fix: reject dangling symlinks escaping sandbox in FsSandbox::resolve (#31)

* fix: reject dangling symlinks escaping sandbox in FsSandbox::resolve

Signed-off-by: danbugs <danilochiarlone@gmail.com>

* fix: follow symlink chains with hop limit, improve test naming and coverage

Signed-off-by: danbugs <danilochiarlone@gmail.com>

---------

Signed-off-by: danbugs <danilochiarlone@gmail.com>

Author: danbugs

host/src/lib.rs

@@ -767,6 +767,41 @@ impl FsSandbox {
         let mut out = resolved_ancestor;
         for name in tail.into_iter().rev() {
             out.push(name);
+            // Walk the symlink chain (with hop limit) to catch escapes
+            // through dangling or chained symlinks.
+            const MAX_SYMLINK_HOPS: usize = 40;
+            let mut cursor = out.clone();
+            for _ in 0..MAX_SYMLINK_HOPS {
+                let Ok(meta) = std::fs::symlink_metadata(&cursor) else {
+                    break;
+                };
+                if !meta.file_type().is_symlink() {
+                    break;
+                }
+                let target = std::fs::read_link(&cursor)?;
+                let abs = if target.is_absolute() {
+                    target
+                } else {
+                    cursor.parent().unwrap_or(&self.root).join(&target)
+                };
+                let mut norm = std::path::PathBuf::new();
+                for c in abs.components() {
+                    match c {
+                        std::path::Component::ParentDir => {
+                            norm.pop();
+                        }
+                        std::path::Component::CurDir => {}
+                        c => norm.push(c),
+                    }
+                }
+                if !norm.starts_with(&self.root) {
+                    return Err(anyhow!(
+                        "symlink target escapes mount root: {:?}",
+                        guest_path
+                    ));
+                }
+                cursor = norm;
+            }
         }
         Ok(out)
     }
@@ -2488,6 +2523,84 @@ mod tests {
         assert!(err.contains("escapes mount root"), "{err}");
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn resolve_rejects_dangling_symlink_escape() {
+        use std::os::unix::fs::symlink;
+        let root = tmpdir("dangling-escape");
+        let outside = tmpdir("dangling-escape-out");
+        symlink(outside.join("nonexistent"), root.join("bad_link")).unwrap();
+        let fs_sb = FsSandbox::new(&root).unwrap();
+        let err = fs_sb.resolve("bad_link").unwrap_err().to_string();
+        assert!(
+            err.contains("escapes mount root"),
+            "expected escape error, got: {err}"
+        );
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn resolve_allows_valid_internal_symlink() {
+        use std::os::unix::fs::symlink;
+        let root = tmpdir("valid-internal");
+        fs::write(root.join("real_file.txt"), "hello").unwrap();
+        symlink(root.join("real_file.txt"), root.join("good_link")).unwrap();
+        let fs_sb = FsSandbox::new(&root).unwrap();
+        let resolved = fs_sb.resolve("good_link").unwrap();
+        assert!(
+            resolved.starts_with(&root),
+            "expected path under root, got: {resolved:?}"
+        );
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn resolve_allows_dangling_symlink_inside_root() {
+        use std::os::unix::fs::symlink;
+        let root = tmpdir("dangling-inside");
+        symlink(root.join("future_file.txt"), root.join("ok_link")).unwrap();
+        let fs_sb = FsSandbox::new(&root).unwrap();
+        let resolved = fs_sb.resolve("ok_link").unwrap();
+        assert!(
+            resolved.starts_with(&root),
+            "expected path under root, got: {resolved:?}"
+        );
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn resolve_rejects_symlink_chain_escape() {
+        use std::os::unix::fs::symlink;
+        let root = tmpdir("chain-escape");
+        let outside = tmpdir("chain-outside");
+        symlink(&outside, root.join("link_b")).unwrap();
+        symlink(root.join("link_b"), root.join("link_a")).unwrap();
+        let fs_sb = FsSandbox::new(&root).unwrap();
+        let err = fs_sb.resolve("link_a").unwrap_err().to_string();
+        assert!(
+            err.contains("escapes mount root"),
+            "expected escape error, got: {err}"
+        );
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn resolve_rejects_chained_dangling_symlink_escape() {
+        use std::os::unix::fs::symlink;
+        let root = tmpdir("chain-dangling");
+        let outside = tmpdir("chain-dangling-out");
+        // link_b -> dangling path outside root
+        symlink(outside.join("nonexistent"), root.join("link_b")).unwrap();
+        // link_a -> link_b (which is under root, but chains outside)
+        symlink(root.join("link_b"), root.join("link_a")).unwrap();
+        let fs_sb = FsSandbox::new(&root).unwrap();
+        let err = fs_sb.resolve("link_a").unwrap_err().to_string();
+        assert!(
+            err.contains("escapes mount root"),
+            "expected escape error, got: {err}"
+        );
+    }
+
     #[test]
     fn resolve_allows_paths_under_the_root() {
         let root = tmpdir("allow");