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

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Message-ID: <20220726233302.zwloxsammnu7clu4@treble>
Date:   Tue, 26 Jul 2022 16:33:02 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Song Liu <song@...nel.org>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        jikos@...nel.org, mbenes@...e.cz, pmladek@...e.com,
        joe.lawrence@...hat.com, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3] livepatch: Clear relocation targets on a module
 removal

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?

2) We can't only fix x86, powerpc also needs a fix.

3) A selftest would be a good idea.

-- 
Josh