linux-kernel - Re: [PATCH] virt: vbox: Only copy_from

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Message-ID: <456c3863-7da9-0b73-144f-dc73582d91c8@redhat.com>
Date:   Tue, 8 May 2018 16:07:58 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virt: vbox: Only copy_from_user the request-header once

Hi,

On 08-05-18 15:46, Hans de Goede 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.
> 
> 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@....edu>
> Signed-off-by: Hans de Goede <hdegoede@...hat.com>

Wenwen just send a v2 of his patch, lets go with his (identical)
version as he has done the hard work of tracking this down.

Regards,

Hans




> ---
>   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;
>   	}
>