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

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Message-ID: <CAPhsuW7tmRt=yx1+i62HPRJJ4Gp8xB_3XvpVxUC=SyGv6iCBEQ@mail.gmail.com>
Date:   Tue, 26 Jul 2022 20:54:38 -0700
From:   Song Liu <song@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     live-patching@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        X86 ML <x86@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3] livepatch: Clear relocation targets on a module removal

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'
> >
> > 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@...hat.com>
> > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> > Signed-off-by: Song Liu <song@...nel.org>
> >
> > ---
> >
> > 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.

>
> 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?

> 3) A selftest would be a good idea.

I will try this.

Thanks,
Song