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

[Vlastimil Babka]

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;
 
-- 
2.38.1