From 369f4ce734570bdfedaa4b5ca50e2a3f6a892728 Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Thu, 3 Sep 2026 23:47:47 +0200 Subject: [PATCH] bpf: Check ancestor frames for rbtree callbacks bpf_rbtree_add() invokes its comparator while the caller holds the root lock. The native insertion code retains raw parent and link pointers across the callback, so the verifier prohibits unlocking, consuming tree nodes, or changing RCU state from that callback. in_rbtree_lock_required_cb() only checks the innermost verifier frame. Static subprogram calls are permitted while holding a spin lock, and such a call pushes a frame without in_callback_fn set. Consequently, all callback restrictions disappear in the nested frame. The subprogram can unlock the tree, remove and drop the node being compared, then relock. Native insertion resumes with the stale parent pointer and links freed memory into the tree. Walk all active frames for the rbtree callback instead. Benign static subprograms remain permitted, while callback restrictions follow execution into nested frames. Fixes: a44b1334aadd ("bpf: Allow calling static subprogs while holding a bpf_spin_lock") Reported-by: Nicholas Carlini Suggested-by: Nicholas Carlini Signed-off-by: Kumar Kartikeya Dwivedi Acked-by: Eduard Zingerman Link: https://lore.kernel.org/r/20260903214758.2727663-2-memxor@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 2ed17edf77f2..2b7e5c9b3ffc 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -10235,9 +10235,10 @@ static void account_current_path(struct bpf_verifier_env *env) frame ? state->frame[frame - 1] : NULL); } -/* Are we currently verifying the callback for a rbtree helper that must - * be called with lock held? If so, no need to complain about unreleased - * lock +/* + * Are we currently verifying the callback for an rbtree kfunc that must + * be called with a lock held, or one of that callback's subprogs? If so, + * no need to complain about an unreleased lock. */ static bool in_rbtree_lock_required_cb(struct bpf_verifier_env *env) { @@ -10245,17 +10246,19 @@ static bool in_rbtree_lock_required_cb(struct bpf_verifier_env *env) struct bpf_insn *insn = env->prog->insnsi; struct bpf_func_state *callee; int kfunc_btf_id; + u32 frame; - if (!state->curframe) - return false; + for (frame = state->curframe; frame; frame--) { + callee = state->frame[frame]; + if (!callee->in_callback_fn) + continue; - callee = state->frame[state->curframe]; + kfunc_btf_id = insn[callee->callsite].imm; + if (is_rbtree_lock_required_kfunc(kfunc_btf_id)) + return true; + } - if (!callee->in_callback_fn) - return false; - - kfunc_btf_id = insn[callee->callsite].imm; - return is_rbtree_lock_required_kfunc(kfunc_btf_id); + return false; } static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg)