Fix SafeTarFile symlinks extraction

tests/integration/archive/tar/__output__/absolute.tar_extract/absolute/absolute-symlink

@@ -1 +1 @@
-absolute/absolute-file
+absolute-file

tests/integration/archive/tar/__output__/absolute.tar_extract/absolute/link_to_etc_shadow

@@ -1 +1 @@
-etc/shadow
+../etc/shadow

tests/integration/archive/tar/__output__/symlinks.tar_extract/test/sbin/cp

@@ -0,0 +1 @@
+../bin/busybox

tests/integration/archive/tar/__output__/symlinks.tar_extract/test/sbin/find

@@ -0,0 +1 @@
+../bin/busybox

tests/integration/archive/tar/__output__/symlinks.tar_extract/test/sbin/ln

@@ -0,0 +1 @@
+../bin/busybox

tests/integration/archive/tar/__output__/symlinks.tar_extract/test/sbin/ls

@@ -0,0 +1 @@
+../bin/busybox

tests/integration/archive/tar/__output__/symlinks.tar_extract/test/sbin/mv

@@ -0,0 +1 @@
+../../bin/busybox

tests/test_file_utils.py

@@ -25,7 +25,7 @@
     round_down,
     round_up,
 )
-from unblob.report import PathTraversalProblem
+from unblob.report import LinkExtractionProblem, PathTraversalProblem
 
 
 @pytest.mark.parametrize(
@@ -503,6 +503,30 @@ def test_create_symlink(self, sandbox: FileSystem):
         assert os.readlink(output_path) == "target file"
         assert sandbox.problems == []
 
+    def test_create_symlink_target_inside_sandbox(self, sandbox: FileSystem):
+        # ./sbin/shell -> ../bin/sh
+        sandbox.mkdir(Path("bin"))
+        sandbox.write_bytes(Path("bin/sh"), b"posix shell")
+        sandbox.mkdir(Path("sbin"))
+        sandbox.create_symlink(Path("../bin/sh"), Path("sbin/shell"))
+
+        output_path = sandbox.root / "sbin/shell"
+        assert output_path.read_bytes() == b"posix shell"
+        assert output_path.exists()
+        assert os.readlink(output_path) == "../bin/sh"
+        assert sandbox.problems == []
+
+    def test_create_symlink_target_outside_sandbox(self, sandbox: FileSystem):
+        # /shell -> ../bin/sh
+        sandbox.mkdir(Path("bin"))
+        sandbox.write_bytes(Path("bin/sh"), b"posix shell")
+        sandbox.create_symlink(Path("../bin/sh"), Path("/shell"))
+
+        assert any(p for p in sandbox.problems if isinstance(p, LinkExtractionProblem))
+        output_path = sandbox.root / "shell"
+        assert not output_path.exists()
+        assert not output_path.is_symlink()
+
     def test_create_symlink_absolute_paths(self, sandbox: FileSystem):
         sandbox.write_bytes(Path("target file"), b"test content")
         sandbox.create_symlink(Path("/target file"), Path("/symlink"))

unblob/handlers/archive/_safe_tarfile.py

@@ -63,7 +63,7 @@ def extract(self, tarinfo: tarfile.TarInfo, extract_root: Path):  # noqa: C901
                 "Absolute path.",
                 "Converted to extraction relative path.",
             )
-            tarinfo.name = f"./{tarinfo.name}"
+            tarinfo.name = str(Path(tarinfo.name).relative_to("/"))
 
         # prevent traversal attempts through file name
         if not is_safe_path(basedir=extract_root, path=extract_root / tarinfo.name):
@@ -77,16 +77,36 @@ def extract(self, tarinfo: tarfile.TarInfo, extract_root: Path):  # noqa: C901
         # prevent traversal attempts through links
         if tarinfo.islnk() or tarinfo.issym():
             if Path(tarinfo.linkname).is_absolute():
+
+                def calculate_linkname():
+                    root = extract_root.resolve()
+                    path = (extract_root / tarinfo.name).resolve()
+
+                    if path.parts[: len(root.parts)] != root.parts:
+                        return None
+
+                    depth = max(0, len(path.parts) - len(root.parts) - 1)
+                    return ("/".join([".."] * depth) or ".") + tarinfo.linkname
+
+                relative_linkname = calculate_linkname()
+                if relative_linkname is None:
+                    self.record_problem(
+                        tarinfo,
+                        "Absolute path conversion to extraction relative failed - would escape root.",
+                        "Skipped.",
+                    )
+                    return
+
+                assert not Path(relative_linkname).is_absolute()
                 self.record_problem(
                     tarinfo,
                     "Absolute path as link target.",
                     "Converted to extraction relative path.",
                 )
-                tarinfo.linkname = f"./{tarinfo.linkname}"
-            if not is_safe_path(
-                basedir=extract_root,
-                path=extract_root / tarinfo.linkname,
-            ):
+                tarinfo.linkname = relative_linkname
+
+            resolved_path = (extract_root / tarinfo.name).parent / tarinfo.linkname
+            if not is_safe_path(basedir=extract_root, path=resolved_path):
                 self.record_problem(
                     tarinfo,
                     "Traversal attempt through link path.",