6.17-stable review patch. If anyone has any objections, please let me know.
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From: Eduard Zingerman eddyz87@gmail.com
[ Upstream commit a3c73d629ea1373af3c0c954d41fd1af555492e3 ]
Syzbot generated a program that triggers a verifier_bug() call in maybe_exit_scc(). maybe_exit_scc() assumes that, when called for a state with insn_idx in some SCC, there should be an instance of struct bpf_scc_visit allocated for that SCC. Turns out the assumption does not hold for speculative execution paths. See example in the next patch.
maybe_scc_exit() is called from update_branch_counts() for states that reach branch count of zero, meaning that path exploration for a particular path is finished. Path exploration can finish in one of three ways: a. Verification error is found. In this case, update_branch_counts() is called only for non-speculative paths. b. Top level BPF_EXIT is reached. Such instructions are never a part of an SCC, so compute_scc_callchain() in maybe_scc_exit() will return false, and maybe_scc_exit() will return early. c. A checkpoint is reached and matched. Checkpoints are created by is_state_visited(), which calls maybe_enter_scc(), which allocates bpf_scc_visit instances for checkpoints within SCCs.
Hence, for non-speculative symbolic execution paths, the assumption still holds: if maybe_scc_exit() is called for a state within an SCC, bpf_scc_visit instance must exist.
This patch removes the verifier_bug() call for speculative paths.
Fixes: c9e31900b54c ("bpf: propagate read/precision marks over state graph backedges") Reported-by: syzbot+3afc814e8df1af64b653@syzkaller.appspotmail.com Closes: https://lore.kernel.org/bpf/68c85acd.050a0220.2ff435.03a4.GAE@google.com/ Signed-off-by: Eduard Zingerman eddyz87@gmail.com Link: https://lore.kernel.org/r/20250916212251.3490455-1-eddyz87@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 9fb1f957a0937..6ad0dc226183a 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1946,9 +1946,24 @@ static int maybe_exit_scc(struct bpf_verifier_env *env, struct bpf_verifier_stat return 0; visit = scc_visit_lookup(env, callchain); if (!visit) { - verifier_bug(env, "scc exit: no visit info for call chain %s", - format_callchain(env, callchain)); - return -EFAULT; + /* + * If path traversal stops inside an SCC, corresponding bpf_scc_visit + * must exist for non-speculative paths. For non-speculative paths + * traversal stops when: + * a. Verification error is found, maybe_exit_scc() is not called. + * b. Top level BPF_EXIT is reached. Top level BPF_EXIT is not a member + * of any SCC. + * c. A checkpoint is reached and matched. Checkpoints are created by + * is_state_visited(), which calls maybe_enter_scc(), which allocates + * bpf_scc_visit instances for checkpoints within SCCs. + * (c) is the only case that can reach this point. + */ + if (!st->speculative) { + verifier_bug(env, "scc exit: no visit info for call chain %s", + format_callchain(env, callchain)); + return -EFAULT; + } + return 0; } if (visit->entry_state != st) return 0;