Re: [PATCH] keys: prevent slab cache merging for key_jar

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Jun 04, 2026 at 01:50:34PM +0100, Mohammed EL Kadiri wrote:
> The key_jar slab cache holds struct key objects containing cryptographic
> keys, authentication tokens, and keyring linkage. This cache currently
> lacks merge prevention, allowing the SLUB allocator to merge it with
> other similarly-sized caches.
> 
> On a default Ubuntu 6.17.0-23-generic system, key_jar has 5 aliases,
> meaning 5 unrelated object types share its slab pages. struct key is
> 224 bytes, placed in 256-byte slabs alongside biovec-16, maple_node,
> ip6_dst_cache, task_delay_info, and kmalloc-256 users.
> 
> Cross-cache heap exploitation is a well-documented attack class
> (CVE-2022-29582, CVE-2022-2588, CVE-2021-22555) where slab cache
> merging enables type confusion between unrelated kernel objects. A
> use-after-free in any subsystem sharing slab pages with key_jar could
> allow an attacker to reclaim a freed slot as a struct key, or corrupt
> an existing key through a dangling pointer to a different type.
> 
> Add SLAB_NO_MERGE to ensure key_jar receives dedicated slab pages,
> eliminating cross-cache attacks targeting struct key. The memory
> overhead is minimal: with 32 objects per slab page and typical key
> usage bounded by system keyring size, the cost of dedicated pages is
> negligible. There is zero performance impact on the allocation hot
> path.
> 
> This follows the precedent set by skbuff_head_cache (net/core/skbuff.c)
> which uses SLAB_NO_MERGE for similar isolation requirements.
> 

~/work/kernel.org/jarkko/linux-tpmdd master*
❯ git log --oneline -1 d0bf7d5759c1d89fb013aa41cca5832e00b9632a
d0bf7d5759c1 mm/slab: introduce kmem_cache flag SLAB_NO_MERGE

~/work/kernel.org/jarkko/linux-tpmdd master*
❯ git describe --contains d0bf7d5759c1d89fb013aa41cca5832e00b9632a
v6.5-rc1~137^2^3~1

So we could probably forward to stable's starting from v6.6+ if that
is necessary / makes sense?

It's not a bug fix but kind of still I think would be a change that
stable kernels are better off with it than without it.

What do you think?

> Signed-off-by: Mohammed EL Kadiri <med08elkadiri@gmail.com>
> ---
>  security/keys/key.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/security/keys/key.c b/security/keys/key.c
> index 3bbdde778631..592b65cf8539 100644
> --- a/security/keys/key.c
> +++ b/security/keys/key.c
> @@ -1275,7 +1275,7 @@ void __init key_init(void)
>  {
>  	/* allocate a slab in which we can store keys */
>  	key_jar = kmem_cache_create("key_jar", sizeof(struct key),
> -			0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
> +			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_NO_MERGE, NULL);
>  
>  	/* add the special key types */
>  	list_add_tail(&key_type_keyring.link, &key_types_list);
> -- 
> 2.43.0
> 

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko