linux-kernel - Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process

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Message-ID: <CAJuCfpEccYMENy_6PrVFTC1_Xx3wV2E2805wsSLENbW+C_mbfg@mail.gmail.com>
Date:   Wed, 13 Jan 2021 10:08:08 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Edgar Arriaga García <edgararriaga@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        linux-mm <linux-mm@...ck.org>, selinux@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

On Wed, Jan 13, 2021 at 6:22 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 12-01-21 09:51:24, Suren Baghdasaryan wrote:
> > On Tue, Jan 12, 2021 at 9:45 AM Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > 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.
> >
> > I haven't seen a written explanation on ptrace modes but when I
> > consulted Jann his explanation was:
> >
> > PTRACE_MODE_READ means you can inspect metadata about processes with
> > the specified domain, across UID boundaries.
> > PTRACE_MODE_ATTACH means you can fully impersonate processes with the
> > specified domain, across UID boundaries.
>
> Maybe this would be a good start to document expectations. Some more
> practical examples where the difference is visible would be great as
> well.

I'll do my best but I'm also not a security expert. Will post the next
version with a draft for the man page (this syscall does not have a
man page yet AFAIKT) and we can iterate on the wording there.

> > He did agree that in this case PTRACE_MODE_ATTACH seems too
> > restrictive (we do not try to gain full control or impersonate a
> > process) and PTRACE_MODE_READ is a better choice.
>
> All that being said, I am not against the changed behavior but I do not
> feel competent to give an ack.

Great. SOunds like the only missing piece is the man page with more
details. I'll work on it but since it's the first time I will be
contributing to man pages it might take me a couple days. Thanks
everyone for the reviews!

> --
> Michal Hocko
> SUSE Labs