linux-hardening - Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

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Message-ID: <CAGsJ_4zY5jwCsJh94OFMi1UqzFu5Ji3OeJZz91p2YKT5jxBc5A@mail.gmail.com>
Date: Thu, 29 Aug 2024 23:53:33 +1200
From: Barry Song <21cnbao@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Michal Hocko <mhocko@...e.com>, Yafang Shao <laoar.shao@...il.com>, akpm@...ux-foundation.org, 
	linux-mm@...ck.org, 42.hyeyoo@...il.com, cl@...ux.com, hailong.liu@...o.com, 
	hch@...radead.org, iamjoonsoo.kim@....com, penberg@...nel.org, 
	rientjes@...gle.com, roman.gushchin@...ux.dev, urezki@...il.com, 
	v-songbaohua@...o.com, virtualization@...ts.linux.dev, 
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

On Thu, Aug 29, 2024 at 10:24 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 8/27/24 09:50, Barry Song wrote:
> > On Tue, Aug 27, 2024 at 7:38 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >>
> >> Ugh, wasn't aware, well spotted. So it means there at least shouldn't be
> >> existing users of __GFP_NOFAIL with order > 1 :)
> >>
> >> But also the check is in the hotpath, even before trying the pcplists, so we
> >> could move it to __alloc_pages_slowpath() while extending it?
> >
> > Agreed. I don't think it is reasonable to check the order and flags in
> > two different places especially rmqueue() has already had
> > gfp_flags & __GFP_NOFAIL operation and order > 1
> > overhead.
> >
> > We can at least extend the current check to make some improvement
> > though I still believe Michal's suggestion of implementing OOPS_ON is a
> > better approach to pursue, as it doesn't crash the entire system
> > while ensuring the problematic process is terminated.
>
> Linus made clear it's not a mm concern. If e.g. hardening people want to
> pursuit that instead, they can.
>
> BTW I think BUG_ON already works like this, if possible only the calling
> process is terminated. panic happens in case of being in a irq context, or

you are right. This is a detail I overlooked in the last discussion.
BUG_ON has already been exactly the case to only terminate the bad
process if it can
(panic_on_oops=N and not in irq context).

> due to panic_on_oops. Which the security people are setting to 1 anyway and
> OOPS_ON would have to observe it too. So AFAICS the only difference from
> BUG_ON would be not panic in the irq context, if panic_on_oops isn't set.

right.

> (as for "no mm locks held" I think it's already satisfied at the points we
> check for __GFP_NOFAIL).

Let me summarize the discussion:

Patch 1/4, which fixes the misuse of combining gfp_nofail and atomic
in vdpa driver, is necessary.
Patch 2/4, which updates the documentation to clarify that
non-blockable gfp_nofail is not
                  supported, is needed.
Patch 3/4: We will replace BUG_ON with WARN_ON_ONCE to warn when the
size is too large,
                 where gfp_nofail will return NULL.
Patch 4/4: We will move the order > 1 check from the current fast path
to the slow path and extend
                 the check of gfp_direct_reclaim flag also in the slow path.

If nobody has an objection, I will prepare v4 as above.

Thanks
Barry