Re: Question on -fwrapv and -fwrapv-pointer

[Martin Uecker <ma.uecker@gmail.com>]

Am Mittwoch, dem 20.09.2023 um 13:40 -0700 schrieb Kees Cook via Gcc:
> 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.

I meant something else: Why wasn't an unsigned type
used in the first place?   If all "counter" variables were
signed and all "modulo" variables unsigned, one could already 
diagnose overflow reliably. 

I was trying to understand if there are generally
valid reasons for using unsigned integers for "counters".
or whether this is just a historical mistake in the
kernel.

(and thank you for your explanations)

Martin

> 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
>