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

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Message-ID: <YFIVwPWTo48ITkHs@dhcp22.suse.cz>
Date:   Wed, 17 Mar 2021 15:44:16 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 17-03-21 14:34:27, Greg KH wrote:
> 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?

What is wrong about using seq_* interface directly?

> 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.

I am not familiar with sysfs, I just got lost in all the indirection but
replacing buffer by the seq_file and operate on that should be possible,
no?

-- 
Michal Hocko
SUSE Labs