Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 17 Jul 2020 07:45:32 -0400
From:   Peilin Ye <yepeilin.cs@gmail.com>
To:     Daniel Colascione <dancol@google.com>
Cc:     timmurray@google.com, selinux@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, viro@zeniv.linux.org.uk, paul@paul-moore.com,
        nnk@google.com, sds@tycho.nsa.gov, lokeshgidra@google.com,
        jmorris@namei.org
Subject: Reporting a use-after-free read bug in userfaultfd_release()
Message-ID: <20200717114532.GA688728@PWN>
Reply-To: 20200401213903.182112-4-dancol@google.com
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi all,

Syzbot reported the following use-after-free bug in
userfaultfd_release():

	https://syzkaller.appspot.com/bug?id=4b9e5aea757b678d9939c364e50212354a3480a6

It seems to be caused by this patch. I took a look at the stack trace.
In the patch:

	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
	if (fd < 0) {
		fput(file);
		goto out;
	}

If get_unused_fd_flags() fails, `ctx` is freed. Later however, before
returning back to userland, userfaultfd_release() is called and tries to
use `ctx` again, causing a use-after-free bug.

The syzbot reproducer does a setrlimit() then a userfaultfd(). The
former sets a hard limit on number of open files to zero, which causes
get_unused_fd_flags() to fail.

Thank you,

Peilin Ye