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

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Message-ID: <YFIikaNixD57o3pk@kroah.com>
Date:   Wed, 17 Mar 2021 16:38:57 +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 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.

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...

thanks,

greg k-h