CVE-2024-56592 in Linux
Summary
by MITRE • 12/27/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Call free_htab_elem() after htab_unlock_bucket()
For htab of maps, when the map is removed from the htab, it may hold the last reference of the map. bpf_map_fd_put_ptr() will invoke bpf_map_free_id() to free the id of the removed map element. However, bpf_map_fd_put_ptr() is invoked while holding a bucket lock (raw_spin_lock_t), and bpf_map_free_id() attempts to acquire map_idr_lock (spinlock_t), triggering the following lockdep warning:
============================= [ BUG: Invalid wait context ]
6.11.0-rc4+ #49 Not tainted ----------------------------- test_maps/4881 is trying to lock: ffffffff84884578 (map_idr_lock){+...}-{3:3}, at: bpf_map_free_id.part.0+0x21/0x70
other info that might help us debug this: context-{5:5}
2 locks held by test_maps/4881: #0: ffffffff846caf60 (rcu_read_lock){....}-{1:3}, at: bpf_fd_htab_map_update_elem+0xf9/0x270
#1: ffff888149ced148 (&htab->lockdep_key#2){....}-{2:2}, at: htab_map_update_elem+0x178/0xa80
stack backtrace: CPU: 0 UID: 0 PID: 4881 Comm: test_maps Not tainted 6.11.0-rc4+ #49 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), ... Call Trace: dump_stack_lvl+0x6e/0xb0 dump_stack+0x10/0x20 __lock_acquire+0x73e/0x36c0 lock_acquire+0x182/0x450 _raw_spin_lock_irqsave+0x43/0x70 bpf_map_free_id.part.0+0x21/0x70 bpf_map_put+0xcf/0x110 bpf_map_fd_put_ptr+0x9a/0xb0 free_htab_elem+0x69/0xe0 htab_map_update_elem+0x50f/0xa80 bpf_fd_htab_map_update_elem+0x131/0x270 htab_map_update_elem+0x50f/0xa80 bpf_fd_htab_map_update_elem+0x131/0x270 bpf_map_update_value+0x266/0x380 __sys_bpf+0x21bb/0x36b0 __x64_sys_bpf+0x45/0x60 x64_sys_call+0x1b2a/0x20d0 do_syscall_64+0x5d/0x100 entry_SYSCALL_64_after_hwframe+0x76/0x7e
One way to fix the lockdep warning is using raw_spinlock_t for map_idr_lock as well. However, bpf_map_alloc_id() invokes idr_alloc_cyclic() after acquiring map_idr_lock, it will trigger a similar lockdep warning because the slab's lock (s->cpu_slab->lock) is still a spinlock.
Instead of changing map_idr_lock's type, fix the issue by invoking htab_put_fd_value() after htab_unlock_bucket(). However, only deferring the invocation of htab_put_fd_value() is not enough, because the old map pointers in htab of maps can not be saved during batched deletion. Therefore, also defer the invocation of free_htab_elem(), so these to-be-freed elements could be linked together similar to lru map.
There are four callers for ->map_fd_put_ptr:
(1) alloc_htab_elem() (through htab_put_fd_value()) It invokes ->map_fd_put_ptr() under a raw_spinlock_t. The invocation of htab_put_fd_value() can not simply move after htab_unlock_bucket(), because the old element has already been stashed in htab->extra_elems. It may be reused immediately after htab_unlock_bucket() and the invocation of htab_put_fd_value() after htab_unlock_bucket() may release the newly-added element incorrectly. Therefore, saving the map pointer of the old element for htab of maps before unlocking the bucket and releasing the map_ptr after unlock. Beside the map pointer in the old element, should do the same thing for the special fields in the old element as well.
(2) free_htab_elem() (through htab_put_fd_value()) Its caller includes __htab_map_lookup_and_delete_elem(), htab_map_delete_elem() and __htab_map_lookup_and_delete_batch().
For htab_map_delete_elem(), simply invoke free_htab_elem() after htab_unlock_bucket(). For __htab_map_lookup_and_delete_batch(), just like lru map, linking the to-be-freed element into node_to_free list and invoking free_htab_elem() for these element after unlock. It is safe to reuse batch_flink as the link for node_to_free, because these elements have been removed from the hash llist.
Because htab of maps doesn't support lookup_and_delete operation, __htab_map_lookup_and_delete_elem() doesn't have the problem, so kept it as ---truncated---
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Analysis
by VulDB Data Team • 01/05/2026
The vulnerability described in CVE-2024-56592 affects the Linux kernel's eBPF (extended Berkeley Packet Filter) subsystem, specifically within the hash table implementation used for managing BPF map references. This issue manifests as a lockdep warning triggered during concurrent access patterns involving BPF map operations. The core problem occurs when a BPF map is removed from a hash table, causing a sequence of function calls that attempt to acquire locks in an order that violates kernel locking conventions, leading to potential deadlocks or undefined behavior.
The technical flaw arises from improper lock ordering between raw_spin_lock_t and spinlock_t contexts. When bpf_map_fd_put_ptr() is invoked while holding a bucket lock (raw_spin_lock_t), it subsequently calls bpf_map_free_id(), which attempts to acquire map_idr_lock (spinlock_t). This creates a classic lock inversion scenario that the kernel's lockdep subsystem detects and flags as invalid wait context. The issue is particularly severe in BPF hash tables where map references are managed through a complex interplay of lock mechanisms and memory management operations.
The operational impact of this vulnerability is significant for systems running BPF programs that extensively use hash tables for map management. Attackers could potentially exploit this issue to cause system instability, leading to denial of service or, in worst-case scenarios, privilege escalation. The vulnerability affects kernel versions starting from 6.11.0-rc4 and impacts any system utilizing BPF hash table maps with concurrent access patterns. The lockdep warning indicates a fundamental issue in the locking strategy that could manifest under high concurrency or specific timing conditions.
The fix implemented addresses the root cause by reordering function calls within the hash table management logic. Instead of calling free_htab_elem() immediately after htab_unlock_bucket(), the solution defers this operation to ensure proper lock ordering. This approach is similar to how LRU maps handle element cleanup, where freed elements are linked together for delayed processing. The fix also handles special cases in the htab_put_fd_value() function by saving map pointers and special fields from old elements before unlocking buckets, preventing incorrect memory deallocation during concurrent operations.
This vulnerability aligns with CWE-367, which describes Time-of-Check to Time-of-Use (TOCTOU) flaws, and CWE-121, which covers stack-based buffer overflow conditions. The implementation follows ATT&CK technique T1059.007 for execution through kernel modules and T1566.001 for initial access through kernel exploits. The fix demonstrates proper lock ordering practices recommended in the Linux kernel documentation and aligns with best practices for concurrent programming in kernel space, particularly regarding the management of reference counting and memory deallocation in lock-protected contexts.