Re: [PATCH v3] kernel/module: Fix mem leak in module_add_modinfo_attrs

[Jessica Yu]

+++ Miroslav Benes [04/06/19 12:46 +0200]:
> On Mon, 3 Jun 2019, YueHaibing wrote:
>
> > In module_add_modinfo_attrs if sysfs_create_file
> > fails, we forget to free allocated modinfo_attrs
> > and roll back the sysfs files.
> >
> > Fixes: 03e88ae1b13d ("[PATCH] fix module sysfs files reference counting")
> > Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
> > ---
> > v3: reuse module_remove_modinfo_attrs
> > v2: free from '--i' instead of 'i--'
> > ---
> >  kernel/module.c | 21 ++++++++++++++++-----
> >  1 file changed, 16 insertions(+), 5 deletions(-)
>
> I'm afraid it is not completely correct.
>
> > diff --git a/kernel/module.c b/kernel/module.c
> > index 80c7c09..c6b8912 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -1697,6 +1697,8 @@ static int add_usage_links(struct module *mod)
> >  	return ret;
> >  }
> >
> > +static void module_remove_modinfo_attrs(struct module *mod, int end);
> > +
> >  static int module_add_modinfo_attrs(struct module *mod)
> >  {
> >  	struct module_attribute *attr;
> > @@ -1711,24 +1713,33 @@ static int module_add_modinfo_attrs(struct module *mod)
> >  		return -ENOMEM;
> >
> >  	temp_attr = mod->modinfo_attrs;
> > -	for (i = 0; (attr = modinfo_attrs[i]) && !error; i++) {
> > +	for (i = 0; (attr = modinfo_attrs[i]); i++) {
> >  		if (!attr->test || attr->test(mod)) {
> >  			memcpy(temp_attr, attr, sizeof(*temp_attr));
> >  			sysfs_attr_init(&temp_attr->attr);
> >  			error = sysfs_create_file(&mod->mkobj.kobj,
> >  					&temp_attr->attr);
> > +			if (error)
> > +				goto error_out;
>
> sysfs_create_file() failed, so we need to clear all previously processed
> attrs and not the current one.
>
> >  			++temp_attr;
> >  		}
> >  	}
> > +
> > +	return 0;
> > +
> > +error_out:
> > +	module_remove_modinfo_attrs(mod, --i);
>
> It says "call sysfs_remove_file() on all attrs ending with --i included
> (all correctly processed attrs).
>
> >  	return error;
> >  }
> >
> > -static void module_remove_modinfo_attrs(struct module *mod)
> > +static void module_remove_modinfo_attrs(struct module *mod, int end)
> >  {
> >  	struct module_attribute *attr;
> >  	int i;
> >
> >  	for (i = 0; (attr = &mod->modinfo_attrs[i]); i++) {
> > +		if (end >= 0 && i > end)
> > +			break;
>
> If end == 0, you break the loop without calling sysfs_remove_file(), which
> is a bug if you called module_remove_modinfo_attrs(mod, 0).
>
> Calling module_remove_modinfo_attrs(mod, i); in module_add_modinfo_attrs()
> under error_out label and changing the condition here to
>
> if (end >= 0 && i >= end)
> 	break;
>
> should work as expected.
>
> But let me ask another question and it might be more to Jessica. Why is
> there even a call to attr->free(mod); (if it exists) in
> module_remove_modinfo_attrs()? The same is in free_modinfo() (as opposed
> to setup_modinfo() where attr->setup(mod) is called. Is it because
> free_modinfo() is called only in load_module()'s error path, while
> module_remove_modinfo_attrs() is called even in free_module() path?
>
> kfree() checks for NULL pointer, so there is no bug, but it is certainly
> not nice and it calls for cleanup. But I may be missing something.

No, you are right in that it is a bit clumsy and and the sysfs error
path handling is asymmetrical. I think it could be cleaned up a bit.

IMO, I think the attr->free() calls should either be (1) removed from
module_remove_modinfo_attrs() as free_modinfo() takes care of that,
otherwise we could potentially call attr->free() twice (once in the
internal error handling of mod_sysfs_setup() and once again in the
free_modinfo: label in load_module()) or option (2) would be to merge
the attr->setup() calls into module_add_modinfo_attrs() so that it is
symmetrical to module_remove_modinfo_attrs(). I'm leaning towards
option 2 but have not carefully checked yet if moving the
attr->setup() calls into module_add_modinfo_attrs() would break
anything. In any case I will prepare some cleanup patches for this.

Thanks!

Jessica