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

[Kees Cook]

On Mon, Jun 10, 2024 at 10:05:44PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 10, 2024 at 10:28:52AM -0700, Kees Cook wrote:
> > On Sat, Jun 01, 2024 at 06:56:15PM +0200, Erick Archer wrote:
> > > Hi everyone,
> > > 
> > > This is an effort to get rid of all multiplications from allocation
> > > functions in order to prevent integer overflows [1][2].
> > 
> > I didn't actually see these 3 patches in this thread nor via lore.
> 
> He managed to break threading between 0/n and the rest.
> 
> > > In the first patch, the "struct amd_uncore_ctx" can be refactored to
> > > use a flex array for the "events" member. This way, the allocation/
> > > freeing of the memory can be simplified. Then, the struct_size()
> > > helper can be used to do the arithmetic calculation for the memory
> > > to be allocated.
> > 
> > I like this patch because it reduces the allocation from 2 to 1. This
> > isn't what Peter might see as "churn": this is an improvement in resource
> > utilization.
> 
> But then he went and used that struct_size() abomination :/
> 
> > I prefer this style, as it makes things unambiguous ("this will never
> > wrap around") without having to check the associated types and doesn't make
> > the resulting binary code different in the "can never overflow" case.
> > 
> > In this particular case:
> > 
> > int size = sizeof(*box) + numshared * sizeof(struct intel_uncore_extra_reg);
> > 
> > "int numshared" comes from struct intel_uncore_type::num_shared_regs,
> > which is:
> > 
> >         unsigned num_shared_regs:8;
> > 
> > And the struct sizes are:
> > 
> > $ pahole -C intel_uncore_box gcc-boot/vmlinux | grep size:
> >         /* size: 488, cachelines: 8, members: 19 */
> > $ pahole -C intel_uncore_extra_reg gcc-boot/vmlinux | grep size:
> >         /* size: 96, cachelines: 2, members: 5 */
> > 
> > So we have:
> > 
> > s32 size = 488 + u8 * 96
> > 
> > Max size here is 24968 so it can never overflow an s32, so I can see
> > why Peter views this as "churn".
> > 
> > I still think the patch is a coding style improvement, but okay.
> 
> I really detest this thing because it makes what was trivially readable
> into something opaque. Get me that type qualifier that traps on overflow
> and write plain C. All this __builtin_overflow garbage is just that,
> unreadable nonsense.

It's more readable than container_of(), IMO. "give me the struct size
for variable VAR, which has a flexible array MEMBER, when we have COUNT
many of them": struct_size(VAR, MEMBER, COUNT). It's more readable, more
robust, and provides saturation in the face of potential wrap-around.

> > This provides __counted_by coverage, and I think this is important to
> > gain in ever place we can. Given that this is part of a ring buffer
> > implementation that is arbitrarily sized, this is exactly the kind of
> > place I'd like to see __counted_by used. This is a runtime robustness
> > improvement, so I don't see this a "churn" at all.
> 
> Again, mixed in with that other crap. Anyway, remind me wth this
> __counted_by thing actually does?

It provides annotation for the compiler to perform run-time bounds
checking on dynamically sized arrays. i.e. CONFIG_FORTIFY_SOURCE and
CONFIG_UBSAN_BOUNDS can actually reason about annotated flexible arrays
instead of just saying "oh no a flexible array, I give up".

> > Peter, for patches 1 and 3, if you'd prefer not to carry them, I could
> > put them in the hardening tree to keep them out of your way. It seems
> > clear you don't want patch 2 at all.
> 
> I prefer to not have struct_size() anywhere at all. Please just write
> readable code.

That ship has sailed, and it has been keeping things at bay for a while
now. As we make progress on making the compiler able to do this more
naturally, we can work on replacing struct_size(), but it's in use
globally and it's useful both for catching runtime mistakes and for
catching compile-time mistakes (the flexible array has to match the
variable's struct).

-Kees

-- 
Kees Cook