bpf, lpm_trie: Allow lookups from sleepable BPF programs

[PATCH bpf] bpf, lpm_trie: Allow lookups from sleepable BPF programs

Posted by Vlad Poenaru 1 week, 2 days ago

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().

A sleepable LSM hook that ends up doing bpf_map_lookup_elem() on 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 does map lookups on an LPM trie, which is increasingly common.

Other map types already use the bpf_rcu_lock_held() helper, which
accepts all three contexts (classic, BH, Tasks Trace). Use it here as
well, matching the established convention.

Fixes: 694cea395fde ("bpf: Allow RCU-protected lookups to happen from bh context")
Cc: stable@vger.kernel.org
Signed-off-by: Vlad Poenaru <vlad.wing@gmail.com>
---
 kernel/bpf/lpm_trie.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index 0f57608b385d..ac36063cb7e6 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -246,7 +246,7 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
 
 	/* Start walking the trie from the root node ... */
 
-	for (node = rcu_dereference_check(trie->root, rcu_read_lock_bh_held());
+	for (node = rcu_dereference_check(trie->root, bpf_rcu_lock_held());
 	     node;) {
 		unsigned int next_bit;
 		size_t matchlen;
@@ -280,7 +280,7 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
 		 */
 		next_bit = extract_bit(key->data, node->prefixlen);
 		node = rcu_dereference_check(node->child[next_bit],
-					     rcu_read_lock_bh_held());
+					     bpf_rcu_lock_held());
 	}
 
 	if (!found)
-- 
2.53.0-Meta

Re: [PATCH bpf] bpf, lpm_trie: Allow lookups from sleepable BPF programs

Posted by Emil Tsalapatis 1 week, 2 days ago

On Fri May 29, 2026 at 1:42 PM EDT, Vlad Poenaru wrote:
> 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().
>
> A sleepable LSM hook that ends up doing bpf_map_lookup_elem() on 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 does map lookups on an LPM trie, which is increasingly common.
>
> Other map types already use the bpf_rcu_lock_held() helper, which
> accepts all three contexts (classic, BH, Tasks Trace). Use it here as
> well, matching the established convention.
>
> Fixes: 694cea395fde ("bpf: Allow RCU-protected lookups to happen from bh context")
> Cc: stable@vger.kernel.org
> Signed-off-by: Vlad Poenaru <vlad.wing@gmail.com>

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

> ---
>  kernel/bpf/lpm_trie.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> index 0f57608b385d..ac36063cb7e6 100644
> --- a/kernel/bpf/lpm_trie.c
> +++ b/kernel/bpf/lpm_trie.c
> @@ -246,7 +246,7 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
>  
>  	/* Start walking the trie from the root node ... */
>  
> -	for (node = rcu_dereference_check(trie->root, rcu_read_lock_bh_held());
> +	for (node = rcu_dereference_check(trie->root, bpf_rcu_lock_held());
>  	     node;) {
>  		unsigned int next_bit;
>  		size_t matchlen;
> @@ -280,7 +280,7 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
>  		 */
>  		next_bit = extract_bit(key->data, node->prefixlen);
>  		node = rcu_dereference_check(node->child[next_bit],
> -					     rcu_read_lock_bh_held());
> +					     bpf_rcu_lock_held());
>  	}
>  
>  	if (!found)