Add support for seccomp thread sync feature

- Adds public functions `seccompiler::apply_filter_all_threads` and
  private `apply_filter_with_flags`
- Moves the body of apply_filter into apply_filter_with_flags
- Uses seccomp call directly in apply_filter, so new Error variant is
  added.
- Error variant also added for TSYNC failures

Resolves #57

Signed-off-by: Harry Stern <harry@harrystern.net>

Author: boustrophedon

CHANGELOG.md

@@ -1,3 +1,11 @@
+# Upcoming Release
+
+- Seccomp is now activated via the seccomp syscall, not prctl
+- A new Error::Seccomp variant is added to indictate seccomp syscall failures
+- Add `apply_filter_all_threads` convenience function which uses the seccomp
+  TSYNC feature to synchronize all threads in the process to the same filter
+- A new Error::ThreadSync variant is added to indicate failure to sync threads
+
 # v0.3.0
 
 ## Changed

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-    "coverage_score": 93.6,
+    "coverage_score": 93.0,
     "exclude_path": "tests/integration_tests.rs,tests/json.rs",
     "crate_features": "json"
 }

src/lib.rs

@@ -208,6 +208,10 @@ pub use backend::{
     SeccompCmpOp, SeccompCondition, SeccompFilter, SeccompRule, TargetArch,
 };
 
+// Until https://github.com/rust-lang/libc/issues/3342 is fixed, define locally
+// From <linux/seccomp.h>
+const SECCOMP_SET_MODE_FILTER: libc::c_int = 1;
+
 // BPF structure definition for filter array.
 // See /usr/include/linux/filter.h .
 #[repr(C)]
@@ -231,6 +235,11 @@ pub enum Error {
     EmptyFilter,
     /// System error related to calling `prctl`.
     Prctl(io::Error),
+    /// System error related to calling `seccomp` syscall.
+    Seccomp(io::Error),
+    /// Returned when calling `seccomp` with the thread sync flag (TSYNC) fails. Contains the pid
+    /// of the thread that caused the failure.
+    ThreadSync(libc::c_long),
     /// Json Frontend Error.
     #[cfg(feature = "json")]
     JsonFrontend(JsonFrontendError),
@@ -243,6 +252,8 @@ impl std::error::Error for Error {
         match self {
             Backend(error) => Some(error),
             Prctl(error) => Some(error),
+            Seccomp(error) => Some(error),
+            ThreadSync(_) => None,
             #[cfg(feature = "json")]
             JsonFrontend(error) => Some(error),
             _ => None,
@@ -264,6 +275,16 @@ impl Display for Error {
             Prctl(errno) => {
                 write!(f, "Error calling `prctl`: {}", errno)
             }
+            Seccomp(errno) => {
+                write!(f, "Error calling `seccomp`: {}", errno)
+            }
+            ThreadSync(pid) => {
+                write!(
+                    f,
+                    "Seccomp filter synchronization failed in thread `{}`",
+                    pid
+                )
+            }
             #[cfg(feature = "json")]
             JsonFrontend(error) => {
                 write!(f, "Json Frontend error: {}", error)
@@ -292,6 +313,30 @@ impl From<JsonFrontendError> for Error {
 ///
 /// [`BpfProgram`]: type.BpfProgram.html
 pub fn apply_filter(bpf_filter: BpfProgramRef) -> Result<()> {
+    apply_filter_with_flags(bpf_filter, 0)
+}
+
+/// Apply a BPF filter to the all threads in the process via the TSYNC feature. Please read the
+/// man page for seccomp (`man 2 seccomp`) for more information.
+///
+/// # Arguments
+///
+/// * `bpf_filter` - A reference to the [`BpfProgram`] to be installed.
+///
+/// [`BpfProgram`]: type.BpfProgram.html
+pub fn apply_filter_all_threads(bpf_filter: BpfProgramRef) -> Result<()> {
+    apply_filter_with_flags(bpf_filter, libc::SECCOMP_FILTER_FLAG_TSYNC)
+}
+
+/// Apply a BPF filter to the calling thread.
+///
+/// # Arguments
+///
+/// * `bpf_filter` - A reference to the [`BpfProgram`] to be installed.
+/// * `flags` - A u64 representing a bitset of seccomp's flags parameter.
+///
+/// [`BpfProgram`]: type.BpfProgram.html
+fn apply_filter_with_flags(bpf_filter: BpfProgramRef, flags: libc::c_ulong) -> Result<()> {
     // If the program is empty, don't install the filter.
     if bpf_filter.is_empty() {
         return Err(Error::EmptyFilter);
@@ -314,14 +359,21 @@ pub fn apply_filter(bpf_filter: BpfProgramRef) -> Result<()> {
     // Safe because the kernel performs a `copy_from_user` on the filter and leaves the memory
     // untouched. We can therefore use a reference to the BpfProgram, without needing ownership.
     let rc = unsafe {
-        libc::prctl(
-            libc::PR_SET_SECCOMP,
-            libc::SECCOMP_MODE_FILTER,
+        libc::syscall(
+            libc::SYS_seccomp,
+            SECCOMP_SET_MODE_FILTER,
+            flags,
             bpf_prog_ptr,
         )
     };
-    if rc != 0 {
-        return Err(Error::Prctl(io::Error::last_os_error()));
+
+    #[allow(clippy::comparison_chain)]
+    // Per manpage, if TSYNC fails, retcode is >0 and equals the pid of the thread that caused the
+    // failure. Otherwise, error code is -1 and errno is set.
+    if rc < 0 {
+        return Err(Error::Seccomp(io::Error::last_os_error()));
+    } else if rc > 0 {
+        return Err(Error::ThreadSync(rc));
     }
 
