[PATCH 0/6] slab: struct slab pointer validation improvements

[PATCH 0/6] slab: struct slab pointer validation improvements

[Vlastimil Babka]

This tries to combine the goals of the first 4 patches from Matthew's
series [1] with points raised during review by Christoph and myself.
Patch 4 from [1] is taken as patch 1 here. In other cases the struct
slab pointer validation is better to move to a place where it can be
performed before touching any of the struct slab fields, rather than
removing it completely.

Further we wrap the validation in a function validate_slab_ptr() and
make the impementation use struct page instead of struct folio to be
compatible with the rest of the series [1]. With further changes towards
memdesc the implementation can change accordingly, if it will still make
sense.

The summary is that we validate pointers from the node partial list when
allocating from it or when validation is triggered from sysfs.  When
freeing, we always obtain the slab pointer in a way that the page type
is tested in the process, so we don't need to validate.

Based on:
https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-6.18/fixes

[1] https://lore.kernel.org/all/20250910115507.1991829-1-willy@infradead.org/

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
Matthew Wilcox (Oracle) (1):
      slab: Remove dead code in free_consistency_checks()

Vlastimil Babka (5):
      slab: wrap debug slab validation in validate_slab_ptr()
      slab: move validate_slab_ptr() from check_slab() to its callers
      slab: move validate_slab_ptr() from alloc_consistency_checks() to its caller
      slab: validate slab before using it in alloc_single_from_partial()
      slab: don't validate slab pointer in free_debug_processing()

 mm/slub.c | 61 +++++++++++++++++++++++++++++++++++++------------------------
 1 file changed, 37 insertions(+), 24 deletions(-)
---
base-commit: 41534d499e50e23571d6b9960498777d93f817ce
change-id: 20250911-slub-slab-validation-0e4f559b0a1d

Best regards,
-- 
Vlastimil Babka <vbabka@suse.cz>

[PATCH 1/6] slab: Remove dead code in free_consistency_checks()

[Vlastimil Babka]

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

We already know that slab is a valid slab as that's checked by the
caller.  In the future, we won't be able to get to a slab pointer
from a non-slab page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 3062f56bf49882538ba5af407de9f69c451f2e29..56143bfd1ae319d384981c810a5ed84af00f4afa 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1684,10 +1684,7 @@ static inline int free_consistency_checks(struct kmem_cache *s,
 		return 0;
 
 	if (unlikely(s != slab->slab_cache)) {
-		if (!folio_test_slab(slab_folio(slab))) {
-			slab_err(s, slab, "Attempt to free object(0x%p) outside of slab",
-				 object);
-		} else if (!slab->slab_cache) {
+		if (!slab->slab_cache) {
 			slab_err(NULL, slab, "No slab cache for object 0x%p",
 				 object);
 		} else {

-- 
2.51.0

[PATCH 2/6] slab: wrap debug slab validation in validate_slab_ptr()

[Vlastimil Babka]

This will make it clear where we currently cast struct slab to folio
only to check the slab type, and allow to change the implementation
later with memdesc conversion.

For now use a struct page based implementation instead of struct folio
to be compatible with further upcoming changes.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 56143bfd1ae319d384981c810a5ed84af00f4afa..5bbfe4ee8d9846ec9a34584c10750388849da3b9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -821,6 +821,15 @@ static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
 	return *(unsigned int *)p;
 }
 
+/*
+ * For debugging context when we want to check if the struct slab pointer
+ * appears to be valid.
+ */
+static 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);
@@ -1453,7 +1462,7 @@ static int check_slab(struct kmem_cache *s, struct slab *slab)
 {
 	int maxobj;
 
-	if (!folio_test_slab(slab_folio(slab))) {
+	if (!validate_slab_ptr(slab)) {
 		slab_err(s, slab, "Not a valid slab page");
 		return 0;
 	}
@@ -1653,7 +1662,7 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
 	return true;
 
 bad:
-	if (folio_test_slab(slab_folio(slab))) {
+	if (validate_slab_ptr(slab)) {
 		/*
 		 * If this is a slab page then lets do the best we can
 		 * to avoid issues in the future. Marking all objects
@@ -2818,7 +2827,7 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
 	slab->inuse++;
 
 	if (!alloc_debug_processing(s, slab, object, orig_size)) {
-		if (folio_test_slab(slab_folio(slab)))
+		if (validate_slab_ptr(slab))
 			remove_partial(n, slab);
 		return NULL;
 	}

-- 
2.51.0

Re: [PATCH 2/6] slab: wrap debug slab validation in validate_slab_ptr()

[Harry Yoo]

On Thu, Sep 11, 2025 at 07:02:35PM +0200, Vlastimil Babka wrote:
> This will make it clear where we currently cast struct slab to folio
> only to check the slab type, and allow to change the implementation
> later with memdesc conversion.
> 
> For now use a struct page based implementation instead of struct folio
> to be compatible with further upcoming changes.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

