Fix ownership change logic

Previously fsgofer was skipping chown call if the uid and gid
were the same as the current user/group. However, when setgid
is set, the group may not be the same as the caller. Instead,
compare the actual uid/gid of the file after it has been
created and change ownership only if needed.

Updates #180

PiperOrigin-RevId: 353118733

2 files changed, 112 insertions, 34 deletions

diff --git a/runsc/fsgofer/fsgofer.go b/runsc/fsgofer/fsgofer.go
index c3bba0973..f25681233 100644
--- a/runsc/fsgofer/fsgofer.go
+++ b/runsc/fsgofer/fsgofer.go
@@ -49,13 +49,6 @@ const (
 	allowedOpenFlags = unix.O_TRUNC
 )
 
-var (
-	// Remember the process uid/gid to skip chown calls when file owner/group
-	// doesn't need to be changed.
-	processUID = p9.UID(os.Getuid())
-	processGID = p9.GID(os.Getgid())
-)
-
 // join is equivalent to path.Join() but skips path.Clean() which is expensive.
 func join(parent, child string) string {
 	if child == "." || child == ".." {
@@ -377,6 +370,23 @@ func fchown(fd int, uid p9.UID, gid p9.GID) error {
 	return unix.Fchownat(fd, "", int(uid), int(gid), linux.AT_EMPTY_PATH|unix.AT_SYMLINK_NOFOLLOW)
 }
 
+func setOwnerIfNeeded(fd int, uid p9.UID, gid p9.GID) (unix.Stat_t, error) {
+	stat, err := fstat(fd)
+	if err != nil {
+		return unix.Stat_t{}, err
+	}
+
+	// Change ownership if not set accordinly.
+	if uint32(uid) != stat.Uid || uint32(gid) != stat.Gid {
+		if err := fchown(fd, uid, gid); err != nil {
+			return unix.Stat_t{}, err
+		}
+		stat.Uid = uint32(uid)
+		stat.Gid = uint32(gid)
+	}
+	return stat, nil
+}
+
 // Open implements p9.File.
 func (l *localFile) Open(flags p9.OpenFlags) (*fd.FD, p9.QID, uint32, error) {
 	if l.isOpen() {
@@ -457,12 +467,7 @@ func (l *localFile) Create(name string, p9Flags p9.OpenFlags, perm p9.FileMode,
 	})
 	defer cu.Clean()
 
-	if uid != processUID || gid != processGID {
-		if err := fchown(child.FD(), uid, gid); err != nil {
-			return nil, nil, p9.QID{}, 0, extractErrno(err)
-		}
-	}
-	stat, err := fstat(child.FD())
+	stat, err := setOwnerIfNeeded(child.FD(), uid, gid)
 	if err != nil {
 		return nil, nil, p9.QID{}, 0, extractErrno(err)
 	}
@@ -505,12 +510,7 @@ func (l *localFile) Mkdir(name string, perm p9.FileMode, uid p9.UID, gid p9.GID)
 	}
 	defer f.Close()
 
-	if uid != processUID || gid != processGID {
-		if err := fchown(f.FD(), uid, gid); err != nil {
-			return p9.QID{}, extractErrno(err)
-		}
-	}
-	stat, err := fstat(f.FD())
+	stat, err := setOwnerIfNeeded(f.FD(), uid, gid)
 	if err != nil {
 		return p9.QID{}, extractErrno(err)
 	}
@@ -900,12 +900,7 @@ func (l *localFile) Symlink(target, newName string, uid p9.UID, gid p9.GID) (p9.
 	}
 	defer f.Close()
 
-	if uid != processUID || gid != processGID {
-		if err := fchown(f.FD(), uid, gid); err != nil {
-			return p9.QID{}, extractErrno(err)
-		}
-	}
-	stat, err := fstat(f.FD())
+	stat, err := setOwnerIfNeeded(f.FD(), uid, gid)
 	if err != nil {
 		return p9.QID{}, extractErrno(err)
 	}
@@ -959,12 +954,7 @@ func (l *localFile) Mknod(name string, mode p9.FileMode, _ uint32, _ uint32, uid
 	}
 	defer child.Close()
 
-	if uid != processUID || gid != processGID {
-		if err := fchown(child.FD(), uid, gid); err != nil {
-			return p9.QID{}, extractErrno(err)
-		}
-	}
-	stat, err := fstat(child.FD())
+	stat, err := setOwnerIfNeeded(child.FD(), uid, gid)
 	if err != nil {
 		return p9.QID{}, extractErrno(err)
 	}
diff --git a/runsc/fsgofer/fsgofer_test.go b/runsc/fsgofer/fsgofer_test.go
index c5daebe5e..99ea9bd32 100644
--- a/runsc/fsgofer/fsgofer_test.go
+++ b/runsc/fsgofer/fsgofer_test.go
@@ -32,6 +32,9 @@ import (
 	"gvisor.dev/gvisor/runsc/specutils"
 )
 
+// Nodoby is the standard UID/GID for the nobody user/group.
+const nobody = 65534
+
 var allOpenFlags = []p9.OpenFlags{p9.ReadOnly, p9.WriteOnly, p9.ReadWrite}
 
 var (
@@ -281,6 +284,92 @@ func TestCreate(t *testing.T) {
 	})
 }
 
+func checkIDs(f p9.File, uid, gid int) error {
+	_, _, stat, err := f.GetAttr(p9.AttrMask{UID: true, GID: true})
+	if err != nil {
+		return fmt.Errorf("GetAttr() failed, err: %v", err)
+	}
+	if want := p9.UID(uid); stat.UID != want {
+		return fmt.Errorf("Wrong UID, want: %v, got: %v", want, stat.UID)
+	}
+	if want := p9.GID(gid); stat.GID != want {
+		return fmt.Errorf("Wrong GID, want: %v, got: %v", want, stat.GID)
+	}
+	return nil
+}
+
+// TestCreateSetGID checks files/dirs/symlinks are created with the proper
+// owner when the parent directory has setgid set,
+func TestCreateSetGID(t *testing.T) {
+	if !specutils.HasCapabilities(capability.CAP_CHOWN) {
+		t.Skipf("Test requires CAP_CHOWN")
+	}
+
+	runCustom(t, []uint32{unix.S_IFDIR}, rwConfs, func(t *testing.T, s state) {
+		// Change group and set setgid to the parent dir.
+		if err := unix.Chown(s.file.hostPath, os.Getuid(), nobody); err != nil {
+			t.Fatalf("Chown() failed: %v", err)
+		}
+		if err := unix.Chmod(s.file.hostPath, 02777); err != nil {
+			t.Fatalf("Chmod() failed: %v", err)
+		}
+
+		t.Run("create", func(t *testing.T) {
+			_, l, _, _, err := s.file.Create("test", p9.ReadOnly, 0777, p9.UID(os.Getuid()), p9.GID(os.Getgid()))
+			if err != nil {
+				t.Fatalf("WriteAt() failed: %v", err)
+			}
+			defer l.Close()
+			if err := checkIDs(l, os.Getuid(), os.Getgid()); err != nil {
+				t.Error(err)
+			}
+		})
+
+		t.Run("mkdir", func(t *testing.T) {
+			_, err := s.file.Mkdir("test-dir", 0777, p9.UID(os.Getuid()), p9.GID(os.Getgid()))
+			if err != nil {
+				t.Fatalf("WriteAt() failed: %v", err)
+			}
+			_, l, err := s.file.Walk([]string{"test-dir"})
+			if err != nil {
+				t.Fatalf("Walk() failed: %v", err)
+			}
+			defer l.Close()
+			if err := checkIDs(l, os.Getuid(), os.Getgid()); err != nil {
+				t.Error(err)
+			}
+		})
+
+		t.Run("symlink", func(t *testing.T) {
+			if _, err := s.file.Symlink("/some/target", "symlink", p9.UID(os.Getuid()), p9.GID(os.Getgid())); err != nil {
+				t.Fatalf("Symlink() failed: %v", err)
+			}
+			_, l, err := s.file.Walk([]string{"symlink"})
+			if err != nil {
+				t.Fatalf("Walk() failed, err: %v", err)
+			}
+			defer l.Close()
+			if err := checkIDs(l, os.Getuid(), os.Getgid()); err != nil {
+				t.Error(err)
+			}
+		})
+
+		t.Run("mknod", func(t *testing.T) {
+			if _, err := s.file.Mknod("nod", p9.ModeRegular|0777, 0, 0, p9.UID(os.Getuid()), p9.GID(os.Getgid())); err != nil {
+				t.Fatalf("Mknod() failed: %v", err)
+			}
+			_, l, err := s.file.Walk([]string{"nod"})
+			if err != nil {
+				t.Fatalf("Walk() failed, err: %v", err)
+			}
+			defer l.Close()
+			if err := checkIDs(l, os.Getuid(), os.Getgid()); err != nil {
+				t.Error(err)
+			}
+		})
+	})
+}
+
 // TestReadWriteDup tests that a file opened in any mode can be dup'ed and
 // reopened in any other mode.
 func TestReadWriteDup(t *testing.T) {
@@ -458,7 +547,7 @@ func TestSetAttrTime(t *testing.T) {
 }
 
 func TestSetAttrOwner(t *testing.T) {
-	if os.Getuid() != 0 {
+	if !specutils.HasCapabilities(capability.CAP_CHOWN) {
 		t.Skipf("SetAttr(owner) test requires CAP_CHOWN, running as %d", os.Getuid())
 	}
 
@@ -477,7 +566,7 @@ func TestSetAttrOwner(t *testing.T) {
 }
 
 func TestLink(t *testing.T) {
-	if os.Getuid() != 0 {
+	if !specutils.HasCapabilities(capability.CAP_DAC_READ_SEARCH) {
 		t.Skipf("Link test requires CAP_DAC_READ_SEARCH, running as %d", os.Getuid())
 	}
 	runCustom(t, allTypes, rwConfs, func(t *testing.T, s state) {
@@ -995,7 +1084,6 @@ func BenchmarkCreateDiffOwner(b *testing.B) {
 	files := make([]p9.File, 0, 500)
 	fds := make([]*fd.FD, 0, 500)
 	gid := p9.GID(os.Getgid())
-	const nobody = 65534
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {