Preserve inherited target stdin authority

crates/lazaret-linux/src/process.rs

@@ -34,6 +34,62 @@ pub struct TargetCommand {
     stdin: Option<File>,
 }
 
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum TargetStdinAccess {
+    ReadOnly,
+    WriteOnly,
+    ReadWrite,
+}
+
+impl TargetStdinAccess {
+    fn from_flags(flags: OFlags) -> io::Result<Self> {
+        if flags.contains(OFlags::PATH) {
+            return Err(io::Error::other(
+                "target stdin descriptor has no read or write authority",
+            ));
+        }
+        match flags & OFlags::ACCMODE {
+            flags if flags == OFlags::RDONLY => Ok(Self::ReadOnly),
+            flags if flags == OFlags::WRONLY => Ok(Self::WriteOnly),
+            flags if flags == OFlags::RDWR => Ok(Self::ReadWrite),
+            _ => Err(io::Error::other(
+                "target stdin descriptor has an unsupported access mode",
+            )),
+        }
+    }
+
+    fn from_descriptor(descriptor: i32) -> io::Result<Self> {
+        let metadata = std::fs::read_to_string(format!("/proc/self/fdinfo/{descriptor}"))?;
+        let flags = metadata
+            .lines()
+            .find_map(|line| line.strip_prefix("flags:"))
+            .map(str::trim)
+            .ok_or_else(|| io::Error::other("target stdin descriptor flags are missing"))
+            .and_then(|flags| {
+                u32::from_str_radix(flags, 8)
+                    .map(OFlags::from_bits_retain)
+                    .map_err(|_| io::Error::other("target stdin descriptor flags are invalid"))
+            })?;
+        Self::from_flags(flags)
+    }
+
+    fn reopen(self, descriptor: i32) -> io::Result<File> {
+        let mut options = File::options();
+        match self {
+            Self::ReadOnly => {
+                options.read(true);
+            }
+            Self::WriteOnly => {
+                options.write(true);
+            }
+            Self::ReadWrite => {
+                options.read(true).write(true);
+            }
+        }
+        options.open(format!("/proc/self/fd/{descriptor}"))
+    }
+}
+
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum ParentEdge {
     ParentDeathKill,
@@ -93,6 +149,7 @@ impl TargetCommand {
     /// # Errors
     /// Returns an error when the descriptor cannot be inherited by the wrapper.
     pub fn with_target_stdin(mut self, stdin: File) -> io::Result<Self> {
+        TargetStdinAccess::from_flags(fcntl_getfl(&stdin)?)?;
         fcntl_setfd(stdin.as_fd(), FdFlags::empty())?;
         self.stdin = Some(stdin);
         Ok(self)
@@ -1373,15 +1430,11 @@ pub fn parse_wrapper_arguments(
             .and_then(|value| value.to_str().and_then(|value| value.parse::<i32>().ok()))
             .filter(|descriptor| *descriptor > 2)
             .ok_or_else(|| io::Error::other("wrapper target stdin descriptor is invalid"))?;
+        let access = TargetStdinAccess::from_descriptor(descriptor)?;
         if arguments.next().as_deref() != Some(OsStr::new("--")) {
             return Err(io::Error::other("wrapper target separator is missing"));
         }
-        Some(
-            File::options()
-                .read(true)
-                .write(true)
-                .open(format!("/proc/self/fd/{descriptor}"))?,
-        )
+        Some(access.reopen(descriptor)?)
     } else if next.as_deref() == Some(OsStr::new("--")) {
         None
     } else {

crates/lazaret-linux/tests/acquisition.rs

@@ -1,5 +1,5 @@
 use std::ffi::OsString;
-use std::fs::File;
+use std::fs::{File, OpenOptions};
 use std::io::{Read, Seek, Write};
 use std::os::fd::AsFd;
 use std::process::{ExitCode, Stdio};
@@ -50,6 +50,7 @@ fn run_all() -> std::io::Result<()> {
     successful_acquisition()?;
     transient_captured_child_preserves_streams_and_exit()?;
     terminal_child_restores_slave_stdin_and_merges_output()?;
+    target_stdin_does_not_gain_write_authority()?;
     acknowledged_transition_precedes_exact_wrapper_release()?;
     failed_acknowledged_transition_refuses_exact_wrapper_release()?;
     lifetime_pipe_descriptor_contract()?;
@@ -61,6 +62,54 @@ fn run_all() -> std::io::Result<()> {
     concurrent_acquisitions_serialize()
 }
 
+fn target_stdin_does_not_gain_write_authority() -> std::io::Result<()> {
+    let path = std::env::temp_dir().join(format!(
+        "lazaret-target-stdin-authority-{}-{}",
+        std::process::id(),
+        SEQUENCE.fetch_add(1, Ordering::Relaxed)
+    ));
+    let mut source = OpenOptions::new()
+        .create_new(true)
+        .write(true)
+        .open(&path)?;
+    source.write_all(b"unchanged")?;
+    drop(source);
+    let stdin = OpenOptions::new().read(true).open(&path)?;
+    let target = TargetCommand::new("/bin/sh")
+        .argument("-c")
+        .argument("printf changed >&0")
+        .with_target_stdin(stdin)?;
+    let expected_parent = current_process_identity()?;
+    let ownership = ProcessOwnershipContext {
+        domain_id: DomainId::new([45; 32]),
+        generation_id: GenerationId::new(46),
+        coordinator_id: CoordinatorId::new([47; 32]),
+        slot: HostProcessOwnershipSlot::SessionTerminalExecClient(
+            lazaret_core::identity::SessionId::new([48; 32]),
+        ),
+    };
+    let process = child_broker().acquire_with_stdio(
+        expected_parent,
+        ownership,
+        &target,
+        ChildStandardIo::silenced(),
+        |_| Ok(()),
+        |staged| Ok(ProcessOwnershipCommitAck::v1(staged)),
+    )?;
+    while !process.has_exited()? {
+        std::thread::yield_now();
+    }
+    let exit = child_broker().reap_status(process)?;
+    let contents = std::fs::read(&path);
+    let _ = std::fs::remove_file(&path);
+    if exit.exit_code == Some(0) || contents? != b"unchanged" {
+        return Err(std::io::Error::other(format!(
+            "read-only target stdin gained write authority: {exit:?}"
+        )));
+    }
+    Ok(())
+}
+
 fn terminal_child_restores_slave_stdin_and_merges_output() -> std::io::Result<()> {
     let master = openpt(OpenptFlags::RDWR | OpenptFlags::NOCTTY | OpenptFlags::CLOEXEC)?;
     grantpt(&master)?;