bpf: dont report verifier bug for missing bpf_scc_visit on speculative path

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: c9e3190 ("bpf: propagate read/precision marks over state graph backedges")
Reported-by: [email protected]
Closes: https://lore.kernel.org/bpf/[email protected]/
Signed-off-by: Eduard Zingerman <[email protected]>

Author: eddyz87

kernel/bpf/verifier.c

@@ -1950,9 +1950,24 @@ 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;