linux-kernel - [PATCH v2 5/6] slab: validate slab before using it in alloc_single_from

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Message-Id: <20250915-slub-slab-validation-v2-5-314690fc1532@suse.cz>
Date: Mon, 15 Sep 2025 15:55:12 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc: Harry Yoo <harry.yoo@...cle.com>, Christoph Lameter <cl@...two.org>, 
 David Rientjes <rientjes@...gle.com>, 
 Roman Gushchin <roman.gushchin@...ux.dev>, 
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
 linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH v2 5/6] slab: validate slab before using it in
 alloc_single_from_partial()

We touch slab->freelist and slab->inuse before checking the slab pointer
is actually sane. Do that validation first, which will be safer. We can
thus also remove the check from alloc_debug_processing().

This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
alloc_single_from_partial() is only called for caches with debugging
enabled so it's acceptable.

In alloc_single_from_new_slab() we just created the struct slab and call
alloc_debug_processing() to mainly set up redzones, tracking etc, while
not really expecting the consistency checks to fail. Thus don't validate
it there.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/slub.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 12ad42f3d2e066b02340f2c30a85422583af3c5d..e5b53d1debddd3fe0f941f579a1043a5b976e50b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -821,6 +821,8 @@ static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
 	return *(unsigned int *)p;
 }
 
+#ifdef CONFIG_SLUB_DEBUG
+
 /*
  * For debugging context when we want to check if the struct slab pointer
  * appears to be valid.
@@ -830,7 +832,6 @@ static inline bool validate_slab_ptr(struct slab *slab)
 	return PageSlab(slab_page(slab));
 }
 
-#ifdef CONFIG_SLUB_DEBUG
 static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
 static DEFINE_SPINLOCK(object_map_lock);
 
@@ -1651,11 +1652,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
 			struct slab *slab, void *object, int orig_size)
 {
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
-		if (!validate_slab_ptr(slab)) {
-			slab_err(s, slab, "Not a valid slab page");
-			return false;
-		}
-
 		if (!alloc_consistency_checks(s, slab, object))
 			goto bad;
 	}
@@ -2825,13 +2821,21 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
 
 	lockdep_assert_held(&n->list_lock);
 
+#ifdef SLUB_DEBUG
+	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+		if (!validate_slab_ptr(slab)) {
+			slab_err(s, slab, "Not a valid slab page");
+			return NULL;
+		}
+	}
+#endif
+
 	object = slab->freelist;
 	slab->freelist = get_freepointer(s, object);
 	slab->inuse++;
 
 	if (!alloc_debug_processing(s, slab, object, orig_size)) {
-		if (validate_slab_ptr(slab))
-			remove_partial(n, slab);
+		remove_partial(n, slab);
 		return NULL;
 	}
 

-- 
2.51.0