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From: "Adrian Ratiu" <adrian.ratiu@collabora.com>
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Date: Wed, 31 Jul 2024 14:15:54 +0100
Cc: "Jeff Xu" <jeffxu@google.com>, linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, kernel@collabora.com, gbiv@google.com, inglorion@google.com, ajordanr@google.com, "Doug Anderson" <dianders@chromium.org>, "Jann Horn" <jannh@google.com>, "Kees Cook" <kees@kernel.org>, "Ard Biesheuvel" <ardb@kernel.org>, "Christian Brauner" <brauner@kernel.org>
To: "Linus Torvalds" <torvalds@linux-foundation.org>
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Subject: =?utf-8?q?Re=3A?= [PATCH v4] =?utf-8?q?proc=3A?= add config & param to 
 block forcing mem writes
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On Wednesday, July 31, 2024 02:18 EEST, Linus Torvalds <torvalds@linux-=
foundation.org> wrote:

> On Tue, 30 Jul 2024 at 16:09, Jeff Xu <jeffxu@google.com> wrote:
> >
> > > +               task =3D get=5Fproc=5Ftask(file=5Finode(file));
> > > +               if (task) {
> > > +                       ptrace=5Factive =3D task->ptrace && task-=
>mm =3D=3D mm && task->parent =3D=3D current;
> >
> > Do we need to call "read=5Flock(&tasklist=5Flock);" ?
> > see comments in ptrace=5Fcheck=5Fattach() of  kernel/ptrace.c
>=20
> Well, technically I guess the tasklist=5Flock should be taken.
>=20
> Practically speaking, maybe just using READ=5FONCE() for these fields
> would really be sufficient.
>=20
> Yes, it could "race" with the task exiting or just detaching, but the
> logic would basically be "at one point we were tracing it", and since
> this fundamentally a "one-point" situation (with the actual =5Faccess=
es=5F
> happening later anyway), logically that should be sufficient.
>=20
> I mean - none of this is about "permissions" per se. We actually did
> the proper *permission* check at open() time regardless of all this
> code. This is more of a further tightening of the rules (ie it has
> gone from "are we allowed to ptrace" to "are we actually actively
> ptracing".
>=20
> I suspect that the main difference between the two situations is
> probably (a) one extra step required and (b) whatever extra system
> call security things people might have which may disable an actual
> ptrace() or whatever..

Either approach is fine with me.

Will leave v4 a few days longer in case others have a stronger
opinion or to gather & address more feedback.

If no one objects by then, I'll send v5 with READ=5FONCE().