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

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Message-ID: <f0b27aab-2adb-444c-97d3-07e69c4c48a7@amd.com>
Date: Mon, 9 Dec 2024 11:46:44 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Borislav Petkov <bp@...en8.de>
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 12/7/2024 1:57 AM, Borislav Petkov wrote:
> 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.

Both get_report()/get_ext_report() are in the unlocked_ioctl(), we will
retain the _ACCOUNT

That leaves us with only one site: snp_init_crypto(), should I fold this change
in current patch ?
Regards
Nikunj