linux-kernel - Re: [PATCH v13 20/22] x86/kexec(): Reset TDX private memory on platforms with TDX erratum

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Date:   Mon, 18 Sep 2023 04:36:33 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v13 20/22] x86/kexec(): Reset TDX private memory on
 platforms with TDX erratum

> > > 
> > 
> > Good point.  Thanks!
> > 
> > Based on my understanding, it should be OK to skip tdx_reset_memory()
> > (or better
> > to) when preserve_context is on.  The second kernel shouldn't touch
> > first
> > kernel's memory anyway otherwise it may corrupt the first kernel
> > state (if it
> > does this maliciously or accidentally, then the first kernel isn't
> > guaranteed to
> > work anyway).  
> 
> I think it may read the memory, is it ok?

Read is fine.  Only "partial write" can poison the memory.

[...]
> 

> 
> Not the most beautiful ifdeffery, I'd just duplicate the
> tdx_reset_memory() call. But not a strong opinion.
> 

Refined to below.  Let me know if you have any further comments?

--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -307,12 +307,19 @@ void machine_kexec(struct kimage *image)
         * all TDX private pages need to be converted back to normal
         * before booting to the new kernel, otherwise the new kernel
         * may get unexpected machine check.
+        *
+        * But skip this when preserve_context is on.  The second kernel
+        * shouldn't write to the first kernel's memory anyway.  Skipping
+        * this also avoids killing TDX in the first kernel, which would
+        * require more complicated handling.
         */
-       tdx_reset_memory();
-
 #ifdef CONFIG_KEXEC_JUMP
        if (image->preserve_context)
                save_processor_state();
+       else
+               tdx_reset_memory();
+#else
+       tdx_reset_memory();
 #endif