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

[Song Liu]

On Tue, Jul 26, 2022 at 8:54 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> On Tue, Jul 26, 2022 at 4:33 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > On Thu, Jul 21, 2022 at 10:51:47AM -0700, 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>
> > >
> > > ---
> > >
> > > Changes from v2:
> > > 1. Rewrite x86 changes to match current code style.
> > > 2. Remove powerpc changes as there is no test coverage in v3.
> > > 3. Only keep 1/3 of v2.
> >
> > 1) All the copy/paste is ugly and IMO guaranteed to eventually introduce
> >    bugs when somebody forgets to update the copy.  Wouldn't it be more
> >    robust to reuse the existing apply_relocate_add() code by making it
> >    more generic somehow, like with a new 'clear' bool arg which sets
> >    'val' to zero?
>
> Agreed. I can give it a try.

I finished this part, though it is not really clean (added if else for
each "case:").

>
> >
> > 2) We can't only fix x86, powerpc also needs a fix.
>
> I have very little experience with powerpc. Would someone be willing to
> help with powerpc part of this?

I guess folks are all busy. Any suggestions on how to test powerpc changes?

>
> > 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.

Thanks,
Song