     Ok(())

tests/integration_tests.rs

@@ -714,7 +714,7 @@ fn test_filter_apply() {
         // Apply seccomp filter.
         assert!(matches!(
             apply_filter(&filter).unwrap_err(),
-            Error::Prctl(_)
+            Error::Seccomp(_)
         ));
     })
     .join()
@@ -756,7 +756,7 @@ fn test_filter_apply() {
 
         assert!(matches!(
             apply_filter(&filter).unwrap_err(),
-            Error::Prctl(_)
+            Error::Seccomp(_)
         ));
 
         // test that seccomp level remains 0 on failure.

tests/multi_thread.rs

@@ -0,0 +1,97 @@
+#![allow(clippy::undocumented_unsafe_blocks)]
+
+/// This test is in a separate top-level test file so that it is isolated from the other tests -
+/// each file in the tests/ directory gets compiled to a separate binary and is run as a separate
+/// process.
+use std::collections::BTreeMap;
+
+use std::sync::mpsc::sync_channel;
+use std::thread;
+
+use seccompiler::{
+    apply_filter_all_threads, BpfProgram, SeccompAction, SeccompFilter, SeccompRule,
+};
+use std::env::consts::ARCH;
+
+fn check_getpid_fails() {
+    let pid = unsafe { libc::getpid() };
+    let errno = std::io::Error::last_os_error().raw_os_error().unwrap();
+
+    assert_eq!(pid, -1, "getpid should return -1 as set in SeccompFilter");
+    assert_eq!(errno, 0, "there should be no errors");
+}
+
+#[test]
+/// Test seccomp's TSYNC functionality, which syncs the current filter to all threads in the
+/// process.
+fn test_tsync() {
+    // These channels will block on send until the receiver has called recv.
+    let (setup_tx, setup_rx) = sync_channel::<()>(0);
+    let (finish_tx, finish_rx) = sync_channel::<()>(0);
+
+    // first check getpid is working
+    let pid = unsafe { libc::getpid() };
+    let errno = std::io::Error::last_os_error().raw_os_error().unwrap();
+
+    assert!(pid > 0, "getpid should return the actual pid");
+    assert_eq!(errno, 0, "there should be no errors");
+
+    // create two threads, one which applies the filter to all threads and another which tries
+    // to call getpid.
+    let seccomp_thread = thread::spawn(move || {
+        let rules = vec![(libc::SYS_getpid, vec![])];
+
+        let rule_map: BTreeMap<i64, Vec<SeccompRule>> = rules.into_iter().collect();
+
+        // Build seccomp filter only disallowing getpid
+        let filter = SeccompFilter::new(
+            rule_map,
+            SeccompAction::Allow,
+            SeccompAction::Errno(1u32),
+            ARCH.try_into().unwrap(),
+        )
+        .unwrap();
+
+        let filter: BpfProgram = filter.try_into().unwrap();
+        apply_filter_all_threads(&filter).unwrap();
+
+        // Verify seccomp is working in this thread
+        check_getpid_fails();
+
+        // seccomp setup done, let the other thread start
+        setup_tx.send(()).unwrap();
+
+        // don't close this thread until the other thread is done asserting. This way we can be
+        // sure the thread that loaded the filter is definitely active when the other thread runs.
+        finish_rx.recv().unwrap();
+        println!("exit seccomp thread");
+    });
+
+    let test_thread = thread::spawn(move || {
+        // wait until seccomp setup is done
+        setup_rx.recv().unwrap();
+
+        // Verify seccomp is working in this thread after disallowing it in other thread
+        check_getpid_fails();
+
+        // let other thread know we've passed
+        finish_tx.send(()).unwrap();
+        println!("exit io thread");
+    });
+
+    let seccomp_res = seccomp_thread.join();
+    assert!(
+        seccomp_res.is_ok(),
+        "seccomp thread failed: {:?}",
+        seccomp_res.unwrap_err()
+    );
+    let test_res = test_thread.join();
+    assert!(
+        test_res.is_ok(),
+        "test thread failed: {:?}",
+        test_res.unwrap_err()
+    );
+
+    // Verify seccomp is working in the parent thread as well
+    check_getpid_fails();
+}