Merge branch 'bpf-fix-and-test-aux-usage-after-do_check_insn'

Luis Gerhorst says:

====================
bpf: Fix and test aux usage after do_check_insn()

Fix cur_aux()->nospec_result test after do_check_insn() referring to the
to-be-analyzed (potentially unsafe) instruction, not the
already-analyzed (safe) instruction. This might allow a unsafe insn to
slip through on a speculative path. Create some tests from the
reproducer [1].

Commit d6f1c85f2253 ("bpf: Fall back to nospec for Spectre v1") should
not be in any stable kernel yet, therefore bpf-next should suffice.

[1] https://lore.kernel.org/bpf/685b3c1b.050a0220.2303ee.0010.GAE@google.com/

Changes since v2:
- Use insn_aux variable instead of introducing prev_aux() as suggested
  by Eduard (and therefore also drop patch 1)
- v2: https://lore.kernel.org/bpf/20250628145016.784256-1-luis.gerhorst@fau.de/

Changes since v1:
- Fix compiler error due to missed rename of prev_insn_idx in first
  patch
- v1: https://lore.kernel.org/bpf/20250628125927.763088-1-luis.gerhorst@fau.de/

Changes since RFC:
- Introduce prev_aux() as suggested by Alexei. For this, we must move
  the env->prev_insn_idx assignment to happen directly after
  do_check_insn(), for which I have created a separate commit. This
  patch could be simplified by using a local prev_aux variable as
  sugested by Eduard, but I figured one might find the new
  assignment-strategy easier to understand (before, prev_insn_idx and
  env->prev_insn_idx were out-of-sync for the latter part of the loop).
  Also, like this we do not have an additional prev_* variable that must
  be kept in-sync and the local variable's usage (old prev_insn_idx, new
  tmp) is much more local. If you think it would be better to not take
  the risk and keep the fix simple by just introducing the prev_aux
  variable, let me know.
- Change WARN_ON_ONCE() to verifier_bug_if() as suggested by Alexei
- Change assertion to check instruction is BPF_JMP[32] as suggested by
  Eduard
- RFC: https://lore.kernel.org/bpf/8734bmoemx.fsf@fau.de/
====================

Link: https://patch.msgid.link/20250705190908.1756862-1-luis.gerhorst@fau.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

3 files changed, 167 insertions, 5 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1e567fff6f23..53007182b46b 100644
--- 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);
diff --git a/tools/testing/selftests/bpf/progs/bpf_misc.h b/tools/testing/selftests/bpf/progs/bpf_misc.h
index 20dce508d8e0..530752ddde8e 100644
--- a/tools/testing/selftests/bpf/progs/bpf_misc.h
+++ b/tools/testing/selftests/bpf/progs/bpf_misc.h
@@ -237,4 +237,8 @@
 #define SPEC_V1
 #endif
 
