Re: [PATCH] arm64/module: use mod->klp_info section header information

[Jessica Yu]

+++ Miroslav Benes [25/10/18 11:00 +0200]:
> On Thu, 25 Oct 2018, Petr Mladek wrote:
>
> > On Tue 2018-10-23 19:55:54, Jessica Yu wrote:
> > > The arm64 module loader keeps a pointer into info->sechdrs to keep track
> > > of section header information for .plt section(s). A pointer to the
> > > relevent section header (struct elf64_shdr) in info->sechdrs is stored
> > > in mod->arch.{init,core}.plt. This pointer may be accessed while
> > > applying relocations in apply_relocate_add() for example. And unlike
> > > normal modules, livepatch modules can call apply_relocate_add() after
> > > module load. But the info struct (and therefore info->sechdrs) gets
> > > freed at the end of load_module() and so mod->arch.{init,core}.plt
> > > becomes an invalid pointer after the module is done loading.
> > >
> > > Luckily, livepatch modules already keep a copy of Elf section header
> > > information in mod->klp_info. So make sure livepatch modules on arm64
> > > have access to the section headers in klp_info and set
> > > mod->arch.{init,core}.plt to the appropriate section header in
> > > mod->klp_info so that they can call apply_relocate_add() even after
> > > module load.
> > >
> > > diff --git a/kernel/module.c b/kernel/module.c
> > > index f475f30eed8c..f3ac04cc9fc3 100644
> > > --- a/kernel/module.c
> > > +++ b/kernel/module.c
> > > @@ -3367,6 +3367,8 @@ int __weak module_finalize(const Elf_Ehdr *hdr,
> > >
> > > static int post_relocation(struct module *mod, const struct load_info *info)
> > > {
> > > +	int err;
> > > +
> > > 	/* Sort exception table now relocations are done. */
> > > 	sort_extable(mod->extable, mod->extable + mod->num_exentries);
> > >
> > > @@ -3377,8 +3379,18 @@ static int post_relocation(struct module *mod, const struct load_info *info)
> > > 	/* Setup kallsyms-specific fields. */
> > > 	add_kallsyms(mod, info);
> > >
> > > +	if (is_livepatch_module(mod)) {
> > > +		err = copy_module_elf(mod, info);
> > > +		if (err < 0)
> > > +			return err;
> > > +	}
> > > +
> > > 	/* Arch-specific module finalizing. */
> > > -	return module_finalize(info->hdr, info->sechdrs, mod);
> > > +	err = module_finalize(info->hdr, info->sechdrs, mod);
> > > +	if (err < 0)
> >
> > 	if (err < 0 && is_livepatch_module(mod))
>
> Ah, right.
>
> > > +		free_module_elf(mod);
> > > +
> > > +	return err;
> > > }
> >
> > Also we need to free the copied stuff in load_module() when
> > anything called after post_relocation() fails. I think
> > that the following would work:
> >
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -3823,6 +3823,8 @@ static int load_module(struct load_info *info, const char __user *uargs,
> >  	kfree(mod->args);
> >   free_arch_cleanup:
> >  	module_arch_cleanup(mod);
> > +	if (is_livepatch_module(mod))
> > +		free_module_elf(mod);
> >   free_modinfo:
> >  	free_modinfo(mod);
> >   free_unload:
>
> Yes, we need to free it somewhere and I missed it. free_arch_cleanup seems
> to be the correct place.

Good catches, thank you both!

> > But I suggest to just move copy_module_elf() up and keep
> > calling it from load_module() directly. It would make
> > the error handling more clear.
>
> Unfortunately it is not that simple. arm64's module_finalize() uses
> mod->klp_info with the patch, so copy_module_elf() must be called before.
> We could move module_finalize() from post_relocation() to load_module()
> and place copy_module_elf() between those two, but I don't know. That's up
> to Jessica.

Yeah, it's a stylistic preference - will shuffle those calls around
and see what looks best. v2 to come shortly.

Thanks!

Jessica