From ffb90589a58e6d18a1b4c0af36568b31852958d0 Mon Sep 17 00:00:00 2001 From: unclejack Date: Tue, 28 Oct 2014 23:18:45 +0200 Subject: [PATCH] pkg/symlink: avoid following out of scope Docker-DCO-1.1-Signed-off-by: Cristian Staretu (github: unclejack) --- symlink/fs.go | 47 +++++++++---- symlink/fs_test.go | 151 ++++++++++++++++++++++++++++++++++++---- symlink/testdata/fs/j/k | 1 + 3 files changed, 172 insertions(+), 27 deletions(-) create mode 120000 symlink/testdata/fs/j/k diff --git a/symlink/fs.go b/symlink/fs.go index d761732..6ce99c6 100644 --- a/symlink/fs.go +++ b/symlink/fs.go @@ -12,6 +12,12 @@ const maxLoopCounter = 100 // FollowSymlink will follow an existing link and scope it to the root // path provided. +// The role of this function is to return an absolute path in the root +// or normalize to the root if the symlink leads to a path which is +// outside of the root. +// Errors encountered while attempting to follow the symlink in path +// will be reported. +// Normalizations to the root don't constitute errors. func FollowSymlinkInScope(link, root string) (string, error) { root, err := filepath.Abs(root) if err != nil { @@ -60,25 +66,36 @@ func FollowSymlinkInScope(link, root string) (string, error) { } return "", err } - if stat.Mode()&os.ModeSymlink == os.ModeSymlink { - dest, err := os.Readlink(prev) - if err != nil { - return "", err - } - if path.IsAbs(dest) { - prev = filepath.Join(root, dest) - } else { - prev, _ = filepath.Abs(prev) - - if prev = filepath.Join(filepath.Dir(prev), dest); len(prev) < len(root) { - prev = filepath.Join(root, filepath.Base(dest)) - } - } - } else { + // let's break if we're not dealing with a symlink + if stat.Mode()&os.ModeSymlink != os.ModeSymlink { break } + + // process the symlink + dest, err := os.Readlink(prev) + if err != nil { + return "", err + } + + if path.IsAbs(dest) { + prev = filepath.Join(root, dest) + } else { + prev, _ = filepath.Abs(prev) + + dir := filepath.Dir(prev) + prev = filepath.Join(dir, dest) + if dir == root && !strings.HasPrefix(prev, root) { + prev = root + } + if len(prev) < len(root) || (len(prev) == len(root) && prev != root) { + prev = filepath.Join(root, filepath.Base(dest)) + } + } } } + if prev == "/" { + prev = root + } return prev, nil } diff --git a/symlink/fs_test.go b/symlink/fs_test.go index d85fd6d..0e2f948 100644 --- a/symlink/fs_test.go +++ b/symlink/fs_test.go @@ -46,6 +46,7 @@ func TestFollowSymLinkUnderLinkedDir(t *testing.T) { if err != nil { t.Fatal(err) } + defer os.RemoveAll(dir) os.Mkdir(filepath.Join(dir, "realdir"), 0700) os.Symlink("realdir", filepath.Join(dir, "linkdir")) @@ -97,25 +98,151 @@ func TestFollowSymLinkRelativeLink(t *testing.T) { } func TestFollowSymLinkRelativeLinkScope(t *testing.T) { - link := "testdata/fs/a/f" + // avoid letting symlink f lead us out of the "testdata" scope + // we don't normalize because symlink f is in scope and there is no + // information leak + { + link := "testdata/fs/a/f" - rewrite, err := FollowSymlinkInScope(link, "testdata") - if err != nil { - t.Fatal(err) + rewrite, err := FollowSymlinkInScope(link, "testdata") + if err != nil { + t.Fatal(err) + } + + if expected := abs(t, "testdata/test"); expected != rewrite { + t.Fatalf("Expected %s got %s", expected, rewrite) + } } - if expected := abs(t, "testdata/test"); expected != rewrite { - t.Fatalf("Expected %s got %s", expected, rewrite) + // avoid letting symlink f lead us out