bpf: Fix aux usage after do_check_insn()

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
insn_aux variable.

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().

In line with Eduard's suggestion, do not introduce prev_aux() because
that requires one to understand that after do_check_insn() call what was
current became previous. This would at-least require a comment.

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/
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250705190908.1756862-2-luis.gerhorst@fau.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1e567fff6f232cd8635ba01d0a97edc72de7f7a6..53007182b46b9c0e014ebc1a2d16abd58c9230f6 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19953,6 +19953,7 @@ static int do_check(struct bpf_verifier_env *env)
 
        for (;;) {
                struct bpf_insn *insn;
+               struct bpf_insn_aux_data *insn_aux;
                int err;
 
                /* reset current history entry on each new instruction */
@@ -19966,6 +19967,7 @@ static int do_check(struct bpf_verifier_env *env)
                }
 
                insn = &insns[env->insn_idx];
+               insn_aux = &env->insn_aux_data[env->insn_idx];
 
                if (++env->insn_processed > BPF_COMPLEXITY_LIMIT_INSNS) {
                        verbose(env,
@@ -20042,7 +20044,7 @@ static int do_check(struct bpf_verifier_env *env)
                /* Reduce verification complexity by stopping speculative path
                 * verification when a nospec is encountered.
                 */
-               if (state->speculative && cur_aux(env)->nospec)
+               if (state->speculative && insn_aux->nospec)
                        goto process_bpf_exit;
 
                err = do_check_insn(env, &do_print_state);
@@ -20050,11 +20052,11 @@ 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;
+                       insn_aux->nospec = true;
                        /* If it was an ADD/SUB insn, potentially remove any
                         * markings for alu sanitization.
                         */
-                       cur_aux(env)->alu_state = 0;
+                       insn_aux->alu_state = 0;
                        goto process_bpf_exit;
                } else if (err < 0) {
                        return err;
@@ -20063,7 +20065,7 @@ static int do_check(struct bpf_verifier_env *env)
                }
                WARN_ON_ONCE(err);
 
-               if (state->speculative && cur_aux(env)->nospec_result) {
+               if (state->speculative && insn_aux->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
@@ -20072,8 +20074,15 @@ 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 != 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);