Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

From: Michal Hocko <mhocko@suse.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: "Oleg Nesterov" <oleg@redhat.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Jann Horn" <jannh@google.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Jeffrey Vander Stoep" <jeffv@google.com>,
	"Minchan Kim" <minchan@kernel.org>,
	"Shakeel Butt" <shakeelb@google.com>,
	"David Rientjes" <rientjes@google.com>,
	"Edgar Arriaga García" <edgararriaga@google.com>,
	"Tim Murray" <timmurray@google.com>,
	linux-mm <linux-mm@kvack.org>,
	selinux@vger.kernel.org, "Linux API" <linux-api@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
Date: Wed, 13 Jan 2021 15:22:02 +0100	[thread overview]
Message-ID: <20210113142202.GC22493@dhcp22.suse.cz> (raw)
In-Reply-To: <CAJuCfpFQz=x-LvONO3c4iqjKP4NKJMgUuiYc8HACKHAv1Omu0w@mail.gmail.com>

On Tue 12-01-21 09:51:24, Suren Baghdasaryan wrote:
> On Tue, Jan 12, 2021 at 9:45 AM Oleg Nesterov <oleg@redhat.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.

> 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.
-- 
Michal Hocko
SUSE Labs