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Date: Fri, 26 Jan 2024 10:55:05 +0100
From: Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To: Richard Weinberger <richard@nod.at>
Cc: Petr Tesarik <petrtesarik@huaweicloud.com>, anton ivanov
 <anton.ivanov@cambridgegreys.com>, Johannes Berg
 <johannes@sipsolutions.net>, linux-um <linux-um@lists.infradead.org>,
 linux-kernel <linux-kernel@vger.kernel.org>, Roberto Sassu
 <roberto.sassu@huaweicloud.com>, Petr Tesarik
 <petr.tesarik1@huawei-partners.com>
Subject: Re: [PATCH 0/2] um: improve UML page fault handling
Message-ID: <20240126105505.38440835@meshulam.tesarici.cz>
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On Fri, 5 Jan 2024 07:51:09 +0100
Petr Tesa=C5=99=C3=ADk <petr@tesarici.cz> wrote:

> Helo Richard,
>=20
> Am Fri, 5 Jan 2024 00:22:11 +0100 (CET)
> schrieb Richard Weinberger <richard@nod.at>:
>=20
> > ----- Urspr=C3=BCngliche Mail ----- =20
> > > Von: "Petr Tesarik" <petrtesarik@huaweicloud.com>
> > > An: "richard" <richard@nod.at>, "anton ivanov" <anton.ivanov@cambridg=
egreys.com>, "Johannes Berg"
> > > <johannes@sipsolutions.net>, "linux-um" <linux-um@lists.infradead.org=
>, "linux-kernel" <linux-kernel@vger.kernel.org>
> > > CC: "Roberto Sassu" <roberto.sassu@huaweicloud.com>, petr@tesarici.cz=
, "Petr Tesarik"
> > > <petr.tesarik1@huawei-partners.com>
> > > Gesendet: Freitag, 15. Dezember 2023 13:14:29
> > > Betreff: [PATCH 0/2] um: improve UML page fault handling   =20
> >  =20
> > > From: Petr Tesarik <petr.tesarik1@huawei-partners.com>
> > >=20
> > > Improve UML handling of segmentation faults in kernel mode. Although
> > > such page faults are generally caused by a kernel bug, it is annoying
> > > if they cause an infinite loop, or panic the kernel. More importantly,
> > > a robust implementation allows to write KUnit tests for various guard
> > > pages, preventing potential kernel self-protection regressions.
> > >=20
> > > Petr Tesarik (2):
> > >  um: do not panic on kernel mode faults
> > >  um: oops on accessing an non-present page in the vmalloc area   =20
> >=20
> > I think this is a good thing to have. =20
>=20
> Thanks for the feedback.
>=20
> > For the implementation side, this needs to use the oops_* helpers
> > from kernel/panic.c and taint the kernel, etc... =20
>=20
> Yes, I did see that coming but wanted to get some confirmation that
> it's worth the effort.
>=20
> > See arch/x86/kernel/dumpstack.c die() and friends. =20
>=20
> This implementation also calls die notifiers, but AFAICS different
> architectures are not very consistent in their use. Do you also
> require die notifiers for the UML implementation?

It seems I won't have time for this in the near future... Can I start
by sending a trivial patch that panics the kernel if kernel mode tries
to access a vmalloc guard page? That's something I can do immediately,
and it's still better than getting page faults in an infinite loop...

Petr T