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Date:   Mon, 11 Jan 2021 17:22:13 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     jannh@...gle.com, keescook@...omium.org, jeffv@...gle.com,
        minchan@...nel.org, mhocko@...e.com, shakeelb@...gle.com,
        rientjes@...gle.com, edgararriaga@...gle.com, timmurray@...gle.com,
        linux-mm@...ck.org, selinux@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement
 for process_madvise

On Mon, 11 Jan 2021 09:06:22 -0800 Suren Baghdasaryan <surenb@...gle.com> 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.

It would be useful to see the proposed manpage update.

process_madvise() was released in 5.10, so this is a
non-backward-compatible change to a released kernel.

I think it would be OK at this stage to feed this into 5.10.x with a
cc:stable and suitable words in the changelog explaining why we're
doing this.

Alternatively we could retain PTRACE_MODE_ATTACH's behaviour and add
PTRACE_MODE_READ&CAP_SYS_NICE alongside that.

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