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

[Kees Cook]

On Tue, May 29, 2018 at 6:27 AM, Andy Whitcroft <robobotbotbot@xxxxxxxxx> 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@xxxxxxx>
>> ---
>>  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@xxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx>

I didn't see this land anywhere? Who's tree is this going through?

-Kees

> ---
>  drivers/block/floppy.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
> index 8ec7235fc93b..7512f6ff7c43 100644
> --- a/drivers/block/floppy.c
> +++ b/drivers/block/floppy.c
> @@ -3470,6 +3470,8 @@ static int fd_locked_ioctl(struct block_device *bdev, fmode_t mode, unsigned int
>                                           (struct floppy_struct **)&outparam);
>                 if (ret)
>                         return ret;
> +               memcpy(&inparam.g, outparam, offsetof(struct floppy_struct, name));
> +               outparam = &inparam.g;
>                 break;
>         case FDMSGON:
>                 UDP->flags |= FTD_MSG;
> --
> 2.17.0
>



-- 
Kees Cook
Pixel Security