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
