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From: Eric Biggers <ebiggers@kernel.org>
To: Aleksa Sarai <cyphar@cyphar.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>,
	viro@zeniv.linux.org.uk, stephen.smalley.work@gmail.com,
	casey@schaufler-ca.com, jmorris@namei.org,
	kaleshsingh@google.com, dancol@dancol.org, surenb@google.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	nnk@google.com, jeffv@google.com, calin@google.com,
	kernel-team@android.com, yanfei.xu@windriver.com,
	syzbot+75867c44841cb6373570@syzkaller.appspotmail.com
Subject: Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and remove O_CLOEXEC
Date: Tue, 4 Aug 2020 21:08:06 -0700	[thread overview]
Message-ID: <20200805040806.GB1136@sol.localdomain> (raw)
In-Reply-To: <20200805034758.lrobunwdcqtknsvz@yavin.dot.cyphar.com>

On Wed, Aug 05, 2020 at 01:47:58PM +1000, Aleksa Sarai wrote:
> On 2020-08-04, Lokesh Gidra <lokeshgidra@google.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).

- Eric

  reply	other threads:[~2020-08-05  4:08 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-04 20:31 Lokesh Gidra
2020-08-04 20:45 ` Eric Biggers
2020-08-04 20:49   ` Lokesh Gidra
2020-08-04 20:58     ` Eric Biggers
2020-08-04 23:34       ` Lokesh Gidra
2020-08-05  3:47 ` Aleksa Sarai
2020-08-05  4:08   ` Eric Biggers [this message]
2020-08-05  4:54     ` Lokesh Gidra
2020-08-06 23:59     ` Aleksa Sarai

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