linux-kernel - Re: [PATCH v3 3/6] android: binder: Move buffer out of area shared with user space

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Message-ID: <20170830092904.b43bmlveo5g4c6rd@mwanda>
Date:   Wed, 30 Aug 2017 12:29:05 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Sherry Yang <sherryy@...roid.com>,
        Arve Hjønnevåg <arve@...roid.com>
Cc:     linux-kernel@...r.kernel.org,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Riley Andrews <riandrews@...roid.com>, maco@...gle.com,
        tkjos@...gle.com
Subject: Re: [PATCH v3 3/6] android: binder: Move buffer out of area shared
 with user space

On Tue, Aug 29, 2017 at 05:46:59PM -0700, Sherry Yang wrote:
> Binder driver allocates buffer meta data in a region that is mapped
> in user space. These meta data contain pointers in the kernel.
> 
> This patch allocates buffer meta data on the kernel heap that is
> not mapped in user space, and uses a pointer to refer to the data mapped.
> 
> Also move alloc->buffers initialization from mmap to init since it's
> now used even when mmap failed or was not called.
> 
> Signed-off-by: Sherry Yang <sherryy@...roid.com>
> ---

The difference between v2 and v3 is that we've shifted some
initialization around to fix the crashing bug that kbuild found.  You
should not that difference here under the --- cut off.

>  drivers/android/binder_alloc.c          | 146 +++++++++++++++++++-------------
>  drivers/android/binder_alloc.h          |   2 +-
>  drivers/android/binder_alloc_selftest.c |  11 ++-
>  3 files changed, 91 insertions(+), 68 deletions(-)

But really we still need to have some answers or discussion about the
questions that Greg and I raised.  Greg asked if the other Android devs
had Acked this.  Please ping Arve to Ack this.

I was curious about the security impact or why we were writing this
patch 3/6.  It seems we are fixing an information disclosure bug.  Or is
it something worse than that?  Or have I misunderstood entirely.

We probably original put the buffers in userspace for accounting reasons
so we could kill programs that used too much RAM.  This patch doesn't
create a problem with that hopefully?  We're just moving the metadata to
kernel space?

regards,
dan carpenter