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Date:   Sun, 9 Dec 2018 14:53:09 -0700
From:   Tycho Andersen <tycho@tycho.ws>
To:     Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, linux-sparse@vger.kernel.org,
        kernel-hardening@lists.openwall.com, linux-kernel@vger.kernel.org
Subject: Re: [RFC v1] copy_{to,from}_user(): only inline when !__CHECKER__
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On Sun, Dec 09, 2018 at 10:39:52PM +0100, Luc Van Oostenryck wrote:
> On Sun, Dec 09, 2018 at 02:25:23PM -0700, Tycho Andersen wrote:
> > Hi Al,
> > 
> > On Sun, Dec 09, 2018 at 09:02:21PM +0000, Al Viro wrote:
> > > On Sun, Dec 09, 2018 at 01:44:49PM -0700, Tycho Andersen wrote:
> > > > While working on some additional copy_to_user() checks for sparse, I
> > > > noticed that sparse's current copy_to_user() checks are not triggered. This
> > > > is because copy_to_user() is declared as __always_inline, and sparse
> > > > specifically looks for a call instruction to copy_to_user() when it tries
> > > > to apply the checks.
> > > > 
> > > > A quick fix is to explicitly not inline when __CHECKER__ is defined, so
> > > > that sparse will be able to analyze all the copy_{to,from}_user calls.
> > > > There may be some refactoring in sparse that we can do to fix this,
> > > > although it's not immediately obvious to me how, hence the RFC-ness of this
> > > > patch.
> > > 
> > > Which sparse checks do not trigger?  Explain, please - as it is, I had been
> > > unable to guess what could "specifically looks for a call instruction" refer
> > > to.
> > 
> > In sparse.c there's check_call_instruction(), which is triggered when
> > there's an instruction of OP_CALL type in the basic block. This simply
> > compares against the name of the call target to determine whether or
> > not to call check_ctu().
> > 
> > I think what's happening here is that the call is getting inlined, and
> > so the OP_CALL goes away, and check_call_instruction() never gets
> > called.
> 
> Yes, it's what's happening.
> 
> There are several more or less bad/good solutions, like:
> * add raw_copy_{to,from}_user() in the list of checked function
>   (not inlined in most archs).

But they are inlined on x86 :\

> * add a new annotation to force sparse to check the byte count
>   (I'm thinking about __range__/OP_RANGE or something similar).

Yes, I was playing around with inventing some check like this without
the need for an annotation. It's not clear to me if it's going to work
or not yet, though :). Top two patches here are what I was playing
with:

https://github.com/tych0/sparse/commits/check-as-infoleaks

> * do these checks before functions are inlined (but then some
>   constant count could not yet be seen as constant).

Yeah, I guess I was wondering if there was some more clever location
in sparse itself we could move these to.

Tycho