Re: UBSAN: object-size-mismatch in wg_xmit

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Jan 11, 2021 at 7:15 PM Jeffrey Walton <noloader@gmail.com> wrote:
> > > > ...
> > > > FTR, I've disabled the following UBSAN configs:
> > > > UBSAN_MISC
> > > > UBSAN_DIV_ZERO
> > > > UBSAN_BOOL
> > > > UBSAN_OBJECT_SIZE
> > > > UBSAN_SIGNED_OVERFLOW
> > > > UBSAN_UNSIGNED_OVERFLOW
> > > > UBSAN_ENUM
> > > > UBSAN_ALIGNMENT
> > > > UBSAN_UNREACHABLE
> > > >
> > > > Only these are enabled now:
> > > > UBSAN_BOUNDS
> > > > UBSAN_SHIFT
> > > >
> > > > This is commit:
> > > > https://github.com/google/syzkaller/commit/2c1f2513486f21d26b1942ce77ffc782677fbf4e
> > >
> > > I think the commit cut too deep.
> > >
> > > The overflows are important if folks are building with compilers other than GCC.
> > >
> > > The aligned data accesses are important on platforms like MIPS64 and Sparc64.
> > >
> > > Object size is important because it catches destination buffer overflows.
> > >
> > > I don't know what's in miscellaneous. There may be something useful in there.
> >
> > Hi Jeff,
> >
> > See the commit for reasons why each of these is disabled.
> > E.g. object size, somebody first needs to fix bugs like this one.
> > While things like skbuff have these UBs on trivial workloads, there is
> > no point in involving fuzzing and making it crash on this trivial bug
> > all the time and stopping doing any other kernel testing as the
> > result.
>
> Going off-topic a bit, what would you suggest for UBSAN_OBJECT_SIZE?
>
> It seems to me object size checking is being conflated with object
> type. It seems to me they need to be split: UBSAN_OBJECT_SIZE for
> actual object sizes, and UBSAN_OBJECT_TYPE for the casts.
>
> I still have a bitter taste in my mouth from
> https://www.cvedetails.com/bugtraq-bid/57602/libupnp-Multiple-Buffer-Overflow-Vulnerabilities.html.
> I hate to see buffer checks go away. (And I realize the kernel folks
> are more skilled than the guy who wrote libupnp).
>
> Jeff

I've filed https://bugs.llvm.org/show_bug.cgi?id=48726 for this. Does
it capture what you are asking? Let's move the discussion re ubsan
there.

However, in the first place I am suggesting fixing the code. E.g. for
sk_buff I would assume it's relatively easily fixable. A
formally legal fix I think should put sk_buff_head into sk_buff and
use it, no downsides and will eliminate the confusing "should go
first" comments.
Or as an workaround maybe we could make __skb_queue_before accept
sk_buff_head and cast the other way around.