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 <[email protected]>
Yes please. :) Reviewed-by: Kees Cook <[email protected]> -- Kees Cook
