From 175827e04f4be53f3dfb57edf12d0d49b18fd939 Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov Date: Wed, 19 Jun 2024 16:53:54 -0700 Subject: [PATCH] bpf: Fix may_goto with negative offset. [ Upstream commit 2b2efe1937ca9f8815884bd4dcd5b32733025103 ] Zac's syzbot crafted a bpf prog that exposed two bugs in may_goto. The 1st bug is the way may_goto is patched. When offset is negative it should be patched differently. The 2nd bug is in the verifier: when current state may_goto_depth is equal to visited state may_goto_depth it means there is an actual infinite loop. It's not correct to prune exploration of the program at this point. Note, that this check doesn't limit the program to only one may_goto insn, since 2nd and any further may_goto will increment may_goto_depth only in the queued state pushed for future exploration. The current state will have may_goto_depth == 0 regardless of number of may_goto insns and the verifier has to explore the program until bpf_exit. Fixes: 011832b97b31 ("bpf: Introduce may_goto instruction") Reported-by: Zac Ecob Signed-off-by: Alexei Starovoitov Signed-off-by: Andrii Nakryiko Signed-off-by: Daniel Borkmann Acked-by: Eduard Zingerman Closes: https://lore.kernel.org/bpf/CAADnVQL-15aNp04-cyHRn47Yv61NXfYyhopyZtUyxNojUZUXpA@mail.gmail.com/ Link: https://lore.kernel.org/bpf/20240619235355.85031-1-alexei.starovoitov@gmail.com Signed-off-by: Sasha Levin --- kernel/bpf/verifier.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 2233bf50a9012..ab558eea1c9ee 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -17308,11 +17308,11 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) goto skip_inf_loop_check; } if (is_may_goto_insn_at(env, insn_idx)) { - if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) { + if (sl->state.may_goto_depth != cur->may_goto_depth && + states_equal(env, &sl->state, cur, RANGE_WITHIN)) { update_loop_entry(cur, &sl->state); goto hit; } - goto skip_inf_loop_check; } if (calls_callback(env, insn_idx)) { if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) @@ -19853,7 +19853,10 @@ static int do_misc_fixups(struct bpf_verifier_env *env) stack_depth_extra = 8; insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_AX, BPF_REG_10, stack_off); - insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 2); + if (insn->off >= 0) + insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 2); + else + insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off - 1); insn_buf[2] = BPF_ALU64_IMM(BPF_SUB, BPF_REG_AX, 1); insn_buf[3] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_AX, stack_off); cnt = 4; -- 2.47.3