linux-kernel - Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count to counter

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Date:   Wed, 7 Oct 2020 13:43:22 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Shuah Khan <skhan@...uxfoundation.org>, gregkh@...uxfoundation.org,
        rafael@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count
 to counter_atomic32

On Wed, Oct 07, 2020 at 09:38:47PM +0200, Johannes Berg wrote:
> On Wed, 2020-10-07 at 13:33 -0600, Shuah Khan wrote:
> > On 10/7/20 12:15 PM, Kees Cook wrote:
> > > On Tue, Oct 06, 2020 at 02:44:35PM -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.
> > > > 
> > > > devcd_count is used to track dev_coredumpm device count and used in
> > > > device name string. It doesn't guard object lifetimes, device usage
> > > > counts, device open counts, and pm states. There is very little chance
> > > > of this counter overflowing. Convert it to use counter_atomic32.
> > > > 
> > > > This conversion doesn't change the overflow wrap around behavior.
> > > > 
> > > > Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
> > > 
> > > I actually wonder if this should use refcount_t just because it is
> > > designed to be an alway-unique value. It is hard to imagine ever causing
> > > this to overflow, but why not let it be protected?
> > > 
> > 
> > This is one of the cases where devcd_count doesn't guard lifetimes,
> > however if it ever overflows, refcount_t is a better choice.
> > 
> > If we decide refcount_t is a better choice, I can drop this patch
> > and send refcount_t conversion patch instead.
> > 
> > Greg! Any thoughts on refcount_t for this being a better choice?
> 
> I'm not Greg, but ... there's a 5 minute timeout. So in order to cause a
> clash you'd have to manage to overflow the counter within a 5 minute
> interval, otherwise you can actually reuse the numbers starting again
> from 0 without any ill effect.

That's not true as far as I can see: there's no reset in here. It's a
global heap variable with function-level visibility (note the "static"),
so it is only ever initialized once:

void dev_coredumpm(struct device *dev, struct module *owner,
                   void *data, size_t datalen, gfp_t gfp,
                   ssize_t (*read)(char *buffer, loff_t offset, size_t count,
                                   void *data, size_t datalen),
                   void (*free)(void *data))
{
        static atomic_t devcd_count = ATOMIC_INIT(0);
	...
        dev_set_name(&devcd->devcd_dev, "devcd%d",
                     atomic_inc_return(&devcd_count));
	...
}

https://godbolt.org/z/T6Wfcj

-- 
Kees Cook