linux-kernel - Re: [PATCH] floppy: Do not copy a kernel pointer to user memory in FDGETPRM ioctl

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Date:   Thu, 20 Sep 2018 09:10:24 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Joe Lawrence <joe.lawrence@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Andy Whitcroft <robobotbotbot@...il.com>,
        Brian Belleville <bbellevi@....edu>,
        Jiri Kosina <jikos@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] floppy: Do not copy a kernel pointer to user memory in
 FDGETPRM ioctl

On 9/20/18 8:01 AM, Joe Lawrence wrote:
> On Mon, Aug 27, 2018 at 12:45:25AM -0700, Kees Cook wrote:
>> On Tue, May 29, 2018 at 6:27 AM, Andy Whitcroft <robobotbotbot@...il.com> wrote:
>>> On Wed, Mar 07, 2018 at 04:02:45PM -0800, Brian Belleville wrote:
>>>> The final field of a floppy_struct is the field "name", which is a
>>>> pointer to a string in kernel memory. The kernel pointer should not be
>>>> copied to user memory. The FDGETPRM ioctl copies a floppy_struct to
>>>> user memory, including the "name" field. This pointer cannot be used
>>>> by the user, and it will leak a kernel address to user-space, which
>>>> will reveal the location of kernel code and data and undermine KASLR
>>>> protection. Instead, copy the floppy_struct except for the "name"
>>>> field.
>>>>
>>>> Signed-off-by: Brian Belleville <bbellevi@....edu>
>>>> ---
>>>>  drivers/block/floppy.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
>>>> index eae484a..4d4a422 100644
>>>> --- a/drivers/block/floppy.c
>>>> +++ b/drivers/block/floppy.c
>>>> @@ -3470,6 +3470,7 @@ static int fd_locked_ioctl(struct block_device *bdev, fmode_t mode, unsigned int
>>>>                                         (struct floppy_struct **)&outparam);
>>>>               if (ret)
>>>>                       return ret;
>>>> +             size = offsetof(struct floppy_struct, name);
>>>>               break;
>>>>       case FDMSGON:
>>>>               UDP->flags |= FTD_MSG;
>>>
>>> I am not sure it is reasonable to simply set size here to the length of the
>>> valid data.  Though in the real world everyonne should be using the defines
>>> and those should include the full length, the code itself does not require
>>> this, it only prevents overly long reads.  So I think it is possible to do
>>> this read with a shorter userspace buffer; with this change we would
>>> then write beyond the end of the buffer.
>>>
>>> This also seems to introduce a slight behavioural difference between the
>>> primary and compat calls.  The compat call already elides the name but it
>>> also is copying into a new structure for return and this is pre-cleared,
>>> so the name will always be null for the compat case and undefined for
>>> the primary ioctl.
>>>
>>> Perhaps the below patch would be more appropriate.
>>>
>>> -apw
>>>
>>> From ddb8c77229a9507fa5575c910d2847e123a9c94c Mon Sep 17 00:00:00 2001
>>> From: Andy Whitcroft <apw@...onical.com>
>>> Date: Tue, 29 May 2018 13:04:15 +0100
>>> Subject: [PATCH 1/1] floppy: Do not copy a kernel pointer to user memory in
>>>  FDGETPRM ioctl
>>>
>>> The final field of a floppy_struct is the field "name", which is a pointer
>>> to a string in kernel memory.  The kernel pointer should not be copied to
>>> user memory.  The FDGETPRM ioctl copies a floppy_struct to user memory,
>>> including this "name" field.  This pointer cannot be used by the user
>>> and it will leak a kernel address to user-space, which will reveal the
>>> location of kernel code and data and undermine KASLR protection.
>>>
>>> Model this code after the compat ioctl which copies the returned data
>>> to a previously cleared temporary structure on the stack (excluding the
>>> name pointer) and copy out to userspace from there.  As we already have
>>> an inparam union with an appropriate member and that memory is already
>>> cleared even for read only calls make use of that as a temporary store.
>>>
>>> Based on an initial patch by Brian Belleville.
>>>
>>> CVE-2018-7755
>>> Signed-off-by: Andy Whitcroft <apw@...onical.com>
>>
>> I didn't see this land anywhere? Who's tree is this going through?
>>
>> -Kees
>>
> 
> Looks like a pretty simple fix, but still don't see this one anywhere...
> maybe Jiri or Jens could pick it up (as per get_maintainer output :)

Applied for 4.19.

-- 
Jens Axboe