linux-kernel - Re: [PATCH v15 01/13] x86/sev: Carve out and export SNP guest messaging init routines

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Message-ID: <20241206202752.GCZ1NeSMYTZ4ZDcfGJ@fat_crate.local>
Date: Fri, 6 Dec 2024 21:27:52 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Nikunj A. Dadhania" <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, 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 v15 01/13] x86/sev: Carve out and export SNP guest
 messaging init routines

On Thu, Dec 05, 2024 at 11:53:53AM +0530, Nikunj A. Dadhania wrote:
> > * get_report - I don't think so:
> > 
> >         /*      
> >          * The intermediate response buffer is used while decrypting the
> >          * response payload. Make sure that it has enough space to cover the
> >          * authtag.
> >          */
> >         resp_len = sizeof(report_resp->data) + mdesc->ctx->authsize;
> >         report_resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);
> > 
> > That resp_len is limited and that's on the guest_ioctl path which cannot
> > happen concurrently?
> 
> It is a trusted allocation, but should it be accounted as it is part of
> the userspace ioctl path ?

Well, it is unlocked_ioctl() and snp_guest_ioctl() is not taking any locks.
What's stopping anyone from writing a nasty little program which hammers the
sev-guest on the ioctl interface until the OOM killer activates?

IOW, this should probably remain _ACCOUNT AFAICT.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette