linux-kernel - Re: [PATCH v3 2/2] binder: Validate the default binderfs device names.

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Message-ID: <20190904110732.e2ddiact3jvd73pt@wittgenstein>
Date:   Wed, 4 Sep 2019 13:07:33 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Hridya Valsaraju <hridya@...gle.com>, kernel-team@...roid.com
Subject: Re: [PATCH v3 2/2] binder: Validate the default binderfs device
 names.

On Wed, Sep 04, 2019 at 12:49:39PM +0200, Greg Kroah-Hartman wrote:
> On Wed, Sep 04, 2019 at 12:44:32PM +0200, Christian Brauner wrote:
> > On Wed, Sep 04, 2019 at 09:19:29AM +0200, Greg Kroah-Hartman wrote:
> > > On Fri, Aug 09, 2019 at 11:41:12AM -0700, Hridya Valsaraju wrote:
> > > > On Fri, Aug 9, 2019 at 11:14 AM Christian Brauner
> > > > <christian.brauner@...ntu.com> wrote:
> > > > >
> > > > > On Fri, Aug 09, 2019 at 04:55:08PM +0200, Greg Kroah-Hartman wrote:
> > > > > > On Thu, Aug 08, 2019 at 03:27:26PM -0700, Hridya Valsaraju wrote:
> > > > > > > Length of a binderfs device name cannot exceed BINDERFS_MAX_NAME.
> > > > > > > This patch adds a check in binderfs_init() to ensure the same
> > > > > > > for the default binder devices that will be created in every
> > > > > > > binderfs instance.
> > > > > > >
> > > > > > > Co-developed-by: Christian Brauner <christian.brauner@...ntu.com>
> > > > > > > Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> > > > > > > Signed-off-by: Hridya Valsaraju <hridya@...gle.com>
> > > > > > > ---
> > > > > > >  drivers/android/binderfs.c | 12 ++++++++++++
> > > > > > >  1 file changed, 12 insertions(+)
> > > > > > >
> > > > > > > diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
> > > > > > > index aee46dd1be91..55c5adb87585 100644
> > > > > > > --- a/drivers/android/binderfs.c
> > > > > > > +++ b/drivers/android/binderfs.c
> > > > > > > @@ -570,6 +570,18 @@ static struct file_system_type binder_fs_type = {
> > > > > > >  int __init init_binderfs(void)
> > > > > > >  {
> > > > > > >     int ret;
> > > > > > > +   const char *name;
> > > > > > > +   size_t len;
> > > > > > > +
> > > > > > > +   /* Verify that the default binderfs device names are valid. */
> > > > > >
> > > > > > And by "valid" you only mean "not bigger than BINDERFS_MAX_NAME, right?
> > > > > >
> > > > > > > +   name = binder_devices_param;
> > > > > > > +   for (len = strcspn(name, ","); len > 0; len = strcspn(name, ",")) {
> > > > > > > +           if (len > BINDERFS_MAX_NAME)
> > > > > > > +                   return -E2BIG;
> > > > > > > +           name += len;
> > > > > > > +           if (*name == ',')
> > > > > > > +                   name++;
> > > > > > > +   }
> > > > > >
> > > > > > We already tokenize the binderfs device names in binder_init(), why not
> > > > > > check this there instead?  Parsing the same string over and over isn't
> > > > > > the nicest.
> > > > >
> > > > > non-binderfs binder devices do not have their limit set to
> > > > > BINDERFS_NAME_MAX. That's why the check has likely been made specific to
> > > > > binderfs binder devices which do have that limit.
> > > > 
> > > > 
> > > > Thank you Greg and Christian, for taking another look. Yes,
> > > > non-binderfs binder devices not having this limitation is the reason
> > > > why the check was made specific to binderfs devices. Also, when
> > > > CONFIG_ANDROID_BINDERFS is set, patch 1/2 disabled the same string
> > > > being parsed in binder_init().
> > > > 
> > > > >
> > > > > But, in practice, 255 is the standard path-part limit that no-one really
> > > > > exceeds especially not for stuff such as device nodes which usually have
> > > > > rather standard naming schemes (e.g. binder, vndbinder, hwbinder, etc.).
> > > > > So yes, we can move that check before both the binderfs binder device
> > > > > and non-binderfs binder device parsing code and treat it as a generic
> > > > > check.
> > > > > Then we can also backport that check as you requested in the other mail.
> > > > > Unless Hridya or Todd have objections, of course.
> > > > 
> > > > I do not have any objections to adding a generic check in binder_init() instead.
> > > 
> > > Was this patchset going to be redone based on this?
> > 
> > No, we decided to leave this check specific to binderfs for now because
> > the length limit only applies to binderfs devices. If you really want to
> > have this check in binder we can send a follow-up. I would prefer to
> > take the series as is.
> > 
> > Btw, for the two binderfs series from Hridya, do you want me to get a
> > branch ready and send you a PR for both of them together?
> 
> Patches in email is fine, but can someone resend this one as I no longer
> have this series in my queue anymore?

Done!
Christian