Verify Git plugin SHA checkouts (#34644)

## Why

Git can interpret a requested commit SHA as a branch name when the remote's
default branch has the same name. This can cause a marketplace plugin source
to materialize a different commit than the one it pinned.

## What changed

Resolve `HEAD` after checking out a SHA-pinned Git plugin source and reject the
source when the resolved commit does not exactly match the requested SHA.
Ref-name checkouts retain their existing behavior.

## Testing

Add a regression test with a default branch named after another commit's SHA
and verify that materialization rejects the mismatched checkout.

GitOrigin-RevId: c19cbd98ee6167dad7b2ee72e283b3bd30b59713

codex-rs/core-plugins/src/loader.rs

@@ -1523,8 +1523,16 @@ fn clone_git_plugin_source(
             /*cwd*/ None,
         )?;
     }
-    if let Some(target) = sha.or(ref_name) {
-        run_git(&["checkout", target], Some(destination))?;
+    if let Some(sha) = sha {
+        run_git(&["checkout", sha], Some(destination))?;
+        let checked_out_sha = run_git_output(&["rev-parse", "HEAD"], Some(destination))?;
+        if !checked_out_sha.eq_ignore_ascii_case(sha) {
+            return Err(format!(
+                "checked out Git SHA {checked_out_sha} does not match requested SHA {sha}"
+            ));
+        }
+    } else if let Some(ref_name) = ref_name {
+        run_git(&["checkout", ref_name], Some(destination))?;
     } else if sparse_checkout_path.is_some() {
         run_git(&["checkout"], Some(destination))?;
     }
@@ -1532,6 +1540,10 @@ fn clone_git_plugin_source(
 }
 
 fn run_git(args: &[&str], cwd: Option<&Path>) -> Result<(), String> {
+    run_git_output(args, cwd).map(drop)
+}
+
+fn run_git_output(args: &[&str], cwd: Option<&Path>) -> Result<String, String> {
     let mut command = Command::new("git");
     command.args(args);
     command.env("GIT_TERMINAL_PROMPT", "0");
@@ -1543,7 +1555,7 @@ fn run_git(args: &[&str], cwd: Option<&Path>) -> Result<(), String> {
         .output()
         .map_err(|err| format!("failed to run git {}: {err}", args.join(" ")))?;
     if output.status.success() {
-        return Ok(());
+        return Ok(String::from_utf8_lossy(&output.stdout).trim().to_string());
     }
 
     Err(format!(

codex-rs/core-plugins/src/loader_tests.rs

@@ -474,6 +474,7 @@ fn materialize_git_subdir_uses_sparse_checkout() {
     run_git(&["config", "user.name", "Test User"], Some(repo.path())).expect("configure git name");
     run_git(&["add", "."], Some(repo.path())).expect("stage git repo");
     run_git(&["commit", "-m", "init"], Some(repo.path())).expect("commit git repo");
+    let sha = run_git_output(&["rev-parse", "HEAD"], Some(repo.path())).expect("resolve commit");
 
     let materialized = materialize_marketplace_plugin_source(
         codex_home.path(),
@@ -481,7 +482,7 @@ fn materialize_git_subdir_uses_sparse_checkout() {
             url: repo.path().display().to_string(),
             path: Some("plugins/toolkit".to_string()),
             ref_name: None,
-            sha: None,
+            sha: Some(sha),
         },
     )
     .expect("materialize git source");
@@ -500,3 +501,46 @@ fn materialize_git_subdir_uses_sparse_checkout() {
     assert!(!checkout_root.join("root.txt").exists());
     assert!(!checkout_root.join("plugins/other/marker.txt").exists());
 }
+
+#[test]
+fn materialize_git_source_rejects_sha_that_resolves_to_hostile_default_branch() {
+    let codex_home = tempfile::tempdir().expect("create codex home");
+    let repo = tempfile::tempdir().expect("create git repo");
+    run_git(&["init"], Some(repo.path())).expect("init git repo");
+    run_git(
+        &["config", "user.email", "test@example.com"],
+        Some(repo.path()),
+    )
+    .expect("configure git email");
+    run_git(&["config", "user.name", "Test User"], Some(repo.path())).expect("configure git name");
+
+    fs::write(repo.path().join("marker.txt"), "benign").expect("write benign marker");
+    run_git(&["add", "."], Some(repo.path())).expect("stage git repo");
+    run_git(&["commit", "-m", "benign"], Some(repo.path())).expect("commit benign revision");
+    let benign_sha =
+        run_git_output(&["rev-parse", "HEAD"], Some(repo.path())).expect("resolve commit A");
+
+    fs::write(repo.path().join("marker.txt"), "malicious").expect("write malicious marker");
+    run_git(&["add", "."], Some(repo.path())).expect("stage malicious revision");
+    run_git(&["commit", "-m", "malicious"], Some(repo.path())).expect("commit malicious revision");
+    let malicious_sha =
+        run_git_output(&["rev-parse", "HEAD"], Some(repo.path())).expect("resolve commit B");
+    run_git(&["branch", "-m", &benign_sha], Some(repo.path()))
+        .expect("name default branch after commit A");
+
+    let err = materialize_marketplace_plugin_source(
+        codex_home.path(),
+        &MarketplacePluginSource::Git {
+            url: repo.path().display().to_string(),
+            path: None,
+            ref_name: None,
+            sha: Some(benign_sha.clone()),
+        },
+    )
+    .expect_err("hostile default branch must not satisfy SHA pinning");
+
+    assert_eq!(
+        err,
+        format!("checked out Git SHA {malicious_sha} does not match requested SHA {benign_sha}")
+    );
+}