linux-hardening - Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

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Date:   Wed, 17 Mar 2021 13:08:21 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Lee Duncan <lduncan@...e.com>, Chris Leech <cleech@...hat.com>,
        Adam Nichols <adam@...mm-co.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

On Tue 16-03-21 12:08:02, Kees Cook wrote:
> On Tue, Mar 16, 2021 at 09:31:23AM +0100, Michal Hocko wrote:
[...]
> > Also this cannot really be done for configurations with a very limited
> > vmalloc space (32b for example). Those systems are more and more rare
> > but you shouldn't really allow userspace to deplete the vmalloc space.
>
> This sounds like two objections:
> - 32b has a small vmalloc space
> - userspace shouldn't allow depletion of vmalloc space
>
> I'd be happy to make this 64b only. For the latter, I would imagine
> there are other vmalloc-exposed-to-userspace cases, but yes, this would
> be much more direct. Is that a problem in practice?

vmalloc space shouldn't be a problem for 64b systems but I am not sure
how does vmalloc scale with many small allocations. There were some
changes by Uladzislau who might give us more insight (CCed).

> > I would be also curious to see how vmalloc scales with huge number of
> > single page allocations which would be easy to trigger with this patch.
>
> Right -- what the best way to measure this (and what would be "too
> much")?

Proc is used quite heavily for all sorts of monitoring so I would be
worried about a noticeable slow down.

Btw. I still have problems with the approach. seq_file is intended to
provide safe way to dump values to the userspace. Sacrificing
performance just because of some abuser seems like a wrong way to go as
Al pointed out earlier. Can we simply stop the abuse and disallow to
manipulate the buffer directly? I do realize this might be more tricky
for reasons mentioned in other emails but this is definitely worth
doing.

-- 
Michal Hocko
SUSE Labs