bpfloader - move sysctl setting from rc to binary

this allows tightening the sepolicy for 'proc_bpf' in:
  https://android-review.git.corp.google.com/c/platform/system/sepolicy/+/2323635
  'sepolicy - move proc bpf writes from bpfloader.rc to bpfloader binary'

While we're at it, this also allows us to actually verify
that these sysctls are being successfully set.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ibde9d817b690395e3eb12f6b5acdf3060aca67b9

1 files changed, 38 insertions, 0 deletions

diff --git a/bpfloader/BpfLoader.cpp b/bpfloader/BpfLoader.cpp
index fd261b5..c1be22a 100644
--- a/bpfloader/BpfLoader.cpp
+++ b/bpfloader/BpfLoader.cpp
@@ -235,10 +235,48 @@ void createSysFsBpfSubDir(const char* const prefix) {
     }
 }
 
+// Technically 'value' doesn't need to be newline terminated, but it's best
+// to include a newline to match 'echo "value" > /proc/sys/...foo' behaviour,
+// which is usually how kernel devs test the actual sysctl interfaces.
+int writeProcSysFile(const char *filename, const char *value) {
+    android::base::unique_fd fd(open(filename, O_WRONLY | O_CLOEXEC));
+    if (fd < 0) {
+        const int err = errno;
+        ALOGE("open('%s', O_WRONLY | O_CLOEXEC) -> %s", filename, strerror(err));
+        return -err;
+    }
+    int len = strlen(value);
+    int v = write(fd, value, len);
+    if (v < 0) {
+        const int err = errno;
+        ALOGE("write('%s', '%s', %d) -> %s", filename, value, len, strerror(err));
+        return -err;
+    }
+    if (v != len) {
+        // In practice, due to us only using this for /proc/sys/... files, this can't happen.
+        ALOGE("write('%s', '%s', %d) -> short write [%d]", filename, value, len, v);
+        return -EINVAL;
+    }
+    return 0;
+}
+
 int main(int argc, char** argv) {
     (void)argc;
     android::base::InitLogging(argv, &android::base::KernelLogger);
 
+    // Linux 5.16-rc1 changed the default to 2 (disabled but changeable), but we need 0 (enabled)
+    // (this writeFile is known to fail on at least 4.19, but always defaults to 0 on pre-5.13,
+    // on 5.13+ it depends on CONFIG_BPF_UNPRIV_DEFAULT_OFF)
+    if (writeProcSysFile("/proc/sys/kernel/unprivileged_bpf_disabled", "0\n") &&
+        android::bpf::isAtLeastKernelVersion(5, 13, 0)) return 1;
+
+    // Enable the eBPF JIT -- but do note that on 64-bit kernels it is likely
+    // already force enabled by the kernel config option BPF_JIT_ALWAYS_ON
+    if (writeProcSysFile("/proc/sys/net/core/bpf_jit_enable", "1\n")) return 1;
+
+    // Enable JIT kallsyms export for privileged users only
+    if (writeProcSysFile("/proc/sys/net/core/bpf_jit_kallsyms", "1\n")) return 1;
+
     // This is ugly... but this allows InProcessTethering which runs as system_server,
     // instead of as network_stack to access /sys/fs/bpf/tethering, which would otherwise
     // (due to genfscon rules) have fs_bpf_tethering selinux context, which is restricted