From 2382971aaaef5bf85a651234c64906f59580b8be Mon Sep 17 00:00:00 2001 From: "Vlastimil Babka (SUSE)" Date: Wed, 10 Jun 2026 17:40:03 +0200 Subject: [PATCH] mm/slab: do not limit zeroing to orig_size when only red zoning is enabled commit 648927ceb84021a25a0fbd5673740956f318d534 upstream. When init (zeroing) on allocation is requested, for kmalloc() we generally have to zero the full object size even if a smaller size is requested, in order to provide krealloc()'s __GFP_ZERO guarantees. But if we track the requested size, krealloc() uses that information to do the right thing, so we can zero only the requested size. With red zoning also enabled, any extra size became part of the red zone, so it must not be zeroed and thus we must zero only the requested size. However the current check is imprecise, and will trigger also when only SLAB_RED_ZONE is enabled without SLAB_STORE_USER (which enables tracking the requested size). This means enabling red zoning alone can compromise krealloc()'s __GFP_ZERO contract. Fix this by using slub_debug_orig_size() instead, which is the exact check for whether the requested size is tracked. We don't need to care if red zoning is also enabled or not. Also update and expand the comment accordingly. Fixes: 9ce67395f5a0 ("mm/slub: only zero requested size of buffer for kzalloc when debug enabled") Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260610-slab_alloc_flags-v2-1-7190909db118@kernel.org Reviewed-by: Harry Yoo (Oracle) Reviewed-by: Hao Li Signed-off-by: Vlastimil Babka (SUSE) Signed-off-by: Greg Kroah-Hartman --- mm/slub.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 5fdec3b83706..71e09c675047 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -5004,15 +5004,17 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru, gfp_t init_flags = flags & gfp_allowed_mask; /* - * For kmalloc object, the allocated memory size(object_size) is likely - * larger than the requested size(orig_size). If redzone check is - * enabled for the extra space, don't zero it, as it will be redzoned - * soon. The redzone operation for this extra space could be seen as a - * replacement of current poisoning under certain debug option, and - * won't break other sanity checks. + * For kmalloc object, the allocated size (object_size) can be larger + * than the requested size (orig_size). We however need to zero the + * whole object_size to handle possible later krealloc() with + *__GFP_ZERO properly. + * + * But if we keep track of the requested size, krealloc() uses that + * information. Additionally if red zoning is enabled, the extra space + * is also red zone, so we should not overwrite it. So limit zeroing to + * orig_size if we track it. */ - if (kmem_cache_debug_flags(s, SLAB_STORE_USER | SLAB_RED_ZONE) && - (s->flags & SLAB_KMALLOC)) + if (slub_debug_orig_size(s)) zero_size = orig_size; /*