[PATCH] cred: prevent slab cache merging for cred_jar

[Kees Cook]

On Sat, Jun 06, 2026 at 03:25:58PM +0100, Mohammed EL Kadiri wrote:
> The cred_jar slab cache holds struct cred objects, which contain
> process credentials: uid, gid, euid, egid, and capability sets.
> Overwriting any of these fields is sufficient for privilege escalation.
> 
> On a default Ubuntu 6.17.0-23-generic system, cred_jar (named "cred"
> in sysfs) has 2 aliases, meaning 2 unrelated object types share its
> slab pages (object_size=184, objs_per_slab=42).
> 
> Cross-cache heap exploitation relies on slab cache merging to achieve
> type confusion between unrelated kernel objects. CVE-2022-29582
> demonstrates this technique: an io_uring use-after-free is leveraged
> across cache boundaries through page-level reallocation, ultimately
> achieving root. struct cred is a primary target in this class of
> attacks due to the direct privilege escalation that results from
> corrupting any of its identity or capability fields.
> 
> Add SLAB_NO_MERGE to ensure cred_jar receives dedicated slab pages,
> so that freed credential slots can only be reallocated as struct cred
> objects. The memory overhead is minimal: one struct cred exists per
> task, and with 42 objects per slab page, 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)
> and key_jar (security/keys/key.c) which use SLAB_NO_MERGE for similar
> isolation requirements.
> 
> Signed-off-by: Mohammed EL Kadiri <med08elkadiri at gmail.com>

Yes please. :)

Reviewed-by: Kees Cook <kees at kernel.org>

-- 
Kees Cook