Messages in this thread | | | Date | Wed, 7 Oct 2020 11:27:29 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v2 10/11] drivers/misc/vmw_vmci: convert num guest devices counter to counter_atomic32 |
| |
On Tue, Oct 06, 2020 at 02:44:41PM -0600, Shuah Khan wrote: > counter_atomic* is introduced to be used when a variable is used as > a simple counter and doesn't guard object lifetimes. This clearly > differentiates atomic_t usages that guard object lifetimes. > > counter_atomic* variables will wrap around to 0 when it overflows and > should not be used to guard resource lifetimes, device usage and > open counts that control state changes, and pm states. > > atomic_t variable used to count number of vmci guest devices is used > as just as counter and it doesn't control object lifetimes or state > management. Overflow doesn't appear to be problem for this use. > > Convert it to use counter_atomic32. > > This conversion doesn't change the overflow wrap around behavior. > > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
I'm not convinced this isn't both managing lifetime and already buggy. Specifically, I'm looking at how vmci_guest_code_active() is used -- it's being tested before making calls? Is this safe?
> --- > drivers/misc/vmw_vmci/vmci_guest.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > diff --git a/drivers/misc/vmw_vmci/vmci_guest.c b/drivers/misc/vmw_vmci/vmci_guest.c > index cc8eeb361fcd..86ae27b05fc2 100644 > --- a/drivers/misc/vmw_vmci/vmci_guest.c > +++ b/drivers/misc/vmw_vmci/vmci_guest.c > @@ -20,6 +20,7 @@ > #include <linux/smp.h> > #include <linux/io.h> > #include <linux/vmalloc.h> > +#include <linux/counters.h> > > #include "vmci_datagram.h" > #include "vmci_doorbell.h" > @@ -68,11 +69,11 @@ struct pci_dev *vmci_pdev; > static struct vmci_guest_device *vmci_dev_g; > static DEFINE_SPINLOCK(vmci_dev_spinlock); > > -static atomic_t vmci_num_guest_devices = ATOMIC_INIT(0); > +static struct counter_atomic32 vmci_num_guest_devices = COUNTER_ATOMIC_INIT(0); > > bool vmci_guest_code_active(void) > { > - return atomic_read(&vmci_num_guest_devices) != 0; > + return counter_atomic32_read(&vmci_num_guest_devices) != 0;
Shouldn't this be "> 0" ?
> } > > u32 vmci_get_vm_context_id(void) > @@ -624,7 +625,7 @@ static int vmci_guest_probe_device(struct pci_dev *pdev, > > dev_dbg(&pdev->dev, "Registered device\n"); > > - atomic_inc(&vmci_num_guest_devices); > + counter_atomic32_inc(&vmci_num_guest_devices); > > /* Enable specific interrupt bits. */ > cmd = VMCI_IMR_DATAGRAM; > @@ -684,7 +685,7 @@ static void vmci_guest_remove_device(struct pci_dev *pdev) > > dev_dbg(&pdev->dev, "Removing device\n"); > > - atomic_dec(&vmci_num_guest_devices); > + counter_atomic32_dec(&vmci_num_guest_devices);
If there is a bug elsewhere and vmci_guest_remove_device() (or probe) gets called too many times, shouldn't we protect the rest of this stack from having vmci_num_guest_devices go negative (and therefore non-zero)?
This really seems like it should be refcount_t to me, though I have no idea what the races between the dec() and the read() might mean in this code generally.
-- Kees Cook
| |