APPLIED/A/B/C: [trusty/linux xenial/linux artful/linux bionic/linux 1/1] floppy: Do not copy a kernel pointer to user memory in FDGETPRM ioctl

[Juerg Haefliger]

And applied to artful, bionic and cosmic master-next.

...Juerg

On 05/29/2018 03:38 PM, Andy Whitcroft 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 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 at canonical.com>
> ---
>  drivers/block/floppy.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
> index eae484acfbbc..0a860603a5b6 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;
> 


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