Messages in this thread | | | Subject | Re: [PATCH] floppy: Do not copy a kernel pointer to user memory in FDGETPRM ioctl | From | Jens Axboe <> | Date | Thu, 20 Sep 2018 09:10:24 -0600 |
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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@gmail.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@uci.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@canonical.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@canonical.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
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