linux-kernel - [PATCH 0/3] Deny the patched modules to be removed

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Date:   Thu,  7 Jun 2018 11:29:46 +0200
From:   Miroslav Benes <mbenes@...e.cz>
To:     jikos@...nel.org, jpoimboe@...hat.com, jeyu@...nel.org
Cc:     pmladek@...e.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>
Subject: [PATCH 0/3] Deny the patched modules to be removed

Quoting the second patch's changelog:

    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 reverse the relocation patching (clear all relocation
    targets on x86_64, or return back nops on powerpc) in
    klp_unpatch_object(). The solution is not universal and is too much
    arch-specific.
    
    We decided to deny the patched modules to be removed. If it proves to be
    a major drawback for users, we can still implement a different approach.

There were many subtle details to check, so it may be broken somewhere.
Hopefully it is not, but please review carefully.

Miroslav Benes (3):
  livepatch: Nullify obj->mod in klp_module_coming()'s error path
  livepatch: Deny the patched modules to be removed
  module: Remove superfluous call to klp_module_going()

 kernel/livepatch/core.c | 25 +++++++++++++++++++++++--
 kernel/module.c         |  1 -
 2 files changed, 23 insertions(+), 3 deletions(-)

-- 
2.17.0