Messages in this thread Patch in this message | | | Date | Tue, 29 May 2018 14:27:22 +0100 | From | Andy Whitcroft <> | Subject | Re: [PATCH] floppy: Do not copy a kernel pointer to user memory in FDGETPRM ioctl |
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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> --- 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
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