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Date:   Tue, 12 Jan 2021 18:45:08 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        jannh@...gle.com, keescook@...omium.org, jeffv@...gle.com,
        minchan@...nel.org, 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 01/12, Michal Hocko wrote:
>
> On Mon 11-01-21 09:06:22, Suren Baghdasaryan wrote:
>
> > 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.
>
> I have to say that ptrace modes are rather obscure to me. So I cannot
> really judge whether MODE_READ is sufficient. My understanding has
> always been that this is requred to RO access to the address space. But
> this operation clearly has a visible side effect. Do we have any actual
> documentation for the existing modes?
>
> I would be really curious to hear from Jann and Oleg (now Cced).

Can't comment, sorry. I never understood these security checks and never tried.
IIUC only selinux/etc can treat ATTACH/READ differently and I have no idea what
is the difference.

Oleg.

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