bpf: Allow LPM map access from sleepable BPF programs

Overview

In the Linux kernel, the following vulnerability has been resolved:

bpf: Allow LPM map access from sleepable BPF programs

trie_lookup_elem() annotates its rcu_dereference_check() walks with only rcu_read_lock_bh_held(). Because rcu_dereference_check(p, c) resolves to "c || rcu_read_lock_held()", this passes for XDP/NAPI and classic RCU readers but fails for sleepable BPF programs, which enter via __bpf_prog_enter_sleepable() and hold only rcu_read_lock_trace().

trie_update_elem() and trie_delete_elem() have the same problem in a different form: they walk the trie with plain rcu_dereference(), which asserts rcu_read_lock_held() unconditionally. Both are reachable from sleepable BPF programs via the bpf_map_update_elem / bpf_map_delete_elem helpers, and from the syscall path under classic rcu_read_lock(). In the writer paths the trie is actually protected by trie->lock (an rqspinlock taken across the walk); we never relied on the RCU read-side lock to keep nodes alive there.

A sleepable LSM hook that ends up touching an LPM trie therefore triggers lockdep on debug kernels:

============================= WARNING: suspicious RCU usage 7.1.0-... Tainted: G E

kernel/bpf/lpm_trie.c:249 suspicious rcu_dereference_check() usage! 1 lock held by net_tests/540: #0: (rcu_tasks_trace_srcu_struct){....}-{0:0}, at: _bpf_prog_enter_sleepable+0x26/0x280 Call Trace: dump_stack_lvl lockdep_rcu_suspicious trie_lookup_elem bpf_prog...enforce_security_socket_connect bpf_trampoline... security_socket_connect __sys_connect do_syscall_64

This is lockdep-only -- no UAF, since Tasks Trace RCU does serialize against the trie's reclaim path -- but it spams the console once per distinct callsite on every debug kernel running a sleepable BPF LSM that touches an LPM trie, which is increasingly common.

For the lookup path, switch the rcu_dereference_check() annotation from rcu_read_lock_bh_held() to bpf_rcu_lock_held(), which accepts all three contexts (classic, BH, Tasks Trace). Other map types already follow this convention.

For trie_update_elem() and trie_delete_elem(), annotate the walks as rcu_dereference_protected(*p, 1) -- matching trie_free() in the same file -- since trie->lock is held across the walk. rqspinlock has no lockdep_map, so the predicate degenerates to '1' rather than lockdep_is_held(&trie->lock); the protection is real but not machine-verifiable. trie_get_next_key() also uses bare rcu_dereference() but is reachable only from the BPF syscall, which holds classic rcu_read_lock() before dispatching, so it is left untouched.

Affected

• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < f0967d4f1ba4323a3cb7dc8fdba74dd3a8caaf04
• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < 304ca50582f0c047370f85e13caec456f78c9fcc
• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < ec662a8b2cde01e76b37ccd4b992d0342299e69c
• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < 9bfdf4b81b0e56d47bc6c46c34a46638be716695
• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < 57454944737f3ad9a8703aecbbb79713b513a94b
• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < bd6ad9a6b30498d845413e863fb95c6fab3babe3
• Linux >= 694cea395fded425008e93cd90cfdf7a451674af < 2f884d371fafea137afea504d49ee4a7c8d7985b
• Linux 5.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.