Handle UMOUNT_NOFOLLOW in VFS2 umount(2).

Reported-by: syzbot+f9ecb181a4b3abdde9b9@syzkaller.appspotmail.com
Reported-by: syzbot+8c5cb9d7a044a91a513b@syzkaller.appspotmail.com
PiperOrigin-RevId: 406951359

2 files changed, 36 insertions, 2 deletions

diff --git a/pkg/sentry/syscalls/linux/vfs2/mount.go b/pkg/sentry/syscalls/linux/vfs2/mount.go
index 4d73d46ef..fd0ab4c76 100644
--- a/pkg/sentry/syscalls/linux/vfs2/mount.go
+++ b/pkg/sentry/syscalls/linux/vfs2/mount.go
@@ -136,14 +136,14 @@ func Umount2(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysca
 	if err != nil {
 		return 0, nil, err
 	}
-	tpop, err := getTaskPathOperation(t, linux.AT_FDCWD, path, disallowEmptyPath, nofollowFinalSymlink)
+	tpop, err := getTaskPathOperation(t, linux.AT_FDCWD, path, disallowEmptyPath, shouldFollowFinalSymlink(flags&linux.UMOUNT_NOFOLLOW == 0))
 	if err != nil {
 		return 0, nil, err
 	}
 	defer tpop.Release(t)
 
 	opts := vfs.UmountOptions{
-		Flags: uint32(flags),
+		Flags: uint32(flags &^ linux.UMOUNT_NOFOLLOW),
 	}
 
 	return 0, nil, t.Kernel().VFS().UmountAt(t, creds, &tpop.pop, &opts)
diff --git a/test/syscalls/linux/mount.cc b/test/syscalls/linux/mount.cc
index 3c7311782..e2a41d172 100644
--- a/test/syscalls/linux/mount.cc
+++ b/test/syscalls/linux/mount.cc
@@ -115,6 +115,40 @@ TEST(MountTest, OpenFileBusy) {
   EXPECT_THAT(umount(dir.path().c_str()), SyscallFailsWithErrno(EBUSY));
 }
 
+TEST(MountTest, UmountNoFollow) {
+  SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
+
+  auto const dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
+
+  auto const mountPoint = NewTempAbsPathInDir(dir.path());
+  ASSERT_THAT(mkdir(mountPoint.c_str(), 0777), SyscallSucceeds());
+
+  // Create a symlink in dir which will point to the actual mountpoint.
+  const std::string symlinkInDir = NewTempAbsPathInDir(dir.path());
+  EXPECT_THAT(symlink(mountPoint.c_str(), symlinkInDir.c_str()),
+              SyscallSucceeds());
+
+  // Create a symlink to the dir.
+  const std::string symlinkToDir = NewTempAbsPath();
+  EXPECT_THAT(symlink(dir.path().c_str(), symlinkToDir.c_str()),
+              SyscallSucceeds());
+
+  // Should fail with ELOOP when UMOUNT_NOFOLLOW is specified and the last
+  // component is a symlink.
+  auto mount = ASSERT_NO_ERRNO_AND_VALUE(
+      Mount("", mountPoint, "tmpfs", 0, "mode=0700", 0));
+  EXPECT_THAT(umount2(symlinkInDir.c_str(), UMOUNT_NOFOLLOW),
+              SyscallFailsWithErrno(EINVAL));
+  EXPECT_THAT(unlink(symlinkInDir.c_str()), SyscallSucceeds());
+
+  // UMOUNT_NOFOLLOW should only apply to the last path component. A symlink in
+  // non-last path component should be just fine.
+  EXPECT_THAT(umount2(JoinPath(symlinkToDir, Basename(mountPoint)).c_str(),
+                      UMOUNT_NOFOLLOW),
+              SyscallSucceeds());
+  mount.Release();
+}
+
 TEST(MountTest, UmountDetach) {
   SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));