linux-kernel - Re: [PATCH v1 1/2] ptrace: Set PF_SUPERPRIV when checking capability

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Date:   Fri, 30 Oct 2020 16:47:47 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Eric Paris <eparis@...hat.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Will Drewry <wad@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v1 1/2] ptrace: Set PF_SUPERPRIV when checking capability

On Fri, Oct 30, 2020 at 1:39 PM Mickaël Salaün <mic@...ikod.net> wrote:
> Commit 69f594a38967 ("ptrace: do not audit capability check when outputing
> /proc/pid/stat") replaced the use of ns_capable() with
> has_ns_capability{,_noaudit}() which doesn't set PF_SUPERPRIV.
>
> Commit 6b3ad6649a4c ("ptrace: reintroduce usage of subjective credentials in
> ptrace_has_cap()") replaced has_ns_capability{,_noaudit}() with
> security_capable(), which doesn't set PF_SUPERPRIV neither.
>
> Since commit 98f368e9e263 ("kernel: Add noaudit variant of ns_capable()"), a
> new ns_capable_noaudit() helper is available.  Let's use it!
>
> As a result, the signature of ptrace_has_cap() is restored to its original one.
>
> Cc: Christian Brauner <christian.brauner@...ntu.com>
> Cc: Eric Paris <eparis@...hat.com>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Serge E. Hallyn <serge@...lyn.com>
> Cc: Tyler Hicks <tyhicks@...ux.microsoft.com>
> Cc: stable@...r.kernel.org
> Fixes: 6b3ad6649a4c ("ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()")
> Fixes: 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>

Yeah... I guess this makes sense. (We'd have to undo or change it if
we ever end up needing to use a different set of credentials, e.g.
from ->f_cred, but I guess that's really something we should avoid
anyway.)

Reviewed-by: Jann Horn <jannh@...gle.com>

with one nit:


[...]
>  /* Returns 0 on success, -errno on denial. */
>  static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  {
> -       const struct cred *cred = current_cred(), *tcred;
> +       const struct cred *const cred = current_cred(), *tcred;

This is an unrelated change, and almost no kernel code marks local
pointer variables as "const". I would drop this change from the patch.

>         struct mm_struct *mm;
>         kuid_t caller_uid;
>         kgid_t caller_gid;