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

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Message-ID: <YFIFY7mj65sStba1@kroah.com>
Date:   Wed, 17 Mar 2021 14:34:27 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.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 Wed, Mar 17, 2021 at 01:08:21PM +0100, Michal Hocko wrote:
> 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.

We have to provide a buffer to "write into" somehow, so what is the best
way to stop "abuse" like this?

Right now, we do have helper functions, sysfs_emit(), that know to stop
the overflow of the buffer size, but porting the whole kernel to them is
going to take a bunch of churn, for almost no real benefit except a
potential random driver that might be doing bad things here that we have
not noticed yet.

Other than that, suggestions are welcome!

thanks,

greg k-h