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x86/nmi: Make register_nmi_handler() more robust

Ani Sinha requested to merge anisinha/kernel-centos-stream-9:bz2181172 into main

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2181172

   x86/nmi: Make register_nmi_handler() more robust
    
    Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2181172
    
    register_nmi_handler() has no sanity check whether a handler has been
    registered already. Such an unintended double-add leads to list corruption
    and hard to diagnose problems during the next NMI handling.
    
    Init the list head in the static NMI action struct and check it for being
    empty in register_nmi_handler().
    
      [ bp: Fixups. ]
    
    Reported-by: Sean Christopherson <seanjc@google.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Link: https://lore.kernel.org/lkml/20220511234332.3654455-1-seanjc@google.com
    (cherry picked from commit a7fed5c0431dbfa707037848830f980e0f93cfb3)

Signed-off-by: Ani Sinha anisinha@redhat.com

Edited by Mohamed Gamal Morsy

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