[PATCH bpf-next v2 2/3] bpf: Fix aux usage after do_check_insn()

[Luis Gerhorst <luis.gerhorst@fau.de>]

We must terminate the speculative analysis if the just-analyzed insn had
nospec_result set. Using cur_aux() here is wrong because insn_idx might
have been incremented by do_check_insn(). Therefore, introduce and use
prev_aux().

Also change cur_aux(env)->nospec in case do_check_insn() ever manages to
increment insn_idx but still fail.

Change the warning to check the insn class (which prevents it from
triggering for ldimm64, for which nospec_result would not be
problematic) and use verifier_bug_if().

Fixes: d6f1c85f2253 ("bpf: Fall back to nospec for Spectre v1")
Reported-by: Paul Chaignon <paul.chaignon@gmail.com>
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Reported-by: syzbot+dc27c5fb8388e38d2d37@syzkaller.appspotmail.com
Link: https://lore.kernel.org/bpf/685b3c1b.050a0220.2303ee.0010.GAE@google.com/
Link: https://lore.kernel.org/bpf/4266fd5de04092aa4971cbef14f1b4b96961f432.camel@gmail.com/
Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
---
 kernel/bpf/verifier.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b33bc37d5372..9d066e4b8248 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11216,6 +11216,11 @@ static struct bpf_insn_aux_data *cur_aux(const struct bpf_verifier_env *env)
 	return &env->insn_aux_data[env->insn_idx];
 }
 
+static struct bpf_insn_aux_data *prev_aux(const struct bpf_verifier_env *env)
+{
+	return &env->insn_aux_data[env->prev_insn_idx];
+}
+
 static bool loop_flag_is_zero(struct bpf_verifier_env *env)
 {
 	struct bpf_reg_state *regs = cur_regs(env);
@@ -19955,11 +19960,11 @@ static int do_check(struct bpf_verifier_env *env)
 			/* Prevent this speculative path from ever reaching the
 			 * insn that would have been unsafe to execute.
 			 */
-			cur_aux(env)->nospec = true;
+			prev_aux(env)->nospec = true;
 			/* If it was an ADD/SUB insn, potentially remove any
 			 * markings for alu sanitization.
 			 */
-			cur_aux(env)->alu_state = 0;
+			prev_aux(env)->alu_state = 0;
 			goto process_bpf_exit;
 		} else if (err < 0) {
 			return err;
@@ -19968,7 +19973,7 @@ static int do_check(struct bpf_verifier_env *env)
 		}
 		WARN_ON_ONCE(err);
 
-		if (state->speculative && cur_aux(env)->nospec_result) {
+		if (state->speculative && prev_aux(env)->nospec_result) {
 			/* If we are on a path that performed a jump-op, this
 			 * may skip a nospec patched-in after the jump. This can
 			 * currently never happen because nospec_result is only
@@ -19977,8 +19982,15 @@ static int do_check(struct bpf_verifier_env *env)
 			 * never skip the following insn. Still, add a warning
 			 * to document this in case nospec_result is used
 			 * elsewhere in the future.
+			 *
+			 * All non-branch instructions have a single
+			 * fall-through edge. For these, nospec_result should
+			 * already work.
 			 */
-			WARN_ON_ONCE(env->insn_idx != env->prev_insn_idx + 1);
+			if (verifier_bug_if(BPF_CLASS(insn->code) == BPF_JMP ||
+					    BPF_CLASS(insn->code) == BPF_JMP32, env,
+					    "speculation barrier after jump instruction may not have the desired effect"))
+				return -EFAULT;
 process_bpf_exit:
 			mark_verifier_state_scratched(env);
 			err = update_branch_counts(env, env->cur_state);
-- 
2.49.0