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

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Message-ID: <ef59bbc78d3a71dba51bfea5f452718d3fa0499c.camel@decadent.org.uk>
Date:   Tue, 10 Jul 2018 00:14:46 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Andy Whitcroft <robobotbotbot@...il.com>,
        Brian Belleville <bbellevi@....edu>
Subject: Re: [PATCH] floppy: Do not copy a kernel pointer to user memory in
 FDGETPRM ioctl

On Tue, 2018-05-29 at 14:27 +0100, Andy Whitcroft wrote:
[...]
> >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>

Reviewed-and-tested-by: Ben Hutchings <ben@...adent.org.uk>

Ben.

> ---
>  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;
-- 
Ben Hutchings
If you seem to know what you are doing, you'll be given more to do.


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