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

Song Liu
2022-12-13T08:31:14Z

On Fri, Dec 9, 2022 at 4:55 AM Miroslav Benes <mbenes@suse.cz> 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@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.
> >
> > Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> > Signed-off-by: Song Liu <song@kernel.org>
>
> 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.

After reading the code (no testing), I think we don't need any logic for
ppc32 and s390.

We need clear_relocate_add() to handle module reload failure.
The failure happens when we

1) call apply_relocate_add() on klp load (or module first load,
if klp was loaded first);
2) do nothing when the module is unloaded;
3) call apply_relocate_add() on module reload, which failed.

This failure happens in the sanity check in
apply_relocate_add().

For x86, the check is something like:
if (*(s32 *)loc != 0)
goto invalid_relocation;

For ppc64, the check is in restore_r2():

if (*instruction != PPC_RAW_NOP()) {
pr_err("%s: Expected nop after call, got %08x at %pS\n",
me->name, *instruction, instruction);
return 0;
}

I don't think we have similar checks for ppc32 and s390, so
clear_relocate_add() is not needed for the two.

OTOH, we can argue that clear_relocate_add() should undo
everything apply_relocate_add() did. But I do think that
will be an overkill.

WDYT?

Thanks,
Song