From a6f0643e4f63cfaa0d5d4a69de4f132eac4b8fe4 Mon Sep 17 00:00:00 2001 From: Matt Bobrowski Date: Sun, 28 Jun 2026 20:11:03 +0000 Subject: 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 Signed-off-by: Matt Bobrowski Signed-off-by: Daniel Borkmann Reviewed-by: Emil Tsalapatis Reviewed-by: Amery Hung Link: https://lore.kernel.org/bpf/20260628201103.3624525-1-mattbobrowski@google.com --- include/linux/bpf_lsm.h | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'include/linux') 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 @@ -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; -- cgit v1.2.3