of the "testdata/fs" scope + // we don't normalize because symlink f is in scope and there is no + // information leak + { + link := "testdata/fs/a/f" + + rewrite, err := FollowSymlinkInScope(link, "testdata/fs") + if err != nil { + t.Fatal(err) + } + + if expected := abs(t, "testdata/fs/test"); expected != rewrite { + t.Fatalf("Expected %s got %s", expected, rewrite) + } } - link = "testdata/fs/b/h" + // avoid letting symlink g (pointed at by symlink h) take out of scope + // TODO: we should probably normalize to scope here because ../[....]/root + // is out of scope and we leak information + { + link := "testdata/fs/b/h" - rewrite, err = FollowSymlinkInScope(link, "testdata") - if err != nil { - t.Fatal(err) + rewrite, err := FollowSymlinkInScope(link, "testdata") + if err != nil { + t.Fatal(err) + } + + if expected := abs(t, "testdata/root"); expected != rewrite { + t.Fatalf("Expected %s got %s", expected, rewrite) + } } - if expected := abs(t, "testdata/root"); expected != rewrite { - t.Fatalf("Expected %s got %s", expected, rewrite) + // avoid letting allowing symlink e lead us to ../b + // normalize to the "testdata/fs/a" + { + link := "testdata/fs/a/e" + + rewrite, err := FollowSymlinkInScope(link, "testdata/fs/a") + if err != nil { + t.Fatal(err) + } + + if expected := abs(t, "testdata/fs/a"); expected != rewrite { + t.Fatalf("Expected %s got %s", expected, rewrite) + } + } + + // avoid letting symlink -> ../directory/file escape from scope + // normalize to "testdata/fs/j" + { + link := "testdata/fs/j/k" + + rewrite, err := FollowSymlinkInScope(link, "testdata/fs/j") + if err != nil { + t.Fatal(err) + } + + if expected := abs(t, "testdata/fs/j"); expected != rewrite { + t.Fatalf("Expected %s got %s", expected, rewrite) + } + } + + // make sure we don't allow escaping to / + // normalize to dir + { + dir, err := ioutil.TempDir("", "docker-fs-test") + if err != nil { + t.Fatal(err) + } + defer os.RemoveAll(dir) + + linkFile := filepath.Join(dir, "foo") + os.Mkdir(filepath.Join(dir, ""), 0700) + os.Symlink("/", linkFile) + + rewrite, err := FollowSymlinkInScope(linkFile, dir) + if err != nil { + t.Fatal(err) + } + + if rewrite != dir { + t.Fatalf("Expected %s got %s", dir, rewrite) + } + } + + // make sure we don't allow escaping to / + // normalize to dir + { + dir, err := ioutil.TempDir("", "docker-fs-test") + if err != nil { + t.Fatal(err) + } + defer os.RemoveAll(dir) + + linkFile := filepath.Join(dir, "foo") + os.Mkdir(filepath.Join(dir, ""), 0700) + os.Symlink("/../../", linkFile) + + rewrite, err := FollowSymlinkInScope(linkFile, dir) + if err != nil { + t.Fatal(err) + } + + if rewrite != dir { + t.Fatalf("Expected %s got %s", dir, rewrite) + } + } + + // make sure we stay in scope without leaking information + // this also checks for escaping to / + // normalize to dir + { + dir, err := ioutil.TempDir("", "docker-fs-test") + if err != nil { + t.Fatal(err) + } + defer os.RemoveAll(dir) + + linkFile := filepath.Join(dir, "foo") + os.Mkdir(filepath.Join(dir, ""), 0700) + os.Symlink("../../", linkFile) + + rewrite, err := FollowSymlinkInScope(linkFile, dir) + if err != nil { + t.Fatal(err) + } + + if rewrite != dir { + t.Fatalf("Expected %s got %s", dir, rewrite) + } } } diff --git a/symlink/testdata/fs/j/k b/symlink/testdata/fs/j/k new file mode 120000 index 0000000..f559e8f --- /dev/null +++ b/symlink/testdata/fs/j/k @@ -0,0 +1 @@ +../i/a \ No newline at end of file