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Message-ID: <20190121214127.t3opb6cffaz4ibp5@ltop.local>
Date: Mon, 21 Jan 2019 22:41:28 +0100
From: Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To: Tycho Andersen <tycho@...ho.ws>
Cc: linux-sparse@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC v1 0/4] static analysis of copy_to_user()
On Mon, Jan 21, 2019 at 08:05:10AM +1300, Tycho Andersen wrote:
> Hey Luc,
>
> On Fri, Dec 21, 2018 at 11:47:19PM +0100, Luc Van Oostenryck wrote:
> > On Thu, Dec 20, 2018 at 12:59:27PM -0700, Tycho Andersen wrote:
> > > Hi all,
> > >
> > > A while ago I talked with various people about whether some static
> > > analsys of copy_to_user() could be productive in finding infoleaks.
> > > Unfortunately, due to the various issues outlined in the patch notes, it
> > > doesn't seem like it is. Perhaps these checks are useful to put in just
> > > to future proof ourselves against these sorts of issues, though.
> > >
> > > Anyway, here's the code. Thoughts welcome!
> >
> > Hi,
> >
> > I'm taking the first patch directly but I won't be able to look
> > closer at the other patches until next week.
>
> Any chance you can take a peek at these?
Hi,
Sorry, I've had few available time the last weeks.
I had look at them shortly after you send them but
I haven't yet made my mind about them.
I'm quite reluctant to add complexity (the AST walking)
if it doesn't bring much benefit if any.
In, short the problems are:
1) duplication of the AST walking
2) unreliable type because of using void *
3) unreliable size because array to pointer degeneracy
There is some solutions, though:
1) what *could* be done is to add a method 'check'
to struct symbol_op and call it, *for example*,
just after op->expand() in expand_symbol_call()
(and add a mechanism to set this method for the symbol
corresponding to copy_to_user()).
Otherwise, splitting the AST walking from sparse.c
and making it something generic would be preferable.
Another approach could be keep the check via OP_CALL
but doing it just after linearization, before the
optimization destroy the types (and add, if needed,
some flag to force linearize_cast() keep absolutely
all type info).
2) this one seems pretty hopeless
3) the current calls degenerate()/create_pointer()
do indeed destroy the original type and (at first sight)
no 'addressof' should exist anymore after evaluation.
This is inconsistent with the existence of expand_addressof().
By changing degenerate()/create_pointer() the original
type should stay available.
Sorry again for the late reply,
-- Luc
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