From: Al Viro <viro@zeniv.linux.org.uk>
To: Vladis Dronov <vdronov@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>,
linux-fsdevel@vger.kernel.org, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: fix use-after-free in __fput() when a chardev is removed but a file is still open
Date: Sun, 8 Dec 2019 19:53:40 +0000 [thread overview]
Message-ID: <20191208195340.GX4203@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20191208194907.GW4203@ZenIV.linux.org.uk>
On Sun, Dec 08, 2019 at 07:49:07PM +0000, Al Viro wrote:
> On Mon, Nov 25, 2019 at 01:53:42PM +0100, Vladis Dronov wrote:
> > In a case when a chardev file (like /dev/ptp0) is open but an underlying
> > device is removed, closing this file leads to a use-after-free. This
> > reproduces easily in a KVM virtual machine:
> >
> > # cat openptp0.c
> > int main() { ... fp = fopen("/dev/ptp0", "r"); ... sleep(10); }
>
> > static void __fput(struct file *file)
> > { ...
> > if (file->f_op->release)
> > file->f_op->release(inode, file); <<< cdev is kfree'd here
>
> > if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
> > !(mode & FMODE_PATH))) {
> > cdev_put(inode->i_cdev); <<< cdev fields are accessed here
> >
> > because of:
> >
> > __fput()
> > posix_clock_release()
> > kref_put(&clk->kref, delete_clock) <<< the last reference
> > delete_clock()
> > delete_ptp_clock()
> > kfree(ptp) <<< cdev is embedded in ptp
> > cdev_put
> > module_put(p->owner) <<< *p is kfree'd
> >
> > The fix is to call cdev_put() before file->f_op->release(). This fix the
> > class of bugs when a chardev device is removed when its file is open, for
> > example:
>
> And what's to prevent rmmod coming and freeing ->release code right as you
> are executing it?
FWIW, the bug here seems to be that the lifetime rules of cdev are fucked -
if it can get freed while its ->kobj is still alive, we have something
very wrong there. IOW, you have ptp lifetime controlled by *TWO*
refcounts - that of clk and that of of cdev->kobj. That's doesn't work.
Replace that kfree() with dropping a kobject reference, perhaps, so
that freeing would've been done by its release callback?
next prev parent reply other threads:[~2019-12-08 19:53 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-25 12:53 [PATCH] fs: fix use-after-free in __fput() when a chardev is removed but a file is still open Vladis Dronov
2019-12-08 19:49 ` Al Viro
2019-12-08 19:53 ` Al Viro [this message]
2019-12-27 2:26 ` [PATCH v2] ptp: fix the race between the release of ptp_clock and cdev Vladis Dronov
2019-12-27 15:02 ` Richard Cochran
2019-12-27 17:24 ` Vladis Dronov
2019-12-31 4:19 ` David Miller
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191208195340.GX4203@ZenIV.linux.org.uk \
--to=viro@zeniv.linux.org.uk \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=richardcochran@gmail.com \
--cc=vdronov@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).