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

[Song Liu]

On Fri, Dec 9, 2022 at 4:55 AM Miroslav Benes <mbenes@xxxxxxx> wrote:
>
> 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.

I don't have test environments for s390 and powerpc, so I really don't know
whether I am doing something sane for them.

Would you have time to finish these parts? (Or maybe the whole patch..)

Thanks,
Song