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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
//! Linux process spawning.
use super::Builder;
use super::Child;
use super::FdOp;
use super::SandboxFailureMode;
use crate::unix::errno;
use crate::unix::SyscallResult;
use caps::CapsHashSet;
use landlock::RulesetCreated;
use seccompiler::SeccompFilter;
use std::ffi::CStr;
use std::ffi::CString;
use std::io;
use std::os::unix::prelude::*;
struct CloneContext<'a> {
executable: &'a CStr,
argv: &'a [*const libc::c_char],
envp: &'a [*const libc::c_char],
result: Option<i32>,
// TODO: refactor this to contain BorrowedFds
fd_ops: &'a mut [(i32, FdOp)],
sandbox_failure_mode: SandboxFailureMode,
setsid: bool,
controlling_terminal: Option<BorrowedFd<'a>>,
uid: Option<libc::uid_t>,
gid: Option<libc::uid_t>,
permitted_capabilities: Option<CapsHashSet>,
effective_capabilities: Option<CapsHashSet>,
ambient_capabilities: Option<CapsHashSet>,
inheritable_capabilities: Option<CapsHashSet>,
bounding_capabilities: Option<CapsHashSet>,
landlock_rules: Option<RulesetCreated>,
seccomp_filter: Option<SeccompFilter>,
}
impl Builder<'_> {
pub(super) fn spawn_internal(
&self,
envp: &[CString],
fd_ops: &mut [(i32, FdOp)],
) -> io::Result<Child> {
let mut landlock_rules = None;
if let Some(lr) = &self.linux_builder.landlock_rules {
landlock_rules = Some(lr.try_clone()?);
}
// Build the null-terminated arrays for exec.
let argv = super::c_slice_to_pointers(&self.argv);
let envp = super::c_slice_to_pointers(envp);
let mut context = CloneContext {
executable: &self.executable,
argv: &argv,
envp: &envp,
result: None,
fd_ops: &mut *fd_ops,
sandbox_failure_mode: self.linux_builder.sandbox_failure_mode,
setsid: self.linux_builder.setsid,
controlling_terminal: self.linux_builder.controlling_terminal,
uid: self.uid,
gid: self.gid,
permitted_capabilities: self.linux_builder.permitted_capabilities.clone(),
effective_capabilities: self.linux_builder.effective_capabilities.clone(),
inheritable_capabilities: self.linux_builder.inheritable_capabilities.clone(),
ambient_capabilities: self.linux_builder.ambient_capabilities.clone(),
bounding_capabilities: self.linux_builder.bounding_capabilities.clone(),
landlock_rules,
seccomp_filter: self.linux_builder.seccomp_filter.clone(),
};
// Use CLONE_VM and CLONE_VFORK so that the new process will share the
// current address space and will block this thread until it either
// exits or calls exec.
//
// Use CLONE_PIDFD to get an fd back to use for polling.
let mut flags = self.linux_builder.clone_flags | libc::CLONE_PIDFD | libc::SIGCHLD;
if self.linux_builder.vfork {
flags |= libc::CLONE_VM | libc::CLONE_VFORK;
}
// SAFETY: sysconf has no safety requirements.
let page_size = unsafe { libc::sysconf(libc::_SC_PAGESIZE) } as usize;
// Common page sizes are 4KiB, 16KiB, and 64KiB. The stack size must be a multiple
// of the page size.
let stack_len: usize = std::cmp::max(16 * 1024, page_size);
assert!(stack_len % page_size == 0);
// Create a stack with one guard page.
let stack_len = stack_len + page_size;
// SAFETY: creating a new mapping, which has no safety requirements.
let stack = unsafe {
libc::mmap(
std::ptr::null_mut(),
stack_len,
libc::PROT_READ | libc::PROT_WRITE,
libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
-1,
0,
)
};
if stack == libc::MAP_FAILED {
return Err(errno().into());
}
let mmap_guard = ChildStackGuard(stack, stack_len);
// SAFETY: The stack has been checked to be valid, and its length is more than one page.
unsafe { libc::mprotect(stack, page_size, libc::PROT_NONE) }.syscall_result()?;
let mut pidfd: libc::pid_t = -1;
// SAFETY: The stack is valid for stack len, if the child goes off the
// stack they'll hit our guard page, the flags include PIDFD so passing
// pidfd is valid, and clone_cb takes a CloneContext pointer as its only
// argument.
let pid = unsafe {
libc::clone(
clone_cb,
stack.add(stack_len),
flags,
std::ptr::from_mut(&mut context).cast(),
&mut pidfd,
)
}
.syscall_result()?;
drop(mmap_guard);
// SAFETY: We set the PIDFD flag, and clone returned successfully, so pidfd is now valid.
let pidfd = unsafe { OwnedFd::from_raw_fd(pidfd) };
let mut child = Child {
pid,
pidfd,
status: None,
};
// This can only be done if we are vforking, without sharing another
// type of status object we can't determine if the execve failed or
// the process failed during early initialization.
if self.linux_builder.vfork && context.result != Some(0) {
// The new process failed without successfully calling execve. Reap
// it and return the associated error code (which may come from
// context or from the exit code).
let status = child.wait().unwrap();
let ec = context.result.unwrap_or_else(|| {
status
.code()
.expect("child should not have failed with a signal")
});
return Err(io::Error::from_raw_os_error(ec));
}
Ok(child)
}
}
struct ChildStackGuard(*mut libc::c_void, usize);
impl Drop for ChildStackGuard {
fn drop(&mut self) {
// SAFETY: We know the pointer is valid and the length is correct at
// construction, and we know the child is not running anymore, so it's
// safe to unmap the stack.
unsafe { libc::munmap(self.0, self.1) }
.syscall_result()
.unwrap();
}
}
/// Runs in the cloned process to set up the process environment and exec the
/// new binary.
