Re: [POC 09/23] livepatch: Handle race when livepatches are reloaded during a module load

Julien Thierry
2020-01-22T18:51:56Z

Hi Petr,

On 1/17/20 3:03 PM, Petr Mladek wrote:
> klp_module_coming() might fail to load a livepatch module when
> the related livepatch gets reloaded in the meantime.
>
> Detect this situation by adding a timestamp into struct klp_patch.
> local_clock is enough because klp_mutex must be released and taken
> several times during this scenario.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
> include/linux/livepatch.h | 2 ++
> kernel/livepatch/core.c | 9 +++++----
> 2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index a4567c17a9f2..feb33f023f9f 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -155,6 +155,7 @@ struct klp_state {
> * @obj_list: dynamic list of the object entries
> * @enabled: the patch is enabled (but operation may be incomplete)
> * @forced: was involved in a forced transition
> + * ts: timestamp when the livepatch has been loaded

Nit: Missing '@'.

> * @free_work: patch cleanup from workqueue-context
> * @finish: for waiting till it is safe to remove the patch module
> */
> @@ -171,6 +172,7 @@ struct klp_patch {
> struct list_head obj_list;
> bool enabled;
> bool forced;
> + u64 ts;
> struct work_struct free_work;
> struct completion finish;
> };
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 34e3ee2be7ef..8e693c58b736 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -20,6 +20,7 @@
> #include <linux/moduleloader.h>
> #include <linux/completion.h>
> #include <linux/memory.h>
> +#include <linux/sched/clock.h>
> #include <asm/cacheflush.h>
> #include "core.h"
> #include "patch.h"
> @@ -854,6 +855,7 @@ static int klp_init_patch_early(struct klp_patch *patch)
> kobject_init(&patch->kobj, &klp_ktype_patch);
> patch->enabled = false;
> patch->forced = false;
> + patch->ts = local_clock();
> INIT_WORK(&patch->free_work, klp_free_patch_work_fn);
> init_completion(&patch->finish);
>
> @@ -1324,6 +1326,7 @@ int klp_module_coming(struct module *mod)
> {
> char patch_name[MODULE_NAME_LEN];
> struct klp_patch *patch;
> + u64 patch_ts;
> int ret = 0;
>
> if (WARN_ON(mod->state != MODULE_STATE_COMING))
> @@ -1339,6 +1342,7 @@ int klp_module_coming(struct module *mod)
> continue;
>
> strncpy(patch_name, patch->obj->patch_name, sizeof(patch_name));
> + patch_ts = patch->ts;
> mutex_unlock(&klp_mutex);
>
> ret = klp_try_load_object(patch_name, mod->name);
> @@ -1346,14 +1350,11 @@ int klp_module_coming(struct module *mod)
> * The load might have failed because the patch has
> * been removed in the meantime. In this case, the
> * error might be ignored.
> - *
> - * FIXME: It is not fully proof. The patch might have be
> - * unloaded and loaded again in the mean time.
> */
> mutex_lock(&klp_mutex);
> if (ret) {
> patch = klp_find_patch(patch_name);
> - if (patch)
> + if (patch && patch->ts == patch_ts)
> goto err;

If the timestamps differ, we have found the klp_patch, feels a bit of a
waste to go through the list again without trying to load it right away.

Admittedly this is to solve a race condition which should not even
happen very often...

> ret = 0;
> }
>

Cheers,

--
Julien Thierry