[PATCH 1/2] staging: vchiq: Fix bulk userdata handling

[Dan Carpenter]

On Mon, Jan 04, 2021 at 07:26:42PM +0000, Phil Elwell wrote:
> On 04/01/2021 18:31, Dan Carpenter wrote:
> > On Mon, Jan 04, 2021 at 12:09:27PM +0000, Phil Elwell wrote:
> > > The addition of the local 'userdata' pointer to
> > > vchiq_irq_queue_bulk_tx_rx omitted the case where neither BLOCKING nor
> > > WAITING modes are used, in which case the value provided by the
> > > caller is replaced with a NULL.
> > > 
> > > Fixes: 4184da4f316a ("staging: vchiq: fix __user annotations")
> > > 
> > > Signed-off-by: Phil Elwell <phil at raspberrypi.com>
> > > ---
> > >   drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > > index f500a7043805..2a8883673ba1 100644
> > > --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > > +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > > @@ -958,7 +958,7 @@ static int vchiq_irq_queue_bulk_tx_rx(struct vchiq_instance *instance,
> > >   	struct vchiq_service *service;
> > >   	struct bulk_waiter_node *waiter = NULL;
> > >   	bool found = false;
> > > -	void *userdata = NULL;
> > > +	void *userdata;
> > >   	int status = 0;
> > >   	int ret;
> > > @@ -997,6 +997,8 @@ static int vchiq_irq_queue_bulk_tx_rx(struct vchiq_instance *instance,
> > >   			"found bulk_waiter %pK for pid %d", waiter,
> > >   			current->pid);
> > >   		userdata = &waiter->bulk_waiter;
> > > +	} else {
> > > +		userdata = args->userdata;
> > 
> > "args->userdata" is marked as a user pointer so we really don't want to
> > mix user and kernel pointers here.  Presumably this opens up a large
> > security hole.
> 
> It's an opaque, pointer-sized token that only exists to bereturned to userspace (or not,
> without this patch) - it's hard to see that as a security hole.

I was assuming the bug here was a NULL dereference...  Apparently that's
not the case?  The commit message needs to be updated to be more clear
about how the bug looks like to the user.

Are we using the "&waiter->bulk_waiter" as a "token to be returned to
userspace" as well?  It looks like maybe it is in vchiq_put_completion().
That defeats KASLR and is a different sort of security problem.

Mixing __user pointers and regular pointers is dangerous and has lead to
security problems in this driver in the past.  But also mixing mixing
tokens with pointers just makes the code hard to read.  Instead of
undoing Arnd's work where he split the user space and kernel pointers
apart we should go ahead and spit it up even more.  At least add a giant
FIXME comment and an item in the TODO list so we don't forget to do this
before removing the code from staging.

regards,
dan carpenter