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SubjectRe: [RFC 5/5] user namespaces: Allow ptrace from non-init user namespaces
"Serge E. Hallyn" <serge@hallyn.com> 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.

The uid equality check below is broken.

Eric


> Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
> ---
> include/linux/capability.h | 2 ++
> kernel/ptrace.c | 40 ++++++++++++++++++++++++++++------------
> security/commoncap.c | 26 +++++++++++++++++++++-----
> 3 files changed, 51 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index cc3e976..777a166 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -543,6 +543,8 @@ 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/kernel/ptrace.c b/kernel/ptrace.c
> index 99bbaa3..aed24eb 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -116,6 +116,19 @@ int ptrace_check_attach(struct task_struct *child, int kill)
> return ret;
> }
>
> +static inline int may_ptrace_ns(struct task_struct *t)

Can we name this ptrace_capable? Since you are only
wrapping the capability check? With a name like may_ptrace_ns
I imagine very different semantics.

> +{
> + struct user_namespace *ns;
> + int ret;
> +
> + rcu_read_lock();
> + ns = task_cred_xxx(t, user)->user_ns;
> + ret = ns_capable(ns, CAP_SYS_PTRACE);
> + rcu_read_unlock();
> +
> + return ret;
> +}
> +
> int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> {
> const struct cred *cred = current_cred(), *tcred;
> @@ -134,21 +147,24 @@ 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;

This needs to be:
> + 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 && !may_ptrace_ns(task))
> return -EPERM;
>
> return security_ptrace_access_check(task, mode);
> @@ -198,7 +214,7 @@ int ptrace_attach(struct task_struct *task)
> goto unlock_tasklist;
>
> task->ptrace = PT_PTRACED;
> - if (capable(CAP_SYS_PTRACE))
> + if (may_ptrace_ns(task))
> task->ptrace |= PT_PTRACE_CAP;
>
> __ptrace_link(task, current);
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 9d910e6..bd0bcc6 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -136,12 +136,20 @@ int cap_settime(struct timespec *ts, struct timezone *tz)
> int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
> {
> int ret = 0;
> + struct cred *cred, *tcred;
>
> rcu_read_lock();
> - if (!cap_issubset(__task_cred(child)->cap_permitted,
> - current_cred()->cap_permitted) &&
> + cred = current_cred();
> + tcred = __task_cred(child);
> + if (cred->user->user_ns != tcred->user->user_ns) {

This probably deserves a comment about why cap_issubset isn't
needed here. Aka we implicitly have all caps in child user namespaces
so if we have CAP_SYS_PTRACE we know we have them all.

> + if (!ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
> + ret = -EPERM;
> + goto out;
> + }
> + if (!cap_issubset(tcred->cap_permitted, cred->cap_permitted) &&
> !capable(CAP_SYS_PTRACE))
> ret = -EPERM;
> +out:
> rcu_read_unlock();
> return ret;
> }
> @@ -156,12 +164,20 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
> int cap_ptrace_traceme(struct task_struct *parent)
> {
> int ret = 0;
> + 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))
> + cred = __task_cred(parent);
> + tcred = current_cred();
> + if (cred->user->user_ns != tcred->user->user_ns) {
> + if (!has_ns_capability(parent, tcred->user->user_ns, CAP_SYS_PTRACE))
> + ret = -EPERM;
> + goto out;
> + }
> + if (!cap_issubset(tcred->cap_permitted, cred->cap_permitted) &&
> + !has_ns_capability(parent, tcred->user->user_ns, CAP_SYS_PTRACE))
> ret = -EPERM;
> +out:
> rcu_read_unlock();
> return ret;
> }


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