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b=CKVxHHpk8X3NtAd+6WoAvNfNyINzICT8pCwrYI2TVmm050HhSwfks93/zAutElxza 4tPWhLQ6K1hw4KWdO1wOeyHmbfV936RKqj13Jv0Xi5q35H7AtaxWIT7+1O76wI7gy5 P2IXuwD5Lt7fFADxxf4LO/kAyazaTWluWn1EoucDP6hl3yf68+/GAn8Bcjur1jyXlQ OwU68dwWG3EqtZDNqw/MH/7N9Nu9jEHmK9cXMfib/jwb0NnQReogXh97T6mtcScsAJ L64TFd2h/+Ic6T0Hn0/CaiHYsBkPfDN8dzLMJNb5RYcrbEtQtf0tycscG4oWIfdeQ7 cWXJbEYjCouUQ== Message-ID: <7906bad7-4e64-40c0-a724-f55ac44e12ab@kernel.org> Date: Mon, 8 Jun 2026 09:39:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] docs/mm/slab: document cache isolation with SLAB_NO_MERGE Content-Language: en-US To: Mohammed EL Kadiri , Jonathan Corbet , Andrew Morton Cc: Vlastimil Babka , Matthew Wilcox , David Hildenbrand , Lorenzo Stoakes , Kees Cook , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260606155856.15548-1-med08elkadiri@gmail.com> <20260607070645.9559-1-med08elkadiri@gmail.com> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CB3DC4000A X-Stat-Signature: wwbxio58a9iwaxzuthhh15rkmzjf4xjp X-HE-Tag: 1780904393-625969 X-HE-Meta: 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 WpWJd+3C kh3lilQ/HyhKagY1Gpcvsl3PgbkHyd+3/vYOWGLJY6AIUDMM8aAowJX7oiA/80cqAdYt3FT/PxGSh2W1OH7tya1tax7uB6ulpj+ueDBymRRc1XKGovVFxIQIwVLuJsKBJUbZC7WtQN6WlaonBpNCcot9t/btjXDbHF+2Rhcyx2e7/DxUerKHkujCDY9GTZiB90INWSXhpenP33VJRUSJxqIbcMIgdxYvwHESmjwLZwwD0YvWDME5QiZLK2M/dpJgZ+w6r/VM4CgoIWWHWQqydMwzu+wqjcLbLcavpdpSToRpz8QecHyJ8Y0oTeLPh+q9MQfHQGwN36U2ERIqpiLYxCZhZIKL6sBqPDDpD Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/7/26 09:06, Mohammed EL Kadiri wrote: > Add documentation to slab.rst explaining when and how to use > SLAB_NO_MERGE to protect security-critical slab caches from > cross-cache heap exploitation. > > The document covers: > - When to use SLAB_NO_MERGE and what it communicates > - How to verify merge status on a running system > - Tradeoffs (memory cost vs performance) > - Relationship to CONFIG_RANDOM_KMALLOC_CACHES, SLAB_TYPESAFE_BY_RCU, > and the slab_nomerge boot parameter > > Assisted-by: Claude:claude-opus-4.6 > Signed-off-by: Mohammed EL Kadiri > --- > Changes in v2 (per Jonathan Corbet and Matthew Wilcox feedback): > - Add content to existing slab.rst instead of creating new file > - Fix markup: use plain function() without additional formatting > - Use slab terminology consistently, not SLUB > - Remove How merging works section (implementation internals) > - Remove cross-cache attack class section (redundant) > - Remove Bounded allocation volume criteria > - Rephrase unmergeability guidance per Matthew Wilcox suggestion > - Add Assisted-by tag per coding-assistants.rst > Documentation/mm/slab.rst | 60 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 60 insertions(+) > > diff --git a/Documentation/mm/slab.rst b/Documentation/mm/slab.rst > index 2bcc58ada302..c485bd257c44 100644 > --- a/Documentation/mm/slab.rst > +++ b/Documentation/mm/slab.rst > @@ -4,6 +4,66 @@ > Slab Allocation > =============== > > +Cache isolation with SLAB_NO_MERGE > +=================================== > + > +The slab allocator merges caches with compatible size, alignment, and flags > +to reduce memory fragmentation. While this improves memory efficiency, it > +allows objects of different types to share the same slab. This enables > +cross-cache heap exploitation, where a use-after-free in one object type can > +be leveraged to corrupt an unrelated type. > + > +SLAB_NO_MERGE prevents a cache from being merged, ensuring it receives a > +dedicated slab. A freed slot in an isolated cache can only be reallocated as > +the same object type. > + > +When to use SLAB_NO_MERGE > +-------------------------- > + > +SLAB_NO_MERGE should be considered for caches holding security-critical > +objects whose corruption leads directly to privilege escalation, such as > +credentials, cryptographic keys, or capability sets. > + > +It is harmless to specify SLAB_NO_MERGE even if the cache is already > +unmergeable for other reasons (e.g., it has a constructor or a non-zero > +usersize). The flag communicates intent and ensures the cache remains > +isolated if those other properties change in the future. > + > +Verifying merge status > +----------------------- > + > +To check whether a cache is merged on a running system:: > + > + # Check how many other caches share its slab > + cat /sys/kernel/slab//aliases > + > + # aliases > 0 means other types share this cache's slab > + > +Tradeoffs > +---------- > + > +**Memory**: Isolated caches may have partially-filled slabs that cannot be > +used by other types. The overhead is typically a few extra pages. > + > +**Performance**: Zero impact on kmem_cache_alloc() and kmem_cache_free(). > +The only effect is at boot when the cache is created. > + > +Relationship to other mitigations > +---------------------------------- > + > +CONFIG_RANDOM_KMALLOC_CACHES creates multiple copies of each kmalloc size > +class and randomly assigns allocations among them. It only affects kmalloc() > +users and does not affect named caches created with kmem_cache_create(). > + > +SLAB_TYPESAFE_BY_RCU delays freeing the slab by an RCU grace period. It > +does not delay object slot reuse and does not prevent cross-cache merging. > +It solves a different problem: safe lockless access to freed-and-reallocated > +objects of the same type. > + > +The slab_nomerge boot parameter disables merging for all caches globally. > +SLAB_NO_MERGE provides the same protection selectively for individual caches > +without the global memory cost. Matthew already pointed out in v1 that these last 2 are unnecessary. I think this is approach unproductive, sorry. IIUC a newcomer to the community, with help of a LLM (I however appreciate the openness about it, with Assisted-by: tag), but multiple senior people have to provide input to hopefully make it into an acceptable shape. This SLAB_NO_MERGE flag isn't that crucial to go through all of this. We could just add a comment about security use cases to the comment above #define SLAB_NO_MERGE and that's it. So please considers other ways of getting involved in the community (which is otherwise welcome), as this is not a great one. Thanks. > + > Functions and structures > ======================== >