From 2f3536bff8823d3c5fdbbe15e17bfca696cc2b2e Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Thu, 27 Aug 2026 15:48:23 -0700 Subject: [PATCH] bpf: don't downgrade half-dead scalar zero spills to STACK_ZERO states.c:__clean_func_state() can downgrade scalar zero spill to STACK_ZERO in the following case: *(u64 *)(r10 - 8) = 0; ... checkpoint ... r1 = *(u32 *)(r10 - 4); ... no reads from r10-8 ... Here 4 bytes at r10-8 are dead and verifier changes scalar spill to a combination: 0000pppp (p stands for poison). Such a change breaks precision propagation chains. All places that produce STACK_ZERO should call bpf_mark_chain_precision() for the zero source. This patch fixes the bug in a simplest way possible: avoids converting stack spills of zero to STACK_ZERO. Two smarter approaches are possible: - do bpf_mark_chain_precision() from __clean_func_state() - check slot liveness information in check_stack_write_fixed_off() I investigated both and the changes required are a bit tricky, hence go with a simple fix for the time being. Fixes: be23266b4a08 ("bpf: 4-byte precise clean_verifier_state") Reported-by: Nicholas Carlini Suggested-by: Nicholas Carlini Signed-off-by: Eduard Zingerman Link: https://lore.kernel.org/r/20260827-bug-011-cleanfunc-stack-zero-simple-v1-v1-1-c0e996589a52@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/states.c | 11 ++++------- 1 file changed, 4 insertions(+), 7 deletions(-) diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c index 4e6aafad33bd..66fb11b6c6a7 100644 --- a/kernel/bpf/states.c +++ b/kernel/bpf/states.c @@ -445,22 +445,19 @@ static void __clean_func_state(struct bpf_verifier_env *env, struct bpf_reg_state *spill = &st->stack[i].spilled_ptr; if (lo_live && stype == STACK_SPILL) { - u8 val = STACK_MISC; - if (spill->type != SCALAR_VALUE) continue; - /* - * 8 byte spill of scalar 0 where half slot is dead - * should become STACK_ZERO in lo 4 bytes. + * Can't replace with STACK_ZERO, because + * that requires bpf_mark_chain_precision(). */ if (bpf_register_is_null(spill)) - val = STACK_ZERO; + continue; for (j = 0; j < 4; j++) { u8 *t = &st->stack[i].slot_type[j]; if (*t == STACK_SPILL) - *t = val; + *t = STACK_MISC; } } bpf_mark_reg_not_init(env, spill);