Re: [PATCH 1/1] coresight: prevent deactivate active config while enable the config

[Yeoreum Yun]

Hi James.
>
>
> On 18/12/2024 8:48 am, Yeoreum Yun wrote:
> > While enable active config via cscfg_csdev_enable_active_config(),
> > active config could be deactivated via configfs' sysfs interface.
> > This could make UAF issue in below scenario:
> >
> > CPU0                                          CPU1
> > (perf or sysfs enable)                        load module
> >                                                cscfg_load_config_sets()
> >                                                activate config. // sysfs
> >                                                (sys_active_cnt == 1)
> > ...
> > cscfg_csdev_enable_active_config()
> >    lock(csdev->cscfg_csdev_lock)
> >    // here load config activate by CPU1
> >    unlock(csdev->cscfg_csdev_lock)
> >
> >                                                deactivate config // sysfs
> >                                                (sys_activec_cnt == 0)
>
> Assuming the left side does Perf, are there some steps missing? To get to
> enable_active_config() you first need to pass through etm_setup_aux() ->
> cscfg_activate_config(). That would also increment sys_active_cnt which
> would leave it at 2 if there were two concurrent sessions. Then it would end
> up as 1 here after deactivate, rather than 0.
>
> It's not explicitly mentioned in the sequence but I'm assuming the left and
> right are the same config, but I suppose it could be an issue with different
> configs too.

Sorry! This happens only in sysfs mode. not perf.
I've forgotten update the diagram...

>
> >                                                cscfg_unload_config_sets()
> >                                                unload module
>
> On the left cscfg_activate_config() also bumps the module refcount, so
> unload wouldn't cause a UAF here as far as I can see.
>
In case of perf yes. However sysfs is done via configfs interface.
IOW, while cscfg_csdev_enable_active_config() iterating to find out the
config, it's possible to deactivate config via confis interface,
and unload moudle.

Above diagram explains this when coresight is configured with sysfs &
configfs interface NOT perf.
Sorry!


> >
> >    // access to config_desc which freed
> >    // while unloading module.
> >    cfs_csdev_enable_config
> >
> > To address this, introduce sys_enable_cnt in cscfg_mgr to prevent
> > deactivate while there is enabled configuration.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> > ---
> >   .../hwtracing/coresight/coresight-etm4x-core.c |  3 +++
> >   drivers/hwtracing/coresight/coresight-syscfg.c | 18 ++++++++++++++++--
> >   drivers/hwtracing/coresight/coresight-syscfg.h |  2 ++
> >   3 files changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > index 86893115df17..6218ef40acbc 100644
> > --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > @@ -986,6 +986,9 @@ static void etm4_disable_sysfs(struct coresight_device *csdev)
> >   	smp_call_function_single(drvdata->cpu, etm4_disable_hw, drvdata, 1);
> >   	raw_spin_unlock(&drvdata->spinlock);
> > +
> > +	cscfg_csdev_disable_active_config(csdev);
> > +
> >   	cpus_read_unlock();
> >   	/*
> > diff --git a/drivers/hwtracing/coresight/coresight-syscfg.c b/drivers/hwtracing/coresight/coresight-syscfg.c
> > index a70c1454b410..dfa7dcbaf25d 100644
> > --- a/drivers/hwtracing/coresight/coresight-syscfg.c
> > +++ b/drivers/hwtracing/coresight/coresight-syscfg.c
> > @@ -953,7 +953,8 @@ int cscfg_config_sysfs_activate(struct cscfg_config_desc *config_desc, bool acti
> >   			cscfg_mgr->sysfs_active_config = cfg_hash;
> >   	} else {
> >   		/* disable if matching current value */
> > -		if (cscfg_mgr->sysfs_active_config == cfg_hash) {
> > +		if (cscfg_mgr->sysfs_active_config == cfg_hash &&
> > +		    !atomic_read(&cscfg_mgr->sys_enable_cnt)) {
> >   			_cscfg_deactivate_config(cfg_hash);
>
> So is sys_enable_cnt a global value? If a fix is needed doesn't it need to
> be a per-config refcount?
>
> Say you have two active configs, sys_enable_cnt is now 2, how do you disable
> one without it always skipping here when the other config is enabled?
>
> >   			cscfg_mgr->sysfs_active_config = 0;
> >   		} else
> > @@ -1055,6 +1056,12 @@ int cscfg_csdev_enable_active_config(struct coresight_device *csdev,
> >   	if (!atomic_read(&cscfg_mgr->sys_active_cnt))
> >   		return 0;
> > +	/*
> > +	 * increment sys_enable_cnt first to prevent deactivate the config
> > +	 * while enable active config.
> > +	 */
> > +	atomic_inc(&cscfg_mgr->sys_enable_cnt);
> > +
> >   	/*
> >   	 * Look for matching configuration - set the active configuration
> >   	 * context if found.
> > @@ -1098,6 +1105,10 @@ int cscfg_csdev_enable_active_config(struct coresight_device *csdev,
> >   			raw_spin_unlock_irqrestore(&csdev->cscfg_csdev_lock, flags);
> >   		}
> >   	}
> > +
> > +	if (!config_csdev_active || err)
> > +		atomic_dec(&cscfg_mgr->sys_enable_cnt);
> > +
> >   	return err;
> >   }
> >   EXPORT_SYMBOL_GPL(cscfg_csdev_enable_active_config);
> > @@ -1129,8 +1140,10 @@ void cscfg_csdev_disable_active_config(struct coresight_device *csdev)
> >   	if (config_csdev) {
> >   		if (!config_csdev->enabled)
> >   			config_csdev = NULL;
> > -		else
> > +		else {
> >   			config_csdev->enabled = false;
> > +			atomic_dec(&cscfg_mgr->sys_enable_cnt);
> > +		}
> >   	}
> >   	csdev->active_cscfg_ctxt = NULL;
> >   	raw_spin_unlock_irqrestore(&csdev->cscfg_csdev_lock, flags);
> > @@ -1179,6 +1192,7 @@ static int cscfg_create_device(void)
> >   	INIT_LIST_HEAD(&cscfg_mgr->config_desc_list);
> >   	INIT_LIST_HEAD(&cscfg_mgr->load_order_list);
> >   	atomic_set(&cscfg_mgr->sys_active_cnt, 0);
> > +	atomic_set(&cscfg_mgr->sys_enable_cnt, 0);
> >   	cscfg_mgr->load_state = CSCFG_NONE;
> >   	/* setup the device */
> > diff --git a/drivers/hwtracing/coresight/coresight-syscfg.h b/drivers/hwtracing/coresight/coresight-syscfg.h
> > index 66e2db890d82..2fc397919985 100644
> > --- a/drivers/hwtracing/coresight/coresight-syscfg.h
> > +++ b/drivers/hwtracing/coresight/coresight-syscfg.h
> > @@ -38,6 +38,7 @@ enum cscfg_load_ops {
> >    * @config_desc_list:	List of system configuration descriptors to load into registered devices.
> >    * @load_order_list:    Ordered list of owners for dynamically loaded configurations.
> >    * @sys_active_cnt:	Total number of active config descriptor references.
> > + * @sys_enable_cnt:	Total number of enable of active config descriptor references.
>
> When these are next to each other it makes me wonder why active_cnt isn't
> enough to prevent unloading? Enabled is always a subset of active, so as
> long as you gate unloads or modifications on the existing active count it
> seems fine?
>
> >    * @cfgfs_subsys:	configfs subsystem used to manage configurations.
> >    * @sysfs_active_config:Active config hash used if CoreSight controlled from sysfs.
> >    * @sysfs_active_preset:Active preset index used if CoreSight controlled from sysfs.
> > @@ -50,6 +51,7 @@ struct cscfg_manager {
> >   	struct list_head config_desc_list;
> >   	struct list_head load_order_list;
> >   	atomic_t sys_active_cnt;
> > +	atomic_t sys_enable_cnt;
> >   	struct configfs_subsystem cfgfs_subsys;
> >   	u32 sysfs_active_config;
> >   	int sysfs_active_preset;
>