///
/// This function must not use the heap or call any functions that might. It
/// also has only a small amount of stack space available. It should avoid using
/// OS functionality via the std crate and should use libc directly.
///
/// Returns the exit code for the new process. If this function does not update
/// context's result, then the exit code will be the Linux errno value
/// associated with the error.
//
// N.B. this should be unsafe but the libc crate neglected to mark the clone
// callback appropriately.
extern "C" fn clone_cb(context: *mut libc::c_void) -> libc::c_int {
// SAFETY: Context is temporarily owned by this function, and we know
// we were passed a valid pointer.
let context = unsafe { &mut *(context.cast::<CloneContext<'_>>()) };
if context.setsid {
// SAFETY: setsid has no safety requirements.
if unsafe { libc::setsid() } < 0 {
return errno().0;
}
}
if let Some(fd) = context.controlling_terminal {
// SAFETY: fd is guaranteed to be valid.
if unsafe { libc::ioctl(fd.as_raw_fd(), libc::TIOCSCTTY, 0) } < 0 {
return errno().0;
}
}
// Find the maximum newfd, needed below.
let maxfd = context.fd_ops.iter().map(|(fd, _)| *fd).max();
if let Some(maxfd) = maxfd {
for (newfd, op) in &mut *context.fd_ops {
match op {
FdOp::Close => {}
FdOp::Dup(oldfd) => {
// Ensure oldfd is above the maximum newfd. This is
// necessary to ensure that another operation does not close
// an oldfd targeted by this operation.
if oldfd != newfd && *oldfd < maxfd {
// SAFETY: fd is guaranteed to be valid
let new_oldfd =
unsafe { libc::fcntl(*oldfd, libc::F_DUPFD_CLOEXEC, maxfd) };
if new_oldfd < 0 {
return errno().0;
}
*oldfd = new_oldfd;
}
}
}
}
for (newfd, op) in &*context.fd_ops {
match op {
FdOp::Close => {
// SAFETY: fd is guaranteed to be valid
if unsafe { libc::close(*newfd) } < 0 {
return errno().0;
}
}
FdOp::Dup(oldfd) => {
if *newfd == *oldfd {
// SAFETY: fd is guaranteed to be valid
if unsafe {
libc::fcntl(
*oldfd,
libc::F_SETFD,
libc::fcntl(*oldfd, libc::F_GETFD) & !libc::FD_CLOEXEC,
)
} < 0
{
return errno().0;
}
} else {
// SAFETY: fds are guaranteed to be valid
if unsafe { libc::dup2(*oldfd, *newfd) } < 0 {
return errno().0;
}
}
}
}
}
}
macro_rules! handle_sandbox_failure {
($m:expr, $r:expr) => {
match context.sandbox_failure_mode {
SandboxFailureMode::Silent => {}
SandboxFailureMode::Warn => {
tracing::warn!($m);
}
SandboxFailureMode::Error => {
tracing::error!($m);
return $r;
}
}
};
}
if let Some(landlock_rules) = context.landlock_rules.take() {
if landlock_rules.restrict_self().is_err() {
handle_sandbox_failure!("failed to apply landlock ruleset", libc::ENOTSUP);
}
}
if let Some(gid) = context.gid {
// SAFETY: setresgid has no safety requirements.
if unsafe { libc::setresgid(gid, gid, gid) } < 0 {
handle_sandbox_failure!("failed to change group id", libc::ENOTSUP);
}
}
if let Some(uid) = context.uid {
// SAFETY: setresuid has no safety requirements.
if unsafe { libc::setresuid(uid, uid, uid) } < 0 {
handle_sandbox_failure!("failed to change user id", libc::ENOTSUP);
}
}
macro_rules! set_capabilities {
($t:expr, $v:ident) => {
if let Some($v) = &context.$v {
if caps::set(None, $t, &$v).is_err() {
handle_sandbox_failure!(
std::concat!("failed to apply ", stringify!($t), " capabilities"),
libc::ENOTSUP
);
}
}
};
}
set_capabilities!(caps::CapSet::Bounding, bounding_capabilities);
set_capabilities!(caps::CapSet::Permitted, permitted_capabilities);
set_capabilities!(caps::CapSet::Ambient, ambient_capabilities);
set_capabilities!(caps::CapSet::Inheritable, inheritable_capabilities);
set_capabilities!(caps::CapSet::Effective, effective_capabilities);
if let Some(seccomp_filter) = context.seccomp_filter.take() {
if let Ok(bpf_program) = TryInto::<seccompiler::BpfProgram>::try_into(seccomp_filter) {
if seccompiler::apply_filter(&bpf_program).is_err() {
handle_sandbox_failure!("failed to apply seccomp profile", libc::ENOTSUP);
}
}
}
// Update the result indicating success in case execvpe does not return.
context.result = Some(0);
// N.B. This will only return on error.
// SAFETY: Arguments in the context are valid CStrings, and the two arrays
// are properly null terminated.
unsafe {
libc::execvpe(
context.executable.as_ptr(),
context.argv.as_ptr(),
context.envp.as_ptr(),
)
};
// Update the result with the failure code.
context.result = Some(errno().0);
255
}
impl AsFd for Child {
fn as_fd(&self) -> BorrowedFd<'_> {
self.pidfd.as_fd()
}
}