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 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/
Luis Gerhorst (3): bpf: Update env->prev_insn_idx after do_check_insn() bpf: Fix aux usage after do_check_insn() selftests/bpf: Add Spectre v4 tests
kernel/bpf/verifier.c | 30 ++-- tools/testing/selftests/bpf/progs/bpf_misc.h | 4 + .../selftests/bpf/progs/verifier_unpriv.c | 149 ++++++++++++++++++ 3 files changed, 174 insertions(+), 9 deletions(-)
base-commit: d69bafe6ee2b5eff6099fa26626ecc2963f0f363
To introduce prev_aux(env), env->prev_insn_idx must be up-to-date directly after do_check_insn(). To achieve this, replace prev_insn_idx with a tmp variable (to discourage use) and update env->prev_insn_idx directly after do_check_insn().
A concern would be that some code relied on env->prev_insn_idx still having the old value between do_check_insn() and the old update-assignment. However, outside the do_check() function it is only used through push_jmp_history()/do_check_insn()/is_state_visisted() which are not called in-between the old and new assignment location. This can also be seen from the -O0 call graph for push_jmp_history() [1].
[1] https://sys.cs.fau.de/extern/person/gerhorst/25-06_d69baf_push_jmp_history_O...
Signed-off-by: Luis Gerhorst luis.gerhorst@fau.de --- kernel/bpf/verifier.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index f403524bd215..b33bc37d5372 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -19854,16 +19854,15 @@ static int do_check(struct bpf_verifier_env *env) struct bpf_insn *insns = env->prog->insnsi; int insn_cnt = env->prog->len; bool do_print_state = false; - int prev_insn_idx = -1;
+ env->prev_insn_idx = -1; for (;;) { struct bpf_insn *insn; - int err; + int err, tmp;
/* reset current history entry on each new instruction */ env->cur_hist_ent = NULL;
- env->prev_insn_idx = prev_insn_idx; if (env->insn_idx >= insn_cnt) { verbose(env, "invalid insn idx %d insn_cnt %d\n", env->insn_idx, insn_cnt); @@ -19942,7 +19941,6 @@ static int do_check(struct bpf_verifier_env *env) }
sanitize_mark_insn_seen(env); - prev_insn_idx = env->insn_idx;
/* Reduce verification complexity by stopping speculative path * verification when a nospec is encountered. @@ -19950,7 +19948,9 @@ static int do_check(struct bpf_verifier_env *env) if (state->speculative && cur_aux(env)->nospec) goto process_bpf_exit;
+ tmp = env->insn_idx; err = do_check_insn(env, &do_print_state); + env->prev_insn_idx = tmp; if (error_recoverable_with_nospec(err) && state->speculative) { /* Prevent this speculative path from ever reaching the * insn that would have been unsafe to execute. @@ -19978,13 +19978,13 @@ static int do_check(struct bpf_verifier_env *env) * to document this in case nospec_result is used * elsewhere in the future. */ - WARN_ON_ONCE(env->insn_idx != prev_insn_idx + 1); + WARN_ON_ONCE(env->insn_idx != env->prev_insn_idx + 1); process_bpf_exit: mark_verifier_state_scratched(env); err = update_branch_counts(env, env->cur_state); if (err) return err; - err = pop_stack(env, &prev_insn_idx, &env->insn_idx, + err = pop_stack(env, &env->prev_insn_idx, &env->insn_idx, pop_log); if (err < 0) { if (err != -ENOENT)
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@g... 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);
On Sat, 2025-06-28 at 16:50 +0200, Luis Gerhorst wrote:
[...]
@@ -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;
I don't like the prev_aux() call in this position, as one needs to understand that after do_check_insn() call what was current became previous. This at-least requires a comment. Implementation with a temporary variable (as at the bottom of this email), imo, is less cognitive load.
/* 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;
[...]
---
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a136d9b1b25f..a923614b7104 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -19953,6 +19953,7 @@ static int do_check(struct bpf_verifier_env *env) bool pop_log = !(env->log.level & BPF_LOG_LEVEL2); struct bpf_verifier_state *state = env->cur_state; struct bpf_insn *insns = env->prog->insnsi; + struct bpf_insn_aux_data *insn_aux; int insn_cnt = env->prog->len; bool do_print_state = false; int prev_insn_idx = -1; @@ -19972,6 +19973,7 @@ 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, @@ -20048,7 +20050,7 @@ 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); @@ -20056,11 +20058,11 @@ 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; @@ -20069,7 +20071,7 @@ 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
Add the following tests:
1. A test with an (unimportant) ldimm64 (16 byte insn) and a Spectre-v4--induced nospec that clarifies and serves as a basic Spectre v4 test.
2. Make sure a Spectre v4 nospec_result does not prevent a Spectre v1 nospec from being added before the dangerous instruction (tests that [1] is fixed).
3. Combine the two, which is the combination that triggers the warning in [2]. This is because the unanalyzed stack write has nospec_result set, but the ldimm64 (which was just analyzed) had incremented insn_idx by 2. That violates the assertion that nospec_result is only used after insns that increment insn_idx by 1 (i.e., stack writes).
[1] https://lore.kernel.org/bpf/4266fd5de04092aa4971cbef14f1b4b96961f432.camel@g... [2] https://lore.kernel.org/bpf/685b3c1b.050a0220.2303ee.0010.GAE@google.com/
Signed-off-by: Luis Gerhorst luis.gerhorst@fau.de --- tools/testing/selftests/bpf/progs/bpf_misc.h | 4 + .../selftests/bpf/progs/verifier_unpriv.c | 149 ++++++++++++++++++ 2 files changed, 153 insertions(+)
diff --git a/tools/testing/selftests/bpf/progs/bpf_misc.h b/tools/testing/selftests/bpf/progs/bpf_misc.h index a678463e972c..be7d9bfa8390 100644 --- a/tools/testing/selftests/bpf/progs/bpf_misc.h +++ b/tools/testing/selftests/bpf/progs/bpf_misc.h @@ -235,4 +235,8 @@ #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";
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Eduard Zingerman -
Luis Gerhorst