Re: [PATCH] ptrace: being capable wrt a process requires mapped uids/gids

[Andy Lutomirski <luto@amacapital.net>]

On Sat, Dec 12, 2015 at 12:12 PM, Jann Horn <jann@thejh.net> wrote:
> ptrace_has_cap() checks whether the current process should be
> treated as having a certain capability for ptrace checks
> against another process. Until now, this was equivalent to
> has_ns_capability(current, target_ns, CAP_SYS_PTRACE).
>
> However, if a root-owned process wants to enter a user
> namespace for some reason without knowing who owns it and
> therefore can't change to the namespace owner's uid and gid
> before entering, as soon as it has entered the namespace,
> the namespace owner can attach to it via ptrace and thereby
> gain access to its uid and gid.
>
> While it is possible for the entering process to switch to
> the uid of a claimed namespace owner before entering,
> causing the attempt to enter to fail if the claimed uid is
> wrong, this doesn't solve the problem of determining an
> appropriate gid.
>
> With this change, the entering process can first enter the
> namespace and then safely inspect the namespace's
> properties, e.g. through /proc/self/{uid_map,gid_map},
> assuming that the namespace owner doesn't have access to
> uid 0.
>
> Signed-off-by: Jann Horn <jann@thejh.net>

The analysis and the patch both look correct.

Reviewed-by: Andy Lutomirski <luto@kernel.org>

Eric, care to opine?

--Andy

> ---
>  kernel/ptrace.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index b760bae..c27770d 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -207,12 +207,32 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
>         return ret;
>  }
>
> -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> +static bool ptrace_has_cap(const struct cred *tcred, unsigned int mode)
>  {
> +       struct user_namespace *tns = tcred->user_ns;
> +       struct user_namespace *curns = current_cred()->user_ns;
> +
> +       /* When a root-owned process enters a user namespace created by a
> +        * malicious user, the user shouldn't be able to execute code under
> +        * uid 0 by attaching to the root-owned process via ptrace.
> +        * Therefore, similar to the capable_wrt_inode_uidgid() check,
> +        * verify that all the uids and gids of the target process are
> +        * mapped into the current namespace.
> +        * No fsuid/fsgid check because __ptrace_may_access doesn't do it
> +        * either.
> +        */
> +       if (!kuid_has_mapping(curns, tcred->euid) ||
> +                       !kuid_has_mapping(curns, tcred->suid) ||
> +                       !kuid_has_mapping(curns, tcred->uid)  ||
> +                       !kgid_has_mapping(curns, tcred->egid) ||
> +                       !kgid_has_mapping(curns, tcred->sgid) ||
> +                       !kgid_has_mapping(curns, tcred->gid))
> +               return false;
> +
>         if (mode & PTRACE_MODE_NOAUDIT)
> -               return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> +               return has_ns_capability_noaudit(current, tns, CAP_SYS_PTRACE);
>         else
> -               return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> +               return has_ns_capability(current, tns, CAP_SYS_PTRACE);
>  }
>
>  /* Returns 0 on success, -errno on denial. */
> @@ -241,7 +261,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>             gid_eq(cred->gid, tcred->sgid) &&
>             gid_eq(cred->gid, tcred->gid))
>                 goto ok;
> -       if (ptrace_has_cap(tcred->user_ns, mode))
> +       if (ptrace_has_cap(tcred, mode))
>                 goto ok;
>         rcu_read_unlock();
>         return -EPERM;
> @@ -252,7 +272,7 @@ ok:
>                 dumpable = get_dumpable(task->mm);
>         rcu_read_lock();
>         if (dumpable != SUID_DUMP_USER &&
> -           !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
> +           !ptrace_has_cap(__task_cred(task), mode)) {
>                 rcu_read_unlock();
>                 return -EPERM;
>         }
> --
> 2.1.4
>



-- 
Andy Lutomirski
AMA Capital Management, LLC