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

[Mitchell Tasman]

On 5/22/20 7:06 AM, Dan Carpenter wrote:
> 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.

Is additional validation of buf and its extent necessary and sufficient, e.g. perhaps access_ok(buf, count * PAGE_SIZE) somewhere along the call chain?  Or does vhciq_arm need to take a different approach in the area that Dan Carpenter flagged?

Thank you.

> 
> regards,
> dan carpenter
> 

Regards,
Mitch