linux-kernel - RE: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <SA1PR11MB67347752DDDCB7F608003C0AA8869@SA1PR11MB6734.namprd11.prod.outlook.com>
Date:   Wed, 22 Mar 2023 23:43:43 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in
 NMI caused VM exits when FRED is enabled

> > Like IRET for IDT, ERETS/ERETU are required to end the NMI handler for
> > FRED to unblock NMI ASAP (w/ bit 28 of CS set).
> 
> That's "CS" on the stack correct?  Is bit 28 set manually by software, or is it set
> automatically by hardware?  If it's set by hardware, does "int $2" actually set the
> bit since it's not a real NMI?

Right, It's the "CS" on the stack. The bit 28 is set by the FRED NMI handler:
https://lore.kernel.org/lkml/20230307023946.14516-20-xin3.li@intel.com/

Upon a NMI delivery, the NMI bit is always set by H/W. However, "int $2" does
NOT set it, thus we need to explicitly set it.
 
> > And there are 2 approaches to
> > invoke the FRED NMI handler:
> > 1) execute "int $2", let the h/w do the job.
> > 2) create a FRED NMI stack frame on the current kernel stack with ASM,
> >    and then jump to fred_entrypoint_kernel in arch/x86/entry/entry_64_fred.S.
> >
> > 1) is preferred as we want less ASM.
> 
> Who is "we", and how much assembly are we talking about?  E.g. I personally don't
> mind a trampoline in KVM if it's small and/or can share code with existing assembly
> subroutines.

I ever got such a comment:
https://lore.kernel.org/lkml/8735bpbhat.ffs@tglx/

However, if ASM is also okay, I can work on it.  And I don't think the ASM code
will be big.