Re: [RFC PATCH v3 1/2] mm: restrictedmem: Allow userspace to specify mount for memfd_restricted

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Thanks for reviewing these patches!

"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:

On Fri, Mar 31, 2023 at 11:50:39PM +0000, Ackerley Tng wrote:

...

+static int restrictedmem_create_on_user_mount(int mount_fd)
+{
+	int ret;
+	struct fd f;
+	struct vfsmount *mnt;
+
+	f = fdget_raw(mount_fd);
+	if (!f.file)
+		return -EBADF;
+
+	ret = -EINVAL;
+	if (!is_mount_root(f.file))
+		goto out;
+
+	mnt = f.file->f_path.mnt;
+	if (!is_shmem_mount(mnt))
+		goto out;
+
+	ret = file_permission(f.file, MAY_WRITE | MAY_EXEC);

Why MAY_EXEC?


Christian pointed out that this check does not make sense, I'll be
removing the entire check in the next revision.

+	if (ret)
+		goto out;
+
+	ret = mnt_want_write(mnt);
+	if (unlikely(ret))
+		goto out;
+
+	ret = restrictedmem_create(mnt);
+
+	mnt_drop_write(mnt);
+out:
+	fdput(f);
+
+	return ret;
+}

We need review from fs folks. Look mostly sensible, but I have no
experience in fs.

+
+SYSCALL_DEFINE2(memfd_restricted, unsigned int, flags, int, mount_fd)
+{
+	if (flags & ~RMFD_USERMNT)
+		return -EINVAL;
+
+	if (flags == RMFD_USERMNT) {
+		if (mount_fd < 0)
+			return -EINVAL;
+
+		return restrictedmem_create_on_user_mount(mount_fd);
+	} else {
+		return restrictedmem_create(NULL);
+	}

Maybe restructure with single restrictedmem_create() call?

	struct vfsmount *mnt = NULL;

	if (flags == RMFD_USERMNT) {
		...
		mnt = ...();
	}

	return restrictedmem_create(mnt);

Will do so in the next revision.

+}
+
  int restrictedmem_bind(struct file *file, pgoff_t start, pgoff_t end,
  		       struct restrictedmem_notifier *notifier, bool exclusive)
  {
--
2.40.0.348.gf938b09366-goog



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