linux-kernel - Reporting a use-after-free read bug in userfaultfd

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Message-ID: <20200717114532.GA688728@PWN>
Date:   Fri, 17 Jul 2020 07:45:32 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     timmurray@...gle.com, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, viro@...iv.linux.org.uk, paul@...l-moore.com,
        nnk@...gle.com, sds@...ho.nsa.gov, lokeshgidra@...gle.com,
        jmorris@...ei.org
Subject: Reporting a use-after-free read bug in userfaultfd_release()

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