linux-kernel - Re: [PATCH] x86/sev-es: Do not use copy_from_kernel

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Message-ID: <e037f1c0-aaf6-7951-04ea-6d27e557b61b@intel.com>
Date:   Wed, 6 Sep 2023 16:01:16 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Adam Dunlap <acdunlap@...gle.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nikunj A Dadhania <nikunj@....com>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>, Jacob Xu <jacobhxu@...gle.com>
Subject: Re: [PATCH] x86/sev-es: Do not use copy_from_kernel_nofault in early
 #VC handler

On 9/6/23 15:45, Adam Dunlap wrote:
>  static int vc_fetch_insn_kernel(struct es_em_ctxt *ctxt,
> -				unsigned char *buffer)
> +				unsigned char *buffer, bool is_early)
>  {
> -	return copy_from_kernel_nofault(buffer, (unsigned char *)ctxt->regs->ip, MAX_INSN_SIZE);
> +	if (is_early) {
> +		memcpy(buffer, (unsigned char *)ctxt->regs->ip, MAX_INSN_SIZE);
> +		return 0;
> +	} else {
> +		return copy_from_kernel_nofault(buffer, (unsigned char *)ctxt->regs->ip,
> +			MAX_INSN_SIZE);
> +	}
>  }

This isn't the normal way we do these kinds of things.

If we go with this solution, they next guy who tries
copy_from_kernel_nofault() will hit the same issue, and start plumbing
their own 'is_early' through _their_ little chunk of arch/x86.

Usually, we'll add some gunk in arch/x86/boot/compressed/misc.h to
override the troublesome implementation.  In this case, it would make a
lot of sense to somehow avoid touching boot_cpu_data.x86_virt_bits in
the first place.