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

[Kees Cook <keescook@chromium.org>]

On Thu, Mar 18, 2021 at 09:07:45AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Mar 17, 2021 at 02:30:47PM -0700, Kees Cook wrote:
> > 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.
> 
> But, is it really worth the churn here?
> 
> Yes, strings in C is "hard", but this _should_ be a simple thing for any
> driver to handle:
> 	return sysfs_emit(buffer, "%d\n", my_dev->value);
> 
> To change that to:
> 	return seq_printf(seq, "%d\n", my_dev->value);
> feels very much "don't we have other more valuable things we could be
> doing?"
> 
> So far we have found 1 driver that messed up and overflowed the buffer
> that I know of.  While reworking apis to make it "hard to get wrong" is
> a great goal, the work involved here vs. any "protection" feels very
> low.

I haven't been keeping a list, but it's not the only one. The _other_
reason we need seq_file is so we can perform checks against f_cred for
things like %p obfuscation (as was needed for modules that I hacked
around) and is needed a proper bug fix for the kernel pointer exposure
bug from the same batch. So now I'm up to 3 distinct reasons that the
sysfs API is lacking -- I think it's worth the churn and time.

> How about moving everyone to sysfs_emit() first?  That way it becomes
> much more "obvious" when drivers are doing stupid things with their
> sysfs buffer.  But even then, it would not have caught the iscsi issue
> as that was printing a user-provided string so maybe I'm just feeling
> grumpy about the potential churn here...

I need to fix the prototypes for CFI sanity too. Switching to seq_file
solves 2 problems, and if we have to change the prototype once for that,
we can include the prototype fixes for CFI at the same time to avoid
double the churn.

-- 
Kees Cook