Re: [RFC PATCH v2 1/3] livepatch: Clear relocation targets on a module removal

[Miroslav Benes <mbenes@suse.cz>]

On Wed, 2 Oct 2019, Josh Poimboeuf wrote:

> On Thu, Sep 05, 2019 at 02:45:12PM +0200, Miroslav Benes wrote:
> > 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, or return back nops on powerpc). 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>
> 
> Since we decided to fix late module patching at LPC, the commit message
> and clear_relocate_add() should both probably clarify that these
> functions are hacks which are relatively temporary, until we fix the
> root cause.

It was the plan, but thanks for pointing it out explicitly. I could 
forget.
 
> But this patch gives me a bad feeling :-/  Not that I have a better
> idea.

I know what you are talking about.

> Has anybody seen this problem in the real world?  If not, maybe we'd be
> better off just pretending the problem doesn't exist for now.

I don't think so. You reported the issue originally and I guess it 
happened during the testing. Then there is a report from Huawei, but it 
suggests testing environment too. Reloading modules seems artificial to 
me.

So I agree, we can pretend the issue does not exist and wait for the real 
solution.

Miroslav