linux-kernel - Re: [PATCH v5 4/5] Capability: Complete PTRACE_MODE

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Date:   Wed, 26 Sep 2018 23:26:07 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Casey Schaufler <casey.schaufler@...el.com>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        selinux@...ho.nsa.gov, Dave Hansen <dave.hansen@...el.com>,
        deneen.t.dock@...el.com, kristen@...ux.intel.com,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH v5 4/5] Capability: Complete PTRACE_MODE_SCHED

On Wed, Sep 26, 2018 at 10:35 PM Casey Schaufler
<casey.schaufler@...el.com> wrote:
> Allow a complete ptrace access check with mode PTRACE_MODE_SCHED.
> Disable the inappropriate privilege check in the capability code
> that does incompatible locking.

What's that locking you're talking about?

> Signed-off-by: Casey Schaufler <casey.schaufler@...el.com>
> ---
>  kernel/ptrace.c      | 2 --
>  security/commoncap.c | 2 ++
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 99cfddde6a55..0b6a9df51c3b 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -331,8 +331,6 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>              !ptrace_has_cap(mm->user_ns, mode)))
>             return -EPERM;
>
> -       if (mode & PTRACE_MODE_SCHED)
> -               return 0;
>         return security_ptrace_access_check(task, mode);
>  }
>
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 2e489d6a3ac8..e77457110d05 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -152,6 +152,8 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
>         if (cred->user_ns == child_cred->user_ns &&
>             cap_issubset(child_cred->cap_permitted, *caller_caps))
>                 goto out;
> +       if (mode & PTRACE_MODE_SCHED)
> +               goto out;

So for PTRACE_MODE_SCHED, this function always returns 0, right? If
that's intentional, perhaps you should instead just put "if (mode &
PTRACE_MODE_SCHED) return 0;" at the start of the function, to avoid
taking the RCU read lock in this case.

>         if (ns_capable(child_cred->user_ns, CAP_SYS_PTRACE))
>                 goto out;
>         ret = -EPERM;