+#if defined(__TARGET_ARCH_x86)
+#define SPEC_V4
+#endif
+
 #endif
diff --git a/tools/testing/selftests/bpf/progs/verifier_unpriv.c b/tools/testing/selftests/bpf/progs/verifier_unpriv.c
index 4470541b5e71..28b4f7035ceb 100644
--- a/tools/testing/selftests/bpf/progs/verifier_unpriv.c
+++ b/tools/testing/selftests/bpf/progs/verifier_unpriv.c
@@ -801,4 +801,153 @@ l2_%=:							\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("unpriv: ldimm64 before Spectre v4 barrier")
+__success __success_unpriv
+__retval(0)
+#ifdef SPEC_V4
+__xlated_unpriv("r1 = 0x2020200005642020") /* should not matter */
+__xlated_unpriv("*(u64 *)(r10 -8) = r1")
+__xlated_unpriv("nospec")
+#endif
+__naked void unpriv_ldimm64_spectre_v4(void)
+{
+	asm volatile ("					\
+	r1 = 0x2020200005642020 ll;			\
+	*(u64 *)(r10 -8) = r1;				\
+	r0 = 0;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
+SEC("socket")
+__description("unpriv: Spectre v1 and v4 barrier")
+__success __success_unpriv
+__retval(0)
+#ifdef SPEC_V1
+#ifdef SPEC_V4
+/* starts with r0 == r8 == r9 == 0 */
+__xlated_unpriv("if r8 != 0x0 goto pc+1")
+__xlated_unpriv("goto pc+2")
+__xlated_unpriv("if r9 == 0x0 goto pc+4")
+__xlated_unpriv("r2 = r0")
+/* Following nospec required to prevent following dangerous `*(u64 *)(NOT_FP -64)
+ * = r1` iff `if r9 == 0 goto pc+4` was mispredicted because of Spectre v1. The
+ * test therefore ensures the Spectre-v4--induced nospec does not prevent the
+ * Spectre-v1--induced speculative path from being fully analyzed.
+ */
+__xlated_unpriv("nospec") /* Spectre v1 */
+__xlated_unpriv("*(u64 *)(r2 -64) = r1") /* could be used to leak r2 */
+__xlated_unpriv("nospec") /* Spectre v4 */
+#endif
+#endif
+__naked void unpriv_spectre_v1_and_v4(void)
+{
+	asm volatile ("					\
+	r1 = 0;						\
+	*(u64*)(r10 - 8) = r1;				\
+	r2 = r10;					\
+	r2 += -8;					\
+	r1 = %[map_hash_8b] ll;				\
+	call %[bpf_map_lookup_elem];			\
+	r8 = r0;					\
+	r2 = r10;					\
+	r2 += -8;					\
+	r1 = %[map_hash_8b] ll;				\
+	call %[bpf_map_lookup_elem];			\
+	r9 = r0;					\
+	r0 = r10;					\
+	r1 = 0;						\
+	r2 = r10;					\
+	if r8 != 0 goto l0_%=;				\
+	if r9 != 0 goto l0_%=;				\
+	r0 = 0;						\
+l0_%=:	if r8 != 0 goto l1_%=;				\
+	goto l2_%=;					\
+l1_%=:	if r9 == 0 goto l3_%=;				\
+	r2 = r0;					\
+l2_%=:	*(u64 *)(r2 -64) = r1;				\
+l3_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm_addr(map_hash_8b)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("unpriv: Spectre v1 and v4 barrier (simple)")
+__success __success_unpriv
+__retval(0)
+#ifdef SPEC_V1
+#ifdef SPEC_V4
+__xlated_unpriv("if r8 != 0x0 goto pc+1")
+__xlated_unpriv("goto pc+2")
+__xlated_unpriv("goto pc-1") /* if r9 == 0 goto l3_%= */
+__xlated_unpriv("goto pc-1") /* r2 = r0 */
+__xlated_unpriv("nospec")
+__xlated_unpriv("*(u64 *)(r2 -64) = r1")
+__xlated_unpriv("nospec")
+#endif
+#endif
+__naked void unpriv_spectre_v1_and_v4_simple(void)
+{
+	asm volatile ("					\
+	r8 = 0;						\
+	r9 = 0;						\
+	r0 = r10;					\
+	r1 = 0;						\
+	r2 = r10;					\
+	if r8 != 0 goto l0_%=;				\
+	if r9 != 0 goto l0_%=;				\
+	r0 = 0;						\
+l0_%=:	if r8 != 0 goto l1_%=;				\
+	goto l2_%=;					\
+l1_%=:	if r9 == 0 goto l3_%=;				\
+	r2 = r0;					\
+l2_%=:	*(u64 *)(r2 -64) = r1;				\
+l3_%=:	r0 = 0;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
+SEC("socket")
+__description("unpriv: ldimm64 before Spectre v1 and v4 barrier (simple)")
+__success __success_unpriv
+__retval(0)
+#ifdef SPEC_V1
+#ifdef SPEC_V4
+__xlated_unpriv("if r8 != 0x0 goto pc+1")
+__xlated_unpriv("goto pc+4")
+__xlated_unpriv("goto pc-1") /* if r9 == 0 goto l3_%= */
+__xlated_unpriv("goto pc-1") /* r2 = r0 */
+__xlated_unpriv("goto pc-1") /* r1 = 0x2020200005642020 ll */
+__xlated_unpriv("goto pc-1") /* second part of ldimm64 */
+__xlated_unpriv("nospec")
+__xlated_unpriv("*(u64 *)(r2 -64) = r1")
+__xlated_unpriv("nospec")
+#endif
+#endif
+__naked void unpriv_ldimm64_spectre_v1_and_v4_simple(void)
+{
+	asm volatile ("					\
+	r8 = 0;						\
+	r9 = 0;						\
+	r0 = r10;					\
+	r1 = 0;						\
+	r2 = r10;					\
+	if r8 != 0 goto l0_%=;				\
+	if r9 != 0 goto l0_%=;				\
+	r0 = 0;						\
+l0_%=:	if r8 != 0 goto l1_%=;				\
+	goto l2_%=;					\
+l1_%=:	if r9 == 0 goto l3_%=;				\
+	r2 = r0;					\
+	r1 = 0x2020200005642020 ll;			\
+l2_%=:	*(u64 *)(r2 -64) = r1;				\
+l3_%=:	r0 = 0;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";