Re: [PATCH v4 0/3] Hardening perf subsystem

Peter Zijlstra
2024-06-14T10:17:20Z

On Wed, Jun 12, 2024 at 04:23:31PM -0700, Kees Cook wrote:
> On Thu, Jun 13, 2024 at 12:08:21AM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 12, 2024 at 12:01:19PM -0700, Kees Cook wrote:
> > > I'm happy to take patches. And for this bikeshed, this would be better
> > > named under the size_*() helpers which are trying to keep size_t
> > > calculations from overflowing (by saturating). i.e.:
> > >
> > > size_add_mult(sizeof(*p), sizeof(*p->member), num)
> >
> > Fine I suppose, but what if we want something not size_t? Are we waiting
> > for the type system extension?
>
> Because of C's implicit promotion/truncation, we can't do anything
> sanely with return values of arbitrary type size; we have to capture the
> lvalue type somehow so the checking can happen without C doing silent
> garbage.

So sizeof() returns the native (built-in) size_t, right? If that type
the nooverflow qualifier on, then:

sizeof(*p) + num*sizeof(p->foo[0])

should all get the nooverflow semantics right? Because size_t is
effectively 'nooverflow unsigned long' the multiplication should promote
'num' to some 'long'.

Now, I've re-read the rules and I don't see qualifiers mentioned, so
can't we state that the overflow/nooverflow qualifiers are to be
preserved on (implicit) promotion and when nooverflow and overflow are
combined the 'safe' nooverflow takes precedence?

I mean, when we're adding qualifiers we can make up rules about them
too, right?

If 'people' don't want to adorn the built-in size_t, we can always do
something like:

#define sizeof(x) ((nooverflow unsigned long)(sizeof(x)))

and 'fix' it ourselves.

> > But none of that is showing me generated asm for the various cases. As
> > such, I don't consider myself informed enough.
>
> Gotcha. For the compile-time stuff it's all just looking at
> known-at-compile-time sizes. So for something like this, we get a
> __compiletime_warning() emitted:
>
> const char src[] = "Hello there";
> char dst[10];
>
> strscpy(dst, src); /* Compiler yells since src is bigger than dst. */
>
> For run-time checks it's basically just using the regular WARN()
> infrastructure with __builtin_dynamic_object_size(). Here's a simplified
> userspace example with assert():
>
> https://godbolt.org/z/zMrKnMxn5
>
> The kernel's FORTIFY_SOURCE is much more complex in how it does the
> checking, how it does the reporting (for helping people figure out what's
> gone weird), etc.

Thanks, I'll go have a look at that.