bpf: Reject BPF_MAP_TYPE_INODE_STORAGE creation if BPF LSM is uninitialized

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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/bpf/20260628201103.3624525-1-mattbobrowski@google.com

1 files changed, 4 insertions, 0 deletions

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;