linux-kernel - Re: [PATCH v9 03/24] virt: sev-guest: Make payload a variable length array

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Date: Mon, 24 Jun 2024 15:39:51 +0200
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: "Nikunj A. Dadhania" <nikunj@....com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, kvm@...r.kernel.org, mingo@...hat.com,
	tglx@...utronix.de, dave.hansen@...ux.intel.com, pgonda@...gle.com,
	seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v9 03/24] virt: sev-guest: Make payload a variable length
 array

On Mon, Jun 24, 2024 at 08:00:38AM -0500, Tom Lendacky wrote:
> An alternative to the #defines would be something like:
> 
> struct snp_guest_msg {
> 	struct snp_guest_msg_hdr hdr;
> 	u8 payload[PAGE_SIZE - sizeof(struct snp_guest_msg_hdr)];
>  } __packed;
> 
> Not sure it matters, but does reduce the changes while ensuring the
> payload plus header doesn't exceed a page.

Yeah, because that would've been my next question - the requirement to keep it
<= PAGE_SIZE.

So yeah, Nikunj, please do that also and add a 

	BUILD_BUG_ON(sizeof(struct snp_guest_msg) > PAGE_SIZE);

somewhere in the driver to catch all kinds of funky stuff.

Thx.

-- 
Regards/Gruss,
    Boris.

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