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

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Date:   Wed, 17 Mar 2021 14:30:47 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        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 04:38:57PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Mar 17, 2021 at 04:20:52PM +0100, Michal Hocko wrote:
> > On Wed 17-03-21 15:56:44, Greg KH wrote:
> > > On Wed, Mar 17, 2021 at 03:44:16PM +0100, Michal Hocko wrote:
> > > > 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 every show() callback of sysfs would have to be changed :(
> > 
> > Is this really the case? Would it be too ugly to have an intermediate
> > buffer and then seq_puts it into the seq file inside sysfs_kf_seq_show.
> 
> Oh, good idea.
> 
> > Sure one copy more than necessary but it this shouldn't be a hot path or
> > even visible on small strings. So that might be worth destroying an
> > inherently dangerous seq API (seq_get_buf).
> 
> I'm all for that, let me see if I can carve out some time tomorrow to
> try this out.

The trouble has been that C string APIs are just so impossibly fragile.
We just get too many bugs with it, so we really do need to rewrite the
callbacks to use seq_file, since it has a safe API.

I've been trying to write coccinelle scripts to do some of this
refactoring, but I have not found a silver bullet. (This is why I've
suggested adding the temporary "seq_show" and "seq_store" functions, so
we can transition all the callbacks without a flag day.)

> But, you don't get rid of the "ability" to have a driver write more than
> a PAGE_SIZE into the buffer passed to it.  I guess I could be paranoid
> and do some internal checks (allocate a bunch of memory and check for
> overflow by hand), if this is something to really be concerned about...

Besides the CFI prototype enforcement changes (which I can build into
the new seq_show/seq_store callbacks), the buffer management is the
primary issue: we just can't hand drivers a string (even with a length)
because the C functions are terrible. e.g. just look at the snprintf vs
scnprintf -- we constantly have to just build completely new API when
what we need is a safe way (i.e. obfuscated away from the caller) to
build a string. Luckily seq_file does this already, so leaning into that
is good here.

-- 
Kees Cook