Messages in this thread | | | Date | Wed, 7 Oct 2020 13:43:22 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count to counter_atomic32 |
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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@linuxfoundation.org> > > > > Signed-off-by: Shuah Khan <skhan@linuxfoundation.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
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