Re: [PATCH] virt: vbox: fix a missing-check bug

[Wenwen Wang <wang6495@umn.edu>]

On Tue, May 8, 2018 at 6:46 AM, Hans de Goede <hdegoede@redhat.com> wrote:
> Hi Wenwen,
>
> On 06-05-18 05:30, Wenwen Wang wrote:
>>
>> In vbg_misc_device_ioctl(), the header of the ioctl argument is copied
>> from
>> the userspace pointer 'arg' and saved to the kernel object 'hdr'. Then the
>> 'version', 'size_in', and 'size_out' fields of 'hdr' are verified. For
>> example, if 'hdr.version' is not VBG_IOCTL_HDR_VERSION, an error code
>> -EINVAL will be returned. If 'hdr' can pass all verifications, the whole
>> structure of the ioctl argument is copied once again from 'arg' and saved
>> to 'buf'. Then the function vbg_core_ioctl() is invoked to execute the
>> ioctl command. Given that the 'arg' pointer resides in userspace, a
>> malicious userspace process can race to change the data pointed to by
>> 'arg'
>> between the two copies. By doing so, the user can bypass the verifications
>> on the ioctl argument, which can cause vbg_core_ioctl() to work on
>> unsecure
>> data because it assumes the 'version', 'size_in', and 'size_out' have been
>> verified by vbg_misc_device_ioctl(), as mentioned in the comment in
>> vbg_core_ioctl():
>>
>>          /*
>>           * hdr->version and hdr->size_in / hdr->size_out minimum size are
>>           * already checked by vbg_misc_device_ioctl().
>>           */
>>
>> This patch adds checks after the second copy to ensure the consistency
>> between the data obtained in the two copies. In case an inconsistency is
>> detected, an error code -EINVAL will be returned.
>>
>> Signed-off-by: Wenwen Wang <wang6495@umn.edu>
>
>
> Thank you for finding this. I don't think that doing a second check is
> a good solution, by copy and pasting the checks we run the risk that
> any future additional checks are omitted from one copy of the checks.
>
> Instead I think we should simply avoid the 2nd copy of the header, like
> this:
>
> From 0c50b0dce3cf25a0ee9794c5816d9a0232d29e0a Mon Sep 17 00:00:00 2001
> From: Hans de Goede <hdegoede@redhat.com>
> Date: Tue, 8 May 2018 13:23:01 +0200
> Subject: [PATCH 3/3] virt: vbox: Only copy_from_user the request-header once
>
> In vbg_misc_device_ioctl(), the header of the ioctl argument is copied from
> the userspace pointer 'arg' and saved to the kernel object 'hdr'. Then the
> 'version', 'size_in', and 'size_out' fields of 'hdr' are verified.
>
> Before this commit, after the checks a buffer for the entire request would
> be allocated and then all data including the verified header would be
> copied from the userspace 'arg' pointer again.
>
> Given that the 'arg' pointer resides in userspace, a malicious userspace
> process can race to change the data pointed to by 'arg' between the two
> copies. By doing so, the user can bypass the verifications on the ioctl
> argument.
>
> This commit fixes this by using the already checked copy of the header
> to fill the header part of the allocated buffer and only copying the
> remainder of the data from userspace.
>
> Reported-by: Wenwen Wang <wang6495@umn.edu>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
>  drivers/virt/vboxguest/vboxguest_linux.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/virt/vboxguest/vboxguest_linux.c
> b/drivers/virt/vboxguest/vboxguest_linux.c
> index 398d22693234..6e2a9619192d 100644
> --- a/drivers/virt/vboxguest/vboxguest_linux.c
> +++ b/drivers/virt/vboxguest/vboxguest_linux.c
> @@ -121,7 +121,9 @@ static long vbg_misc_device_ioctl(struct file *filp,
> unsigned int req,
>         if (!buf)
>                 return -ENOMEM;
>
> -       if (copy_from_user(buf, (void *)arg, hdr.size_in)) {
> +       *((struct vbg_ioctl_hdr *)buf) = hdr;
> +       if (copy_from_user(buf + sizeof(hdr), (void *)arg + sizeof(hdr),
> +                          hdr.size_in - sizeof(hdr))) {
>                 ret = -EFAULT;
>                 goto out;
>         }
>
> Do you agree that this would also fix the race window you found?

Thanks for your response. Yes, this fix also works.

Wenwen