linux-kernel - Re: [PATCH v4] ptrace: reintroduce usage of subjective credentials in ptrace_has

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Message-ID: <20200120133115.GA30403@redhat.com>
Date:   Mon, 20 Jan 2020 14:31:16 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, stable@...r.kernel.org,
        Serge Hallyn <serge@...lyn.com>, avagin@...il.com,
        Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH v4] ptrace: reintroduce usage of subjective credentials
 in ptrace_has_cap()

On 01/18, Christian Brauner wrote:
>
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -264,12 +264,17 @@ 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 *cred, struct user_namespace *ns,
> +			   unsigned int mode)
>  {
> +	int ret;
> +
>  	if (mode & PTRACE_MODE_NOAUDIT)
> -		return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> +		ret = security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
>  	else
> -		return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> +		ret = security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NONE);
> +
> +	return ret == 0;
>  }
>  
>  /* Returns 0 on success, -errno on denial. */
> @@ -321,7 +326,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	    gid_eq(caller_gid, tcred->sgid) &&
>  	    gid_eq(caller_gid, tcred->gid))
>  		goto ok;
> -	if (ptrace_has_cap(tcred->user_ns, mode))
> +	if (ptrace_has_cap(cred, tcred->user_ns, mode))
>  		goto ok;
>  	rcu_read_unlock();
>  	return -EPERM;
> @@ -340,7 +345,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	mm = task->mm;
>  	if (mm &&
>  	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
> -	     !ptrace_has_cap(mm->user_ns, mode)))
> +	     !ptrace_has_cap(cred, mm->user_ns, mode)))
>  	    return -EPERM;

I never understood these security checks and thus I don't understand the
security impact. Say, has_capability_noaudit() in __set_oom_adj(). Isn't
it equally wrong?

However, the patch looks "obviously correct" to me.

Reviewed-by: Oleg Nesterov <oleg@...hat.com>