linux-kernel - Re: [PATCH v4 1/3] x86/tdx: Add TDX Guest attestation interface driver

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Date:   Thu, 28 Apr 2022 11:04:59 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Wander Lairson Costa <wander@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] x86/tdx: Add TDX Guest attestation interface
 driver

On 4/28/22 10:56, Sathyanarayanan Kuppuswamy wrote:
> On 4/28/22 10:45 AM, Wander Lairson Costa wrote:
>> On Fri, Apr 22, 2022 at 04:34:16PM -0700, Kuppuswamy Sathyanarayanan
>> wrote:
>>> +static long tdx_get_tdreport(void __user *argp)
>>> +{
>>> +    void *report_buf = NULL, *tdreport_buf = NULL;
>>> +    long ret = 0, err;
>>> +
>>> +    /* Allocate space for report data */
>>> +    report_buf = kmalloc(TDX_REPORT_DATA_LEN, GFP_KERNEL);
>>> