linux-kernel - Re: [PATCH] arm64/module: use mod->klp

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Message-ID: <20181025114228.4zkwvdfwrwhngwod@linux-8ccs>
Date:   Thu, 25 Oct 2018 13:42:28 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Petr Mladek <pmladek@...e.com>, Torsten Duwe <duwe@....de>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH] arm64/module: use mod->klp_info section header
 information

+++ 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