linux-kernel - Re: [PATCH v3] livepatch: Clear relocation targets on a module removal

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Date:   Tue, 2 Aug 2022 09:30:04 -0700
From:   Song Liu <song@...nel.org>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        live-patching@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, X86 ML <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3] livepatch: Clear relocation targets on a module removal

Hi Joe,

On Tue, Aug 2, 2022 at 5:31 AM Joe Lawrence <joe.lawrence@...hat.com> wrote:
>
> On 8/1/22 17:19, Song Liu wrote:
> > On Mon, Aug 1, 2022 at 3:25 AM Petr Mladek <pmladek@...e.com> wrote:
> >>
> >> On Sat 2022-07-30 20:20:22, Song Liu wrote:
> >>> On Sat, Jul 30, 2022 at 3:32 PM Song Liu <song@...nel.org> wrote:
> >>>>
> >>>> On Tue, Jul 26, 2022 at 8:54 PM Song Liu <song@...nel.org> wrote:
> >>>>>
> >>>>> On Tue, Jul 26, 2022 at 4:33 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> >>>>>>
> >>>>>> On Thu, Jul 21, 2022 at 10:51:47AM -0700, Song Liu wrote:
> >>>>>>> From: Miroslav Benes <mbenes@...e.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'
> >>>>>>>
> >>>>>> 3) A selftest would be a good idea.
> >>>>>
> >>>>
> >>>> I found it is pretty tricky to run the selftests inside a qemu VM. How about
> >>>> we test it with modules in samples/livepatch? Specifically, we can add a
> >>>> script try to reload livepatch-shadow-mod.ko.
> >>>
> >>> Actually, livepatch-shadow-mod.ko doesn't have the reload problem before
> >>> the fix. Is this expected?
> >>
> >> Good question. I am afraid that there is no easy way to prepare
> >> the selftest at the moment.
> >>
> >> There are two situations when a symbol from the livepatched module is
> >> relocated:
> >>
> >>
> >> 1. The livepatch might access a symbol exported by the module via
> >>    EXPORT_SYMBOL(). In this case, it is "normal" external symbol
> >>    and it gets relocated by the module loader.
> >>
> >>    But EXPORT_SYMBOL() will create an explicit dependency between the
> >>    livepatch and livepatched module. As a result, the livepatch
> >>    module could be loaded only when the livepatched module is loaded.
> >>    And the livepatched module could not be removed when the livepatch
> >>    module is loaded.
> >>
> >>    In this case, the problem will not exist. Well, the developers
> >>    of the livepatch module will probably want to avoid this
> >>    dependency.
> >>
> >>
> >> 2. The livepatch module might access a non-exported symbol from another
> >>    module using the special elf section for klp relocation, see
> >>    section, see Documentation/livepatch/module-elf-format.rst
> >>
> >>    These symbols are relocated in klp_apply_section_relocs().
> >>
> >>    The problem is that upstream does not have a support to
> >>    create this elf section. There is a patchset for this, see
> >>    https://lore.kernel.org/all/20220216163940.228309-1-joe.lawrence@redhat.com/
> >>    It requires some more review.
> >>
> >>
> >> Resume: I think that we could not prepare the selftest without
> >>         upstreaming klp-convert tool.
> >
> > Thanks for the explanation! I suspected the same issue, but couldn't
> > connect all the logic.
> >
> > I guess the selftests can wait until the klp-convert tool.
> >
>
> Hi Song,
>
> Petr is correct about selftests and these relocations.  Let me know if
> rebasing the klp-convert patchset would be helpful in your testing.
> Otherwise kpatch-build is the only (easy?) way to create klp-relocations
> as far as I know.  (For limited arches anyway.)

I was able to test the patch on x86_64 with kpatch-build. I will look into
klp-convert later (after I fix kpatch-build for some OOT modules..)

I the meanwhile, I guess this patch doesn't need to wait for klp-convert
and the selftest?

Thanks,
Song