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index faf4679..862fc59 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -39,6 +39,9 @@ static inline void put_user_ns(struct user_namespace *ns)
>  uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t uid);
>  gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t gid);
>  
> +int same_or_ancestor_user_ns(struct task_struct *task,
> +	struct task_struct *victim);
> +
>  #else
>  
>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> @@ -66,6 +69,12 @@ static inline gid_t user_ns_map_gid(struct user_namespace *to,
>  	return gid;
>  }
>  
> +static inline int same_or_ancestor_user_ns(struct task_struct *task,
> +	struct task_struct *victim)
> +{
> +	return 1;
> +}
> +
>  #endif
>  
>  #endif /* _LINUX_USER_H */
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 1708b1e..cde4655 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -134,21 +134,24 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  		return 0;
>  	rcu_read_lock();
>  	tcred = __task_cred(task);
> -	if ((cred->uid != tcred->euid ||
> -	     cred->uid != tcred->suid ||
> -	     cred->uid != tcred->uid  ||
> -	     cred->gid != tcred->egid ||
> -	     cred->gid != tcred->sgid ||
> -	     cred->gid != tcred->gid) &&
> -	    !capable(CAP_SYS_PTRACE)) {
> -		rcu_read_unlock();
> -		return -EPERM;
> -	}
> +	if (cred->user->user_ns == tcred->user->user_ns &&
> +	    (cred->uid == tcred->euid &&
> +	     cred->uid == tcred->suid &&
> +	     cred->uid == tcred->uid  &&
> +	     cred->gid == tcred->egid &&
> +	     cred->gid == tcred->sgid &&
> +	     cred->gid == tcred->gid))
> +		goto ok;
> +	if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> +		goto ok;
> +	rcu_read_unlock();
> +	return -EPERM;
> +ok:
>  	rcu_read_unlock();
>  	smp_rmb();
>  	if (task->mm)
>  		dumpable = get_dumpable(task->mm);
> -	if (!dumpable && !capable(CAP_SYS_PTRACE))
> +	if (!dumpable && !task_ns_capable(task, CAP_SYS_PTRACE))
>  		return -EPERM;
>  
>  	return security_ptrace_access_check(task, mode);
> @@ -198,7 +201,7 @@ int ptrace_attach(struct task_struct *task)
>  		goto unlock_tasklist;
>  
>  	task->ptrace = PT_PTRACED;
> -	if (capable(CAP_SYS_PTRACE))
> +	if (task_ns_capable(task, CAP_SYS_PTRACE))
>  		task->ptrace |= PT_PTRACE_CAP;
>  
>  	__ptrace_link(task, current);
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 9da289c..0ef2258 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -129,6 +129,22 @@ gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t
>  	return overflowgid;
>  }
>  
> +int same_or_ancestor_user_ns(struct task_struct *task,
> +	struct task_struct *victim)
> +{
> +	struct user_namespace *u1 = task_cred_xxx(task, user)->user_ns;
> +	struct user_namespace *u2 = task_cred_xxx(victim, user)->user_ns;
> +	for (;;) {
> +		if (u1 == u2)
> +			return 1;
> +		if (u1 == &init_user_ns)
> +			return 0;
> +		u1 = u1->creator->user_ns;
> +	}
> +	/* We never get here */
> +	return 0;
> +}
> +
>  static __init int user_namespaces_init(void)
>  {
>  	user_ns_cachep = KMEM_CACHE(user_namespace, SLAB_PANIC);
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 51fa9ec..12ff65c 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -130,18 +130,34 @@ int cap_settime(struct timespec *ts, struct timezone *tz)
>   * @child: The process to be accessed
>   * @mode: The mode of attachment.
>   *
> + * If we are in the same or an ancestor user_ns and have all the target
> + * task's capabilities, then ptrace access is allowed.
> + * If we have the ptrace capability to the target user_ns, then ptrace
> + * access is allowed.
> + * Else denied.
> + *
>   * Determine whether a process may access another, returning 0 if permission
>   * granted, -ve if denied.
>   */
>  int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
>  {
>  	int ret = 0;
> +	const struct cred *cred, *tcred;
>  
>  	rcu_read_lock();
> -	if (!cap_issubset(__task_cred(child)->cap_permitted,
> -			  current_cred()->cap_permitted) &&
> -	    !capable(CAP_SYS_PTRACE))
> -		ret = -EPERM;
> +	cred = current_cred();
> +	tcred = __task_cred(child);
> +	/*
> +	 * The ancestor user_ns check may be gratuitous, as I think
> +	 * we've already guaranteed that in kernel/ptrace.c.
> +	 */
> +	if (same_or_ancestor_user_ns(current, child) &&
> +	    cap_issubset(tcred->cap_permitted, cred->cap_permitted))
> +		goto out;

I have commented on this before but I took a good hard look this time,
and can comment more intelligently.

The cap_issubset check is for the case where we don't use the
CAP_SYS_PTRACE capability, as such is only valid in the same user
namespace.  Furthermore capabilities really are not comparable between
different user namespaces.  So can you please replace the
same_or_ancestor_user_ns with a simple namespace equality check.
Having the wrong logic in here will just be confusing in the future.

Also could you name tcred child_cread I think that would be clearer in
the test below.

> +	if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> +		goto out;
> +	ret = -EPERM;
> +out:
>  	rcu_read_unlock();
>  	return ret;

I also find it strange that we allow CAP_SYS_PTRACE to allow us to
ptrace processes with more capabilities than ourselves.  But that is an
entirely different issue.

>  }
> @@ -150,18 +166,34 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
>   * cap_ptrace_traceme - Determine whether another process may trace the current
>   * @parent: The task proposed to be the tracer
>   *
> + * If parent is in the same or an ancestor user_ns and has all current's
> + * capabilities, then ptrace access is allowed.
> + * If parent has the ptrace capability to current's user_ns, then ptrace
> + * access is allowed.
> + * Else denied.
> + *
>   * Determine whether the nominated task is permitted to trace the current
>   * process, returning 0 if permission is granted, -ve if denied.
>   */
>  int cap_ptrace_traceme(struct task_struct *parent)
>  {
>  	int ret = 0;
> +	const struct cred *cred, *tcred;
>  
>  	rcu_read_lock();
> -	if (!cap_issubset(current_cred()->cap_permitted,
> -			  __task_cred(parent)->cap_permitted) &&
> -	    !has_capability(parent, CAP_SYS_PTRACE))
> -		ret = -EPERM;
> +	cred = __task_cred(parent);
> +	tcred = current_cred();
> +	/*
> +	 * The ancestor user_ns check may be gratuitous, as I think
> +	 * we've already guaranteed that in kernel/ptrace.c.
> +	 */
> +	if (same_or_ancestor_user_ns(parent, current) &&
> +	    cap_issubset(tcred->cap_permitted, cred->cap_permitted))
> +		goto out;
> +	if (has_ns_capability(parent, tcred->user->user_ns, CAP_SYS_PTRACE))
> +		goto out;
> +	ret = -EPERM;
> +out:
>  	rcu_read_unlock();
>  	return ret;
>  }
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