Permit sandboxee's bpf() to fail

The default policy causes immediate termination of a sandboxee that
calls `bpf`(2).

This does not allow for try-call use of `bpf()` to test for optional
features.

To support such try-call use cases, sandboxes would like to say:

```
  sandbox2::PolicyBuilder builder;
  builder.BlockSyscallWithErrno(__NR_bpf, EPERM);
```

but this doesn't work because the default policy unconditionally treats
`bpf()` as a sandbox violation.

Remove the bpf violation check from the policy if `bpf()` is explicitly
blocked with an errno.

PiperOrigin-RevId: 345239389
Change-Id: I7fcfd3a938c610c8679edf8e1fa0238b32cc9db4

sandboxed_api/sandbox2/policy.cc

@@ -106,18 +106,26 @@ std::vector<sock_filter> Policy::GetDefaultPolicy() const {
     SANDBOX2_TRACE,
     LABEL(&l, past_execveat_l),
 
-    // Forbid some syscalls because unsafe or too risky.
+    // Forbid ptrace because it's unsafe or too risky.
     LOAD_SYSCALL_NR,
     JEQ32(__NR_ptrace, DENY),
-    JEQ32(__NR_bpf, DENY),
-
-    // Disallow clone with CLONE_UNTRACED flag.
-    JNE32(__NR_clone, JUMP(&l, past_clone_untraced_l)),
-    // Regardless of arch, we only care about the lower 32-bits of the flags.
-    ARG_32(0),
-    JA32(CLONE_UNTRACED, DENY),
-    LABEL(&l, past_clone_untraced_l),
   };
+  // If user policy doesn't mention it, then forbid bpf because it's unsafe or
+  // too risky.  This uses LOAD_SYSCALL_NR from above.
+  if (!user_policy_handles_bpf_) {
+    policy.insert(policy.end(), {JEQ32(__NR_bpf, DENY)});
+  }
+  policy.insert(policy.end(),
+                {
+                    // Disallow clone with CLONE_UNTRACED flag.  This uses
+                    // LOAD_SYSCALL_NR from above.
+                    JNE32(__NR_clone, JUMP(&l, past_clone_untraced_l)),
+                    // Regardless of arch, we only care about the lower 32-bits
+                    // of the flags.
+                    ARG_32(0),
+                    JA32(CLONE_UNTRACED, DENY),
+                    LABEL(&l, past_clone_untraced_l),
+                });
 
   if (bpf_resolve_jumps(&l, policy.data(), policy.size()) != 0) {
     LOG(FATAL) << "Cannot resolve bpf jumps";

sandboxed_api/sandbox2/policy.h

@@ -96,6 +96,7 @@ class Policy final {
 
   // The policy set by the user.
   std::vector<sock_filter> user_policy_;
+  bool user_policy_handles_bpf_ = false;
 
   // Get the default policy, which blocks certain dangerous syscalls and
   // mismatched syscall tables.

sandboxed_api/sandbox2/policy_test.cc

@@ -40,7 +40,7 @@ using ::testing::Eq;
 namespace sandbox2 {
 namespace {
 
-std::unique_ptr<Policy> PolicyTestcasePolicy() {
+PolicyBuilder CreatePolicyTestPolicyBuilder() {
   return PolicyBuilder()
       .DisableNamespaces()
       .AllowStaticStartup()
@@ -60,8 +60,11 @@ std::unique_ptr<Policy> PolicyTestcasePolicy() {
 #ifdef __NR_faccessat
       .BlockSyscallWithErrno(__NR_faccessat, ENOENT)
 #endif
-      .BlockSyscallWithErrno(__NR_prlimit64, EPERM)
-      .BuildOrDie();
+      .BlockSyscallWithErrno(__NR_prlimit64, EPERM);
+}
+
+std::unique_ptr<Policy> PolicyTestcasePolicy() {
+  return CreatePolicyTestPolicyBuilder().BuildOrDie();
 }
 
 #ifdef SAPI_X86_64
@@ -150,6 +153,25 @@ TEST(PolicyTest, BpfDisallowed) {
   EXPECT_THAT(result.reason_code(), Eq(__NR_bpf));
 }
 
+// Test that bpf(2) can return EPERM.
+TEST(PolicyTest, BpfPermissionDenied) {
+  SKIP_SANITIZERS_AND_COVERAGE;
+  const std::string path = GetTestSourcePath("sandbox2/testcases/policy");
+  std::vector<std::string> args = {path, "7"};
+  auto executor = absl::make_unique<Executor>(path, args);
+
+  auto policy = CreatePolicyTestPolicyBuilder()
+                    .BlockSyscallWithErrno(__NR_bpf, EPERM)
+                    .BuildOrDie();
+
+  Sandbox2 s2(std::move(executor), std::move(policy));
+  auto result = s2.Run();
+
+  // bpf(2) is not a violation due to explicit policy.  EPERM is expected.
+  ASSERT_THAT(result.final_status(), Eq(Result::OK));
+  EXPECT_THAT(result.reason_code(), Eq(EPERM));
+}
+
 TEST(PolicyTest, IsattyAllowed) {
   SKIP_SANITIZERS_AND_COVERAGE;
   const std::string path = GetTestSourcePath("sandbox2/testcases/policy");

sandboxed_api/sandbox2/policybuilder.cc

@@ -82,6 +82,9 @@ PolicyBuilder& PolicyBuilder::BlockSyscallWithErrno(unsigned int num,
                                                     int error) {
   if (handled_syscalls_.insert(num).second) {
     user_policy_.insert(user_policy_.end(), {SYSCALL(num, ERRNO(error))});
+    if (num == __NR_bpf) {
+      user_policy_handles_bpf_ = true;
+    }
   }
   return *this;
 }
@@ -728,6 +731,7 @@ absl::StatusOr<std::unique_ptr<Policy>> PolicyBuilder::TryBuild() {
   output->collect_stacktrace_on_timeout_ = collect_stacktrace_on_timeout_;
   output->collect_stacktrace_on_kill_ = collect_stacktrace_on_kill_;
   output->user_policy_ = std::move(user_policy_);
+  output->user_policy_handles_bpf_ = user_policy_handles_bpf_;
 
   auto pb_description = absl::make_unique<PolicyBuilderDescription>();
 

sandboxed_api/sandbox2/policybuilder.h

@@ -554,6 +554,7 @@ class PolicyBuilder final {
 
   // Seccomp fields
   std::vector<sock_filter> user_policy_;
+  bool user_policy_handles_bpf_ = false;
   std::set<unsigned int> handled_syscalls_;
 
   // Error handling

sandboxed_api/sandbox2/testcases/policy.cc

@@ -19,6 +19,7 @@
 #include <syscall.h>
 #include <unistd.h>
 
+#include <cerrno>
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
@@ -79,6 +80,10 @@ void TestBpf() {
   exit(EXIT_FAILURE);
 }
 
+void TestBpfError() {
+  exit(syscall(__NR_bpf, 0, nullptr, 0) == -1 ? errno : 0);
+}
+
 void TestIsatty() {
   isatty(0);
 
@@ -119,6 +124,9 @@ int main(int argc, char** argv) {
     case 6:
       TestIsatty();
       break;
+    case 7:
+      TestBpfError();
+      break;
     default:
       printf("Unknown test: %d\n", testno);
       return EXIT_FAILURE;