Re: [PATCH 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

From: Minchan Kim <minchan@kernel.org>
To: Suren Baghdasaryan <surenb@google.com>
Cc: akpm@linux-foundation.org, jannh@google.com,
	keescook@chromium.org, jeffv@google.com, mhocko@suse.com,
	shakeelb@google.com, rientjes@google.com,
	edgararriaga@google.com, timmurray@google.com,
	linux-mm@kvack.org, selinux@vger.kernel.org,
	linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@android.com
Subject: Re: [PATCH 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
Date: Fri, 8 Jan 2021 14:15:37 -0800	[thread overview]
Message-ID: <X/jZibYD1B42D+r7@google.com> (raw)
In-Reply-To: <20210108205857.1471269-1-surenb@google.com>

On Fri, Jan 08, 2021 at 12:58:57PM -0800, Suren Baghdasaryan wrote:
> process_madvise currently requires ptrace attach capability.
> PTRACE_MODE_ATTACH gives one process complete control over another
> process. It effectively removes the security boundary between the
> two processes (in one direction). Granting ptrace attach capability
> even to a system process is considered dangerous since it creates an
> attack surface. This severely limits the usage of this API.
> The operations process_madvise can perform do not affect the correctness
> of the operation of the target process; they only affect where the data
> is physically located (and therefore, how fast it can be accessed).
> What we want is the ability for one process to influence another process
> in order to optimize performance across the entire system while leaving
> the security boundary intact.
> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ
> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata
> and CAP_SYS_NICE for influencing process performance.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

It sounds logical to me.
If security folks don't see any concern and fix below,

Acked-by: Minchan Kim <minchan@kernel.org>

> @@ -1197,12 +1197,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  		goto release_task;
>  	}
>  
> -	mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
> +	/* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */
> +	mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
>  	if (IS_ERR_OR_NULL(mm)) {
>  		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
>  		goto release_task;
>  	}
>  
> +	/*
> +	 * Require CAP_SYS_NICE for influencing process performance. Note that
> +	 * only non-destructive hints are currently supported.
> +	 */
> +	if (!capable(CAP_SYS_NICE)) {
> +		ret = -EPERM;
> +		goto release_task;

mmput?

> +	}
> +
>  	total_len = iov_iter_count(&iter);
>  
>  	while (iov_iter_count(&iter)) {
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
> 