Re: Question on -fwrapv and -fwrapv-pointer

[Kees Cook <keescook@chromium.org>]

On Sat, Sep 16, 2023 at 10:36:52AM +0200, Martin Uecker wrote:
> > On Fri, Sep 15, 2023 at 08:18:28AM -0700, Andrew Pinski wrote:
> > > On Fri, Sep 15, 2023 at 8:12 AM Qing Zhao <qing.zhao@oracle.com> wrote:
> > > >
> > > >
> > > >
> > > > > On Sep 15, 2023, at 3:43 AM, Xi Ruoyao <xry111@xry111.site> wrote:
> > > > >
> > > > > On Thu, 2023-09-14 at 21:41 +0000, Qing Zhao wrote:
> > > > >>>> CLANG already provided -fsanitize=unsigned-integer-overflow. GCC
> > > > >>>> might need to do the same.
> > > > >>>
> > > > >>> NO. There is no such thing as unsigned integer overflow. That option
> > > > >>> is badly designed and the GCC community has rejected a few times now
> > > > >>> having that sanitizer before. It is bad form to have a sanitizer for
> > > > >>> well defined code.
> > > > >>
> > > > >> Even though unsigned integer overflow is well defined, it might be
> > > > >> unintentional, shall we warn user about this?
> > > > >
> > > > > *Everything* could be unintentional and should be warned then.  GCC is a
> > > > > compiler, not an advanced AI educating the programmers.
> > > >
> > > > Well, you are right in some sense. -:)
> > > >
> > > > However, overflow is one important source for security flaws, it’s important  for compilers to detect
> > > > overflows in the programs in general.
> > > 
> > > Except it is NOT an overflow. Rather it is wrapping. That is a big
> > > point here. unsigned wraps and does NOT overflow. Yes there is a major
> > > difference.
> > 
> > Right, yes. I will try to pick my language very carefully. :)
> > 
> > The practical problem I am trying to solve in the 30 million lines of
> > Linux kernel code is that of catching arithmetic wrap-around. The
> > problem is one of evolving the code -- I can't just drop -fwrapv and
> > -fwrapv-pointer because it's not possible to fix all the cases at once.
> > (And we really don't want to reintroduce undefined behavior.)
> > 
> > So, for signed, pointer, and unsigned types, we need:
> > 
> > a) No arithmetic UB -- everything needs to have deterministic behavior.
> >    The current solution here is "-fno-strict-overflow", which eliminates
> >    the UB and makes sure everything wraps.
> > 
> > b) A way to run-time warn/trap on overflow/underflow/wrap-around. This
> >    would work with -fsanitize=[signed-integer|pointer]-overflow except
> >    due to "a)" we always wrap. And there isn't currently coverage like
> >    this for unsigned (in GCC).
> > 
> > Our problem is that the kernel is filled with a mix of places where there
> > is intended wrap-around and unintended wrap-around. We can chip away at
> > fixing the intended wrap-around that we can find with static analyzers,
> > etc, but at the end of the day there is a long tail of finding the places
> > where intended wrap-around is hiding. But when the refactoring is
> > sufficiently completely, we can move the wrap-around warning to a trap,
> > and the kernel will not longer have this class of security flaw.
> > 
> > As a real-world example, here is a bug where a u8 wraps around causing
> > an under-allocation that allowed for a heap overwrite:
> > 
> > https://git.kernel.org/linus/6311071a0562
> > https://elixir.bootlin.com/linux/v6.5/source/net/wireless/nl80211.c#L5422
> > 
> > If there were more than 255 elements in a linked list, the allocation
> > would be too small, and the second loop would write past the end of the
> > allocation. This is a pretty classic allocation underflow and linear
> > heap write overflow security flaw. (And it would be trivially stopped by
> > trapping on the u8 wrap around.)
> > 
> > So, I want to be able to catch that at run-time. But we also have code
> > doing things like "if (ulong + offset < ulong) { ... }":
> > 
> > https://elixir.bootlin.com/linux/v6.5/source/drivers/crypto/axis/artpec6_crypto.c#L1187
> > 
> > This is easy for a static analyzer to find and we can replace it with a
> > non-wrapping test (e.g. __builtin_add_overflow()), but we'll not find
> > them all immediately, especially for the signed and pointer cases.
> > 
> > So, I need to retain the "everything wraps" behavior while still being
> > able to detect when it happens.
> 
> 
> Hi Kees,
> 
> I have a couple of questions:
> 
> Currently, my thinking was that you would use signed integers
> if you want the usual integer arithmetic rules we know from
> elementary school and if you overflow this is clearly a bug 
> you can diagnose with UBsan.
> 
> There are people who think that signed overflow should be
> defined to wrap, but I think this would be a severe
> mistake because then code would start to rely on it, which
> makes it then difficult to differentiate between bugs and
> intended uses (e.g. the unfortunate situation you have 
> with the kernel).

Right -- my goal is to migrate the kernel codebase into using unambiguous
arithmetic. Doing that evolution, though, is the hard part.  :)

At present, the kernel treats all signed and pointer arithmetic as
wrapping, as that makes sure that there is no UB, which causes must more
unexpected problems than universally wrapping does.

> I assume you want to combine UBSan plus wrapping for
> production use?  Or only for testing?   Or in other words:
> why would testing UBSan and production with wrapping
> not be sufficient to find and fix all bugs? 

I want UBSan to catch wrapping in production -- there is going to be
a long tail of code that may be vulnerable to having it be manipulated
into wrapping a calculation.

The stuff that _intentionally_ wraps will stand out very quickly and we
can fix those rapidly. (Many we can find today with static analyzers,
but not all from what I've seen.)

> Wrapping would not be correct because it may lead to
> logic errors or use-after-free etc.  I assume it is still
> preferred because it more deterministic than whatever comes
> out of the optimizer assuming that overflow has UB. Is this
> the reasoning applied here?

Exactly correct.

> For unsigned the intended use case is modulo arithmetic
> where wrapping is the correct behavior. At least, this
> is what I thought so far.. This seems also to be the 
> position of the overall GCC community rejecting 
> -fsanitize=unsigned-integer-overflow.

That is my understanding as well: unsigned wrap is "intended behavior".
But for the Linux kernel this behavior is still "unexpected" in the
majority of places where it may be reachable.

> But then there are also people like Richard Seacord that
> have the position that one should use  "unsigned" for 
> every quantity which can not be negative, which implies
> that then the modulo use case becomes the exception.

Sure -- this is the preferred coding style, but we both have tons of
old code that uses "int", and we still almost always would interpret
wrapping as an unwanted state.

> Related to this I have the following question: In the 
> bug you refer to above, an unsigned variable was used
> for something that is not meant for modulo arithmetic.
> Is this done in the kernel to save space?  I.e. because
> 127 would not be enough as maximum but going to i16 takes
> to much space? or is this for compatibility with some
> on-the-wire protocol? 

I don't know the original rationale -- I expect the author wanted an
unsigned variable but didn't expect to ever encounter > 255 entries.
These kinds of expectations aren't uncommon in the kernel, and sometimes
past expectations get violated when refactoring, etc. Basically I can't
trust the sanity of the codebase, so I have to depend on the compiler to
provide the runtime coverage to avoid these kinds of security flaws.

> Would an attribute for variables help that tells the
> compiler that if stores to the variable wrap around
> then this is not intended and this is an error? 
> 
> u8 x [[nowrap]];
> 
> x = 256; // can be diagnosed ?

I would need the reverse: I want to assume all math to not wrap,
excepting a handful that are designed to.

-fsanitize=nowrap

u8 modulo [[wrapping]];
u8 counter;

modulo = 256; // no diagnose
counter = 256; // diagnose

-- 
Kees Cook