>  mm/slub.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 

-- 
Cheers,
Harry / Hyeonggon

[PATCH 3/6] slab: move validate_slab_ptr() from check_slab() to its callers

[Vlastimil Babka]

We will want to do the validation earlier in some callers or remove it
completely, so extract it from check_slab() first. No functional change.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 5bbfe4ee8d9846ec9a34584c10750388849da3b9..94a089205a86f0667444484e158d307e72cd96e1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1458,15 +1458,15 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
 	return ret;
 }
 
+/*
+ * Checks if the slab state looks sane. Assumes the struct slab pointer
+ * was either obtained in a way that ensures it's valid, or validated
+ * by validate_slab_ptr()
+ */
 static int check_slab(struct kmem_cache *s, struct slab *slab)
 {
 	int maxobj;
 
-	if (!validate_slab_ptr(slab)) {
-		slab_err(s, slab, "Not a valid slab page");
-		return 0;
-	}
-
 	maxobj = order_objects(slab_order(slab), s->size);
 	if (slab->objects > maxobj) {
 		slab_err(s, slab, "objects %u > max %u",
@@ -1633,6 +1633,11 @@ void setup_slab_debug(struct kmem_cache *s, struct slab *slab, void *addr)
 static inline int alloc_consistency_checks(struct kmem_cache *s,
 					struct slab *slab, void *object)
 {
+	if (!validate_slab_ptr(slab)) {
+		slab_err(s, slab, "Not a valid slab page");
+		return 0;
+	}
+
 	if (!check_slab(s, slab))
 		return 0;
 
@@ -3485,6 +3490,11 @@ static inline bool free_debug_processing(struct kmem_cache *s,
 	int cnt = 0;
 
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+		if (!validate_slab_ptr(slab)) {
+			slab_err(s, slab, "Not a valid slab page");
+			goto out;
+		}
+
 		if (!check_slab(s, slab))
 			goto out;
 	}
@@ -6519,6 +6529,11 @@ static void validate_slab(struct kmem_cache *s, struct slab *slab,
 	void *p;
 	void *addr = slab_address(slab);
 
+	if (!validate_slab_ptr(slab)) {
+		slab_err(s, slab, "Not a valid slab page");
+		return;
+	}
+
 	if (!check_slab(s, slab) || !on_freelist(s, slab, NULL))
 		return;
 

-- 
2.51.0

Re: [PATCH 3/6] slab: move validate_slab_ptr() from check_slab() to its callers

[Harry Yoo]

On Thu, Sep 11, 2025 at 07:02:36PM +0200, Vlastimil Babka wrote:
> We will want to do the validation earlier in some callers or remove it
> completely, so extract it from check_slab() first. No functional change.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

>  mm/slub.c | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 

-- 
Cheers,
Harry / Hyeonggon

[PATCH 4/6] slab: move validate_slab_ptr() from alloc_consistency_checks() to its caller

[Vlastimil Babka]

In alloc_debug_processing() we can call validate_slab_ptr() upfront and
then don't need to recheck when alloc_consistency_checks() fails for
other reasons.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 94a089205a86f0667444484e158d307e72cd96e1..909c71372a2f542b6e0d67c12ea683133b246b66 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1633,11 +1633,6 @@ void setup_slab_debug(struct kmem_cache *s, struct slab *slab, void *addr)
 static inline int alloc_consistency_checks(struct kmem_cache *s,
 					struct slab *slab, void *object)
 {
-	if (!validate_slab_ptr(slab)) {
-		slab_err(s, slab, "Not a valid slab page");
-		return 0;
-	}
-
 	if (!check_slab(s, slab))
 		return 0;
 
@@ -1656,6 +1651,11 @@ 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;
 	}
@@ -1667,17 +1667,15 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
 	return true;
 
 bad:
-	if (validate_slab_ptr(slab)) {
-		/*
-		 * If this is a slab page then lets do the best we can
-		 * to avoid issues in the future. Marking all objects
-		 * as used avoids touching the remaining objects.
-		 */
-		slab_fix(s, "Marking all objects used");
-		slab->inuse = slab->objects;
-		slab->freelist = NULL;
-		slab->frozen = 1; /* mark consistency-failed slab as frozen */
-	}
+	/*
+	 * Let's do the best we can to avoid issues in the future. Marking all
+	 * objects as used avoids touching the remaining objects.
+	 */
+	slab_fix(s, "Marking all objects used");
+	slab->inuse = slab->objects;
+	slab->freelist = NULL;
+	slab->frozen = 1; /* mark consistency-failed slab as frozen */
+
 	return false;
 }
 

-- 
2.51.0

Re: [PATCH 4/6] slab: move validate_slab_ptr() from alloc_consistency_checks() to its caller

[Harry Yoo]

On Thu, Sep 11, 2025 at 07:02:37PM +0200, Vlastimil Babka wrote:
> In alloc_debug_processing() we can call validate_slab_ptr() upfront and
> then don't need to recheck when alloc_consistency_checks() fails for
> other reasons.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

>  mm/slub.c | 30 ++++++++++++++----------------
>  1 file changed, 14 insertions(+), 16 deletions(-)

-- 
Cheers,
Harry / Hyeonggon

[PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

[Vlastimil Babka]

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@suse.cz>
---
 mm/slub.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1651,11 +1651,6 @@ 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;
 	}
@@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
 
 	lockdep_assert_held(&n->list_lock);
 
+	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+		if (!validate_slab_ptr(slab)) {
+			slab_err(s, slab, "Not a valid slab page");
+			return NULL;
+		}
+	}
+
 	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);
+	if (!alloc_debug_processing(s, slab, object, orig_size))
 		return NULL;
-	}
 
 	if (slab->inuse == slab->objects) {
 		remove_partial(n, slab);

-- 
2.51.0

Re: [PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

[Harry Yoo]

On Thu, Sep 11, 2025 at 07:02:38PM +0200, Vlastimil Babka wrote:
> 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@suse.cz>
> ---
>  mm/slub.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1651,11 +1651,6 @@ 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;
>  	}
> @@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>  
>  	lockdep_assert_held(&n->list_lock);
>  
> +	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> +		if (!validate_slab_ptr(slab)) {
> +			slab_err(s, slab, "Not a valid slab page");
> +			return NULL;
> +		}
> +	}
> +
>  	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);
> +	if (!alloc_debug_processing(s, slab, object, orig_size))
>  		return NULL;

Is it intentional to not remove the slab from the partial list
when alloc_debug_processing() returns false?

> -	}
>  
>  	if (slab->inuse == slab->objects) {
>  		remove_partial(n, slab);
> 
> -- 
> 2.51.0

-- 
Cheers,
Harry / Hyeonggon

Re: [PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

[Vlastimil Babka]

On 9/12/25 12:48, Harry Yoo wrote:
> On Thu, Sep 11, 2025 at 07:02:38PM +0200, Vlastimil Babka wrote:
>> 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@suse.cz>
>> ---
>>  mm/slub.c | 17 ++++++++---------
>>  1 file changed, 8 insertions(+), 9 deletions(-)
>> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1651,11 +1651,6 @@ 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;
>>  	}
>> @@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>>  
>>  	lockdep_assert_held(&n->list_lock);
>>  
>> +	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>> +		if (!validate_slab_ptr(slab)) {
>> +			slab_err(s, slab, "Not a valid slab page");
>> +			return NULL;
>> +		}
>> +	}
>> +
>>  	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);
>> +	if (!alloc_debug_processing(s, slab, object, orig_size))
>>  		return NULL;
> 
> Is it intentional to not remove the slab from the partial list
> when alloc_debug_processing() returns false?

No, good catch, will fix. Thanks!

>> -	}
>>  
>>  	if (slab->inuse == slab->objects) {
>>  		remove_partial(n, slab);
>> 
>> -- 
>> 2.51.0
> 

[PATCH 6/6] slab: don't validate slab pointer in free_debug_processing()

[Vlastimil Babka]

The struct slab pointer has been obtained one from the object being
freed on all the paths that lead to this function. In all cases this
already includes the test for slab type of the struct page which struct
slab is overlaying. Thus we would not reach this function if it was
not a valid slab pointer in the first place.

One less obvious case is that kmem_cache_free() trusts virt_to_slab()
blindly so it may be NULL if the slab type check is false. But with
SLAB_CONSISTENCY_CHECKS, cache_from_obj() called also from
kmem_cache_free() catches this and returns NULL, which terminates
freeing immediately.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 93df6e82af37c798c3fa5574c9d825f0f4a83013..106dbce64acdf32c1d271ec130c35c0ec0e15630 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3487,11 +3487,6 @@ static inline bool free_debug_processing(struct kmem_cache *s,
 	int cnt = 0;
 
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
-		if (!validate_slab_ptr(slab)) {
-			slab_err(s, slab, "Not a valid slab page");
-			goto out;
-		}
-
 		if (!check_slab(s, slab))
 			goto out;
 	}

-- 
2.51.0

Re: [PATCH 6/6] slab: don't validate slab pointer in free_debug_processing()

[Harry Yoo]

On Thu, Sep 11, 2025 at 07:02:39PM +0200, Vlastimil Babka wrote:
> The struct slab pointer has been obtained one from the object being
> freed on all the paths that lead to this function. In all cases this
> already includes the test for slab type of the struct page which struct
> slab is overlaying. Thus we would not reach this function if it was
> not a valid slab pointer in the first place.
> 
> One less obvious case is that kmem_cache_free() trusts virt_to_slab()
> blindly so it may be NULL if the slab type check is false. But with
> SLAB_CONSISTENCY_CHECKS, cache_from_obj() called also from
> kmem_cache_free() catches this and returns NULL, which terminates
> freeing immediately.

Oh, I thought it'll crash even with debug caches
but it won't and I misread the code.

> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon