Re: [PATCH bpf-next] kallsyms: Disable preemption for find_kallsyms_symbol_value

[Artem Savkov <asavkov@redhat.com>]

On Tue, Apr 04, 2023 at 12:02:54AM +0200, Jiri Olsa wrote:
> Artem reported suspicious RCU usage [1]. The reason is that verifier
> calls find_kallsyms_symbol_value with preemption enabled which will
> trigger suspicious RCU usage warning in rcu_dereference_sched call.
> 
> Disabling preemption in find_kallsyms_symbol_value and adding
> __find_kallsyms_symbol_value function.

Tested-by: Artem Savkov <asavkov@redhat.com>

> Fixes: 31bf1dbccfb0 ("bpf: Fix attaching fentry/fexit/fmod_ret/lsm to modules")
> [1] https://lore.kernel.org/bpf/ZBrPMkv8YVRiWwCR@samus.usersys.redhat.com/
> Reported-by: Artem Savkov <asavkov@redhat.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/module/kallsyms.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> index ab2376a1be88..bdc911dbcde5 100644
> --- a/kernel/module/kallsyms.c
> +++ b/kernel/module/kallsyms.c
> @@ -442,7 +442,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
>  }
>  
>  /* Given a module and name of symbol, find and return the symbol's value */
> -unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
> +static unsigned long __find_kallsyms_symbol_value(struct module *mod, const char *name)
>  {
>  	unsigned int i;
>  	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
> @@ -466,7 +466,7 @@ static unsigned long __module_kallsyms_lookup_name(const char *name)
>  	if (colon) {
>  		mod = find_module_all(name, colon - name, false);
>  		if (mod)
> -			return find_kallsyms_symbol_value(mod, colon + 1);
> +			return __find_kallsyms_symbol_value(mod, colon + 1);
>  		return 0;
>  	}
>  
> @@ -475,7 +475,7 @@ static unsigned long __module_kallsyms_lookup_name(const char *name)
>  
>  		if (mod->state == MODULE_STATE_UNFORMED)
>  			continue;
> -		ret = find_kallsyms_symbol_value(mod, name);
> +		ret = __find_kallsyms_symbol_value(mod, name);
>  		if (ret)
>  			return ret;
>  	}
> @@ -494,6 +494,16 @@ unsigned long module_kallsyms_lookup_name(const char *name)
>  	return ret;
>  }
>  
> +unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
> +{
> +	unsigned long ret;
> +
> +	preempt_disable();
> +	ret = __find_kallsyms_symbol_value(mod, name);
> +	preempt_enable();
> +	return ret;
> +}
> +
>  int module_kallsyms_on_each_symbol(const char *modname,
>  				   int (*fn)(void *, const char *,
>  					     struct module *, unsigned long),
> -- 
> 2.39.2
> 

-- 
 Artem