linux-kernel - Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and remove O

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Date:   Tue, 4 Aug 2020 21:54:03 -0700
From:   Lokesh Gidra <lokeshgidra@...gle.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        casey@...aufler-ca.com, James Morris <jmorris@...ei.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Daniel Colascione <dancol@...col.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Nick Kralevich <nnk@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Calin Juravle <calin@...gle.com>, kernel-team@...roid.com,
        yanfei.xu@...driver.com,
        syzbot+75867c44841cb6373570@...kaller.appspotmail.com
Subject: Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and
 remove O_CLOEXEC

On Tue, Aug 4, 2020 at 9:08 PM Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Wed, Aug 05, 2020 at 01:47:58PM +1000, Aleksa Sarai wrote:
> > On 2020-08-04, Lokesh Gidra <lokeshgidra@...gle.com> wrote:
> > > when get_unused_fd_flags returns error, ctx will be freed by
> > > userfaultfd's release function, which is indirectly called by fput().
> > > Also, if anon_inode_getfile_secure() returns an error, then
> > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx.
> > >
> > > Also, the O_CLOEXEC was inadvertently added to the call to
> > > get_unused_fd_flags() [1].
> >
> > I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all,
> > it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on
> > an existing file descriptor because it's possible for another thread to
> > exec() before you set the flag). Several new syscalls and fd-returning
> > facilities are O_CLOEXEC-by-default now (the most obvious being pidfds
> > and seccomp notifier fds).
>
> Sure, O_CLOEXEC *should* be the default, but this is an existing syscall so it
> has to keep the existing behavior.
>
> > At the very least there should be a new flag added that sets O_CLOEXEC.
>
> There already is one (but these patches broke it).
>
I looked at the existing implementation, and the right thing is to
pass on the 'flags' (that is passed in to the syscall) to fetch 'fd'.

Besides, as you said in the other email thread,
anon_inode_getfile_secure() should be replaced with
anon_inode_getfd_secure(), which will remove this ambiguity.

I'll resend the patch series soon with all the changes that you proposed.
> - Eric