Re: [PATCH v6] livepatch: Clear relocation targets on a module removal

[Miroslav Benes]

Hi,

first thank you for taking over and I also appologize for not replying 
much sooner.

On Thu, 1 Sep 2022, Song Liu wrote:

> From: Miroslav Benes <mbenes@xxxxxxx>
> 
> 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.
> 
> Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
> Signed-off-by: Song Liu <song@xxxxxxxxxx>

Petr has commented on the code aspects. I will just add that s390x was not 
dealt with at the time because there was no live patching support for 
s390x back then if I remember correctly and my notes do not lie. The same 
applies to powerpc32. I think that both should be fixed as well with this 
patch. It might also help to clean up the ifdeffery in the patch a bit.

Miroslav