Re: [PATCH v4 2/6] mm/slab: Plumb kmem_buckets into __do_kmalloc_node()

[Kees Cook]

On Mon, Jun 03, 2024 at 07:06:15PM +0200, Vlastimil Babka wrote:
> On 5/31/24 9:14 PM, Kees Cook wrote:
> > Introduce CONFIG_SLAB_BUCKETS which provides the infrastructure to
> > support separated kmalloc buckets (in the follow kmem_buckets_create()
> > patches and future codetag-based separation). Since this will provide
> > a mitigation for a very common case of exploits, enable it by default.
> 
> Are you sure? I thought there was a policy that nobody is special enough
> to have stuff enabled by default. Is it worth risking Linus shouting? :)

I think it's important to have this enabled given how common the
exploitation methodology is and how cheap this solution is. Regardless,
if you want it "default n", I can change it.

> I found this too verbose and tried a different approach, in the end rewrote
> everything to verify the idea works. So I'll just link to the result in git:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slab-buckets-v4-rewrite
> 
> It's also rebased on slab.git:slab/for-6.11/cleanups with some alloc_hooks()
> cleanups that would cause conflicts otherwkse.
> 
> But the crux of that approach is:
> 
> /*
>  * These macros allow declaring a kmem_buckets * parameter alongside size, which
>  * can be compiled out with CONFIG_SLAB_BUCKETS=n so that a large number of call
>  * sites don't have to pass NULL.
>  */
> #ifdef CONFIG_SLAB_BUCKETS
> #define DECL_BUCKET_PARAMS(_size, _b)   size_t (_size), kmem_buckets *(_b)
> #define PASS_BUCKET_PARAMS(_size, _b)   (_size), (_b)
> #define PASS_BUCKET_PARAM(_b)           (_b)
> #else
> #define DECL_BUCKET_PARAMS(_size, _b)   size_t (_size)
> #define PASS_BUCKET_PARAMS(_size, _b)   (_size)
> #define PASS_BUCKET_PARAM(_b)           NULL
> #endif
> 
> Then we have declaration e.g.
> 
> void *__kmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), gfp_t flags, int node)
>                                 __assume_kmalloc_alignment __alloc_size(1);
> 
> and the function is called like (from code not using buckets)
> return __kmalloc_node_noprof(PASS_BUCKET_PARAMS(size, NULL), flags, node);
> 
> or (from code using buckets)
> #define kmem_buckets_alloc(_b, _size, _flags)   \
>         alloc_hooks(__kmalloc_node_noprof(PASS_BUCKET_PARAMS(_size, _b), _flags, NUMA_NO_NODE))
> 
> And implementation looks like:
> 
> void *__kmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), gfp_t flags, int node)
> {
>         return __do_kmalloc_node(size, PASS_BUCKET_PARAM(b), flags, node, _RET_IP_);
> }
> 
> The size param is always the first, so the __alloc_size(1) doesn't need tweaking.
> size is also used in the macros even if it's never mangled, because it's easy
> to pass one param instead of two, but not zero params instead of one, if we want
> the ending comma not be part of the macro (which would look awkward).
> 
> Does it look ok to you? Of course names of the macros could be tweaked. Anyway feel
> free to use the branch for the followup. Hopefully this way is also compatible with
> the planned codetag based followup.

This looks really nice, thank you! This is well aligned with the codetag
followup, which also needs to have "size" be very easy to find (to the
macros can check for compile-time-constant or not).

I will go work from your branch...

Thanks!

-Kees

-- 
Kees Cook