[PATCH 4/4] Allow unprivileged chroot when safe

[Andy Lutomirski <luto@amacapital.net>]

Chroot can easily be used to subvert setuid programs.  If no_new_privs,
then setuid programs don't gain any privilege, so allow chroot.

Because chroot is an easy way to break out of chroot jail, CAP_SYS_ADMIN
is still required if the caller is already chrooted.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 fs/open.c |   16 ++++++++++++++--
 1 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index f711921..80ca7e2 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -422,6 +422,8 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
 {
 	struct path path;
 	int error;
+	struct fs_struct *fs = current->fs;
+	bool is_chrooted;
 
 	error = user_path_dir(filename, &path);
 	if (error)
@@ -432,13 +434,23 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
 		goto dput_and_out;
 
 	error = -EPERM;
-	if (!capable(CAP_SYS_CHROOT))
+	/*
+	 * Chroot is dangerous unless no_new_privs is set.  But we also
+	 * don't want to allow unprivileged users to break out of chroot
+	 * jail with another chroot call, so we require either CAP_SYS_CHROOT
+	 * unless we're not chrooted already and we have no_new_privs.
+	 */
+	is_chrooted = (fs->root.mnt->mnt_mountpoint !=
+		       fs->root.mnt->mnt_parent->mnt_root ||
+		       fs->root.dentry != fs->root.mnt->mnt_root);
+	if (!(current->no_new_privs && !is_chrooted) &&
+	    !capable(CAP_SYS_CHROOT))
 		goto dput_and_out;
 	error = security_path_chroot(&path);
 	if (error)
 		goto dput_and_out;
 
-	set_fs_root(current->fs, &path);
+	set_fs_root(fs, &path);
 	error = 0;
 dput_and_out:
 	path_put(&path);
-- 
1.7.7.5