From 1f82dffc10ff8e44bd0c2c85ba6e21189b4a5695 Mon Sep 17 00:00:00 2001 From: Dave Marchevsky Date: Thu, 1 Dec 2022 10:34:05 -0800 Subject: [PATCH] bpf: Fix release_on_unlock release logic for multiple refs Consider a verifier state with three acquired references, all with release_on_unlock = true: idx 0 1 2 state->refs = [2 4 6] (with 2, 4, and 6 being the ref ids). When bpf_spin_unlock is called, process_spin_lock will loop through all acquired_refs and, for each ref, if it's release_on_unlock, calls release_reference on it. That function in turn calls release_reference_state, which removes the reference from state->refs by swapping the reference state with the last reference state in refs array and decrements acquired_refs count. process_spin_lock's loop logic, which is essentially: for (i = 0; i < state->acquired_refs; i++) { if (!state->refs[i].release_on_unlock) continue; release_reference(state->refs[i].id); } will fail to release release_on_unlock references which are swapped from the end. Running this logic on our example demonstrates: state->refs = [2 4 6] (start of idx=0 iter) release state->refs[0] by swapping w/ state->refs[2] state->refs = [6 4] (start of idx=1) release state->refs[1], no need to swap as it's the last idx state->refs = [6] (start of idx=2, loop terminates) ref_id 6 should have been removed but was skipped. Fix this by looping from back-to-front, which results in refs that are candidates for removal being swapped with refs which have already been examined and kept. If we modify our initial example such that ref 6 is replaced with ref 7, which is _not_ release_on_unlock, and loop from the back, we'd see: state->refs = [2 4 7] (start of idx=2) state->refs = [2 4 7] (start of idx=1) state->refs = [2 7] (start of idx=0, refs 7 and 4 swapped) state->refs = [7] (after idx=0, 7 and 2 swapped, loop terminates) Signed-off-by: Dave Marchevsky Acked-by: Yonghong Song cc: Kumar Kartikeya Dwivedi Fixes: 534e86bc6c66 ("bpf: Add 'release on unlock' logic for bpf_list_push_{front,back}") Link: https://lore.kernel.org/r/20221201183406.1203621-1-davemarchevsky@fb.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index d0ecc0b18b20..b0db9c10567b 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5738,7 +5738,7 @@ static int process_spin_lock(struct bpf_verifier_env *env, int regno, cur->active_lock.ptr = NULL; cur->active_lock.id = 0; - for (i = 0; i < fstate->acquired_refs; i++) { + for (i = fstate->acquired_refs - 1; i >= 0; i--) { int err; /* Complain on error because this reference state cannot