Re: [PATCH bpf-next] bpf: reject BPF_MAP_TYPE_INODE_STORAGE creation if BPF LSM is uninitialized

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sun Jun 28, 2026 at 4:11 PM EDT, Matt Bobrowski wrote:
> When CONFIG_BPF_LSM=y is set, BPF inode storage maps
> (BPF_MAP_TYPE_INODE_STORAGE) are compiled into the kernel. However,
> if the BPF LSM is not explicitly enabled at boot time (e.g. omitted
> from the "lsm=" boot parameter), lsm_prepare() is never executed for
> the BPF LSM.
>
> Consequently, the BPF inode security blob offset
> (bpf_lsm_blob_sizes.lbs_inode) is never initialized and remains at its
> default compiled size of 8 bytes instead of being updated to a valid
> offset past the reserved struct rcu_head (typically 16 bytes or more).
>
> When a privileged user creates and updates a
> BPF_MAP_TYPE_INODE_STORAGE map, bpf_inode() evaluates
> inode->i_security + 8. This erroneously aliases the struct
> rcu_head.func callback pointer at the beginning of the
> inode->i_security blob. During subsequent map element cleanup or inode
> destruction, writing NULL to owner_storage clears the queued RCU
> callback pointer. When rcu_do_batch() later executes the queued
> callback, it attempts an instruction fetch at address 0x0, triggering
> an immediate kernel panic.
>
> Fix this by introducing a global bpf_lsm_initialized boolean flag
> marked with __ro_after_init. Set this flag to true inside
> bpf_lsm_init() when the LSM framework successfully registers the BPF
> LSM. Gate map allocation in inode_storage_map_alloc() on this flag,
> returning -EOPNOTSUPP if the BPF LSM is in turn uninitialized.
>
> This fail-fast approach prevents userspace from allocating inode
> storage maps when the supporting BPF LSM infrastructure is absent,
> avoiding zombie map states.
>
> Fixes: 8ea636848aca ("bpf: Implement bpf_local_storage for inodes")
> Reported-by: oxsignal <awo@kakao.com>
> Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

It makes sense for since the only users of inode-local storage are
related to LSM (case in point, this whole bug is due to storing the
storage map in the inode's ->security in the first place).

> ---
>  include/linux/bpf_lsm.h        | 4 ++++
>  kernel/bpf/bpf_inode_storage.c | 9 +++++++++
>  security/bpf/hooks.c           | 3 +++
>  3 files changed, 16 insertions(+)
>
> diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> index 143775a27a2a..dda272d78f01 100644
> --- a/include/linux/bpf_lsm.h
> +++ b/include/linux/bpf_lsm.h
> @@ -14,6 +14,8 @@
>  
>  #ifdef CONFIG_BPF_LSM
>  
> +extern bool bpf_lsm_initialized __ro_after_init;
> +
>  #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
>  	RET bpf_lsm_##NAME(__VA_ARGS__);
>  #include <linux/lsm_hook_defs.h>
> @@ -56,6 +58,8 @@ bool bpf_lsm_hook_returns_errno(u32 btf_id);
>  
>  #else /* !CONFIG_BPF_LSM */
>  
> +#define bpf_lsm_initialized false
> +
>  static inline bool bpf_lsm_is_sleepable_hook(u32 btf_id)
>  {
>  	return false;
> diff --git a/kernel/bpf/bpf_inode_storage.c b/kernel/bpf/bpf_inode_storage.c
> index 0da8d923e39d..f9e81060c1f4 100644
> --- a/kernel/bpf/bpf_inode_storage.c
> +++ b/kernel/bpf/bpf_inode_storage.c
> @@ -178,6 +178,15 @@ static int notsupp_get_next_key(struct bpf_map *map, void *key,
>  
>  static struct bpf_map *inode_storage_map_alloc(union bpf_attr *attr)
>  {
> +	/*
> +	 * Do not allow allocation of BPF_MAP_TYPE_INODE_STORAGE if the BPF LSM
> +	 * was not initialized by the LSM framework at boot. Without proper
> +	 * initialization, the BPF inode security blob offset remains unprepared,
> +	 * causing bpf_inode() to calculate an invalid memory offset and corrupt
> +	 * inode->i_security.
> +	 */
> +	if (!bpf_lsm_initialized)
> +		return ERR_PTR(-EOPNOTSUPP);
>  	return bpf_local_storage_map_alloc(attr, &inode_cache);
>  }
>  
> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> index 40efde233f3a..7b98f5d1e2be 100644
> --- a/security/bpf/hooks.c
> +++ b/security/bpf/hooks.c
> @@ -7,6 +7,8 @@
>  #include <linux/bpf_lsm.h>
>  #include <uapi/linux/lsm.h>
>  
> +bool bpf_lsm_initialized __ro_after_init;
> +
>  static struct security_hook_list bpf_lsm_hooks[] __ro_after_init = {
>  	#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
>  	LSM_HOOK_INIT(NAME, bpf_lsm_##NAME),
> @@ -24,6 +26,7 @@ static int __init bpf_lsm_init(void)
>  {
>  	security_add_hooks(bpf_lsm_hooks, ARRAY_SIZE(bpf_lsm_hooks),
>  			   &bpf_lsmid);
> +	bpf_lsm_initialized = true;
>  	pr_info("LSM support for eBPF active\n");
>  	return 0;
>  }