Re: [PATCH v8] livepatch: Clear relocation targets on a module removal

[Miroslav Benes <mbenes@suse.cz>]

Hi,

On Fri, 6 Jan 2023, Song Liu wrote:

> From: Miroslav Benes <mbenes@suse.cz>
> 
> Josh reported a bug:
> 
>   When the object to be patched is a module, and that module is
>   rmmod'ed and reloaded, it fails to load with:
> 
>   module: x86/modules: Skipping invalid relocation target, existing value is nonzero for type 2, loc 00000000ba0302e9, val ffffffffa03e293c
>   livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
>   livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'
> 
>   The livepatch module has a relocation which references a symbol
>   in the _previous_ loading of nfsd. When apply_relocate_add()
>   tries to replace the old relocation with a new one, it sees that
>   the previous one is nonzero and it errors out.
> 
>   On ppc64le, we have a similar issue:
> 
>   module_64: livepatch_nfsd: Expected nop after call, got e8410018 at e_show+0x60/0x548 [livepatch_nfsd]
>   livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
>   livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'
> 
> He also proposed three different solutions. We could remove the error
> check in apply_relocate_add() introduced by commit eda9cec4c9a1
> ("x86/module: Detect and skip invalid relocations"). However the check
> is useful for detecting corrupted modules.
> 
> We could also deny the patched modules to be removed. If it proved to be
> a major drawback for users, we could still implement a different
> approach. The solution would also complicate the existing code a lot.
> 
> We thus decided to reverse the relocation patching (clear all relocation
> targets on x86_64). The solution is not
> universal and is too much arch-specific, but it may prove to be simpler
> in the end.
> 
> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> Signed-off-by: Song Liu <song@kernel.org>
> Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>

I would be fine if you just claimed the authorship (and include my 
Originally-by: tag for example), because you have reworked it quite a lot 
since my first attempts.

> +int klp_apply_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
> +			     const char *shstrtab, const char *strtab,
> +			     unsigned int symndx, unsigned int secndx,
> +			     const char *objname)
> +{
> +	return klp_write_section_relocs(pmod, sechdrs, shstrtab, strtab, symndx,
> +					secndx, objname, true);
>  }

Is this redirection needed somewhere? You could just replace 
klp_apply_section_relocs() with klp_write_section_relocs() in 
include/linux/livepatch.h and kernel/module/main.c.

It may be cleaned up later.

Acked-by: Miroslav Benes <mbenes@suse.cz>

It would be nice to get an Acked-by from a x86 maintainter as well.

Thanks
Miroslav