Re: [PATCH] staging: vchiq_arm: cast with __force as needed

[Dan Carpenter]

On Mon, May 18, 2020 at 08:45:31PM -0400, Mitchell Tasman wrote:
> In several cases where a pointer marked as __user is
> (intentionally) assigned or passed to a non-marked target,
> cast to the target pointer type with a __force directive
> to quiet warnings from sparse.
> 
> Signed-off-by: Mitchell Tasman <tasman@xxxxxxxxxxxx>
> ---
>  .../vc04_services/interface/vchiq_arm/vchiq_2835_arm.c     | 7 ++++---
>  .../staging/vc04_services/interface/vchiq_arm/vchiq_arm.c  | 4 +++-
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
> index c18c6ca0b6c0..38a13e4618a8 100644
> --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
> +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
> @@ -371,14 +371,15 @@ create_pagelist(char __user *buf, size_t count, unsigned short type)
>  	pagelistinfo->scatterlist = scatterlist;
>  	pagelistinfo->scatterlist_mapped = 0;
>  
> -	if (is_vmalloc_addr(buf)) {
> +	if (is_vmalloc_addr((void __force *)buf)) {

Am I reading this correctly???

This is actually a user controlled pointer that comes from the
vchiq_ioctl() when we do VCHIQ_IOC_QUEUE_BULK_TRANSMIT/RECEIVE.  So we
take random pointer from user space and if it happens to point to kernel
space then we trust it and presumably start BULK_TRANSMITing data to
it???

LOL....  This doesn't seem safe at all.

regards,
dan carpenter