RE: [PATCHv6 00/16] x86/tdx: Add kexec support

From: "Huang, Kai" <kai.huang@intel.com>
To: Nikolay Borisov <nik.borisov@suse.com>, Baoquan He <bhe@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Hunter, Adrian" <adrian.hunter@intel.com>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	"Reshetova, Elena" <elena.reshetova@intel.com>,
	"Nakajima, Jun" <jun.nakajima@intel.com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	"Kalra, Ashish" <ashish.kalra@amd.com>,
	"Sean Christopherson" <seanjc@google.com>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>,
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCHv6 00/16] x86/tdx: Add kexec support
Date: Wed, 31 Jan 2024 13:07:29 +0000	[thread overview]
Message-ID: <BL1PR11MB5978633DF36A69F8020818E1F77C2@BL1PR11MB5978.namprd11.prod.outlook.com> (raw)
In-Reply-To: <5d91fc11-0b3b-40f0-872d-abdbb4038f76@suse.com>

> > Runtime disabling kexec looks better than at cmpile time, esp for
> > distros. While from above patch, making using of kexec_load_disabled
> > to achive the runtime disabling may not be so good. Because we have a
> > front door to enable it through:
> >
> > /proc/sys/kernel/kexec_load_disabled
> 
> AFAIU it can't be enabled via this sysctl because the handler for it expects
> only 1 to be written to it:
> 
>       2                 .proc_handler   = proc_dointvec_minmax,
> 
>       1                 .extra1         = SYSCTL_ONE,
> 
>    994                  .extra2         = SYSCTL_ONE,
> 

This is also my understanding.  

The documentation also says once it is turned to disable we cannot turn back again:

kexec_load_disable
===================

A toggle indicating if the syscalls ``kexec_load`` and
``kexec_file_load`` have been disabled.
This value defaults to 0 (false: ``kexec_*load`` enabled), but can be
set to 1 (true: ``kexec_*load`` disabled).
Once true, kexec can no longer be used, and the toggle cannot be set
back to false.
......