Re: [PATCH 01/12] mm, slab: ignore hardened usercopy parameters when disabled

[Kees Cook]

On November 21, 2022 9:11:51 AM PST, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>With CONFIG_HARDENED_USERCOPY not enabled, there are no
>__check_heap_object() checks happening that would use the kmem_cache
>useroffset and usersize fields. Yet the fields are still initialized,
>preventing merging of otherwise compatible caches. Thus ignore the
>values passed to cache creation and leave them zero when
>CONFIG_HARDENED_USERCOPY is disabled.
>
>In a quick virtme boot test, this has reduced the number of caches in
>/proc/slabinfo from 131 to 111.
>
>Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>---
> mm/slab_common.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
>diff --git a/mm/slab_common.c b/mm/slab_common.c
>index 0042fb2730d1..a8cb5de255fc 100644
>--- a/mm/slab_common.c
>+++ b/mm/slab_common.c
>@@ -317,7 +317,8 @@ kmem_cache_create_usercopy(const char *name,
> 	flags &= CACHE_CREATE_MASK;
> 
> 	/* Fail closed on bad usersize of useroffset values. */
>-	if (WARN_ON(!usersize && useroffset) ||
>+	if (!IS_ENABLED(CONFIG_HARDENED_USERCOPY) ||
>+	    WARN_ON(!usersize && useroffset) ||
> 	    WARN_ON(size < usersize || size - usersize < useroffset))
> 		usersize = useroffset = 0;
> 
>@@ -640,6 +641,9 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
> 		align = max(align, size);
> 	s->align = calculate_alignment(flags, align, size);
> 
>+	if (!IS_ENABLED(CONFIG_HARDENED_USERCOPY))
>+		useroffset = usersize = 0;
>+
> 	s->useroffset = useroffset;
> 	s->usersize = usersize;
> 

"Always non-mergeable" is intentional here, but I do see the argument for not doing it under hardened-usercopy.

That said, if you keep this part, maybe go the full step and ifdef away useroffset/usersize's struct member definition and other logic, especially for SLUB_TINY benefits, so 2 ulongs are dropped from the cache struct?

-Kees


-- 
Kees Cook