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From: Kees Cook <keescook@chromium.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Hocko <mhocko@suse.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Lee Duncan <lduncan@suse.com>, Chris Leech <cleech@redhat.com>,
	Adam Nichols <adam@grimm-co.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-hardening@vger.kernel.org,
	Uladzislau Rezki <urezki@gmail.com>
Subject: Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer
Date: Wed, 17 Mar 2021 14:30:47 -0700	[thread overview]
Message-ID: <202103171425.CB0F4619A8@keescook> (raw)
In-Reply-To: <YFIikaNixD57o3pk@kroah.com>

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

  parent reply	other threads:[~2021-03-17 21:31 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-15 17:48 Kees Cook
2021-03-15 18:33 ` Al Viro
2021-03-15 20:43   ` Kees Cook
2021-03-16  7:24     ` Greg Kroah-Hartman
2021-03-16 12:43       ` Al Viro
2021-03-16 12:55         ` Greg Kroah-Hartman
2021-03-16 13:01         ` Michal Hocko
2021-03-16 19:18         ` Kees Cook
2021-03-17 10:44           ` Greg Kroah-Hartman
2021-03-16  8:31 ` Michal Hocko
2021-03-16 19:08   ` Kees Cook
2021-03-17 12:08     ` Michal Hocko
2021-03-17 13:34       ` Greg Kroah-Hartman
2021-03-17 14:44         ` Michal Hocko
2021-03-17 14:56           ` Greg Kroah-Hartman
2021-03-17 15:20             ` Michal Hocko
2021-03-17 15:38               ` Greg Kroah-Hartman
2021-03-17 15:48                 ` Michal Hocko
2021-03-17 21:30                 ` Kees Cook [this message]
2021-03-18  8:07                   ` Greg Kroah-Hartman
2021-03-18 15:51                     ` Kees Cook
2021-03-18 17:56                       ` Greg Kroah-Hartman
2021-03-19 14:07 ` [seq_file] 5fd6060e50: stress-ng.eventfd.ops_per_sec -49.1% regression kernel test robot
2021-03-19 19:31   ` Kees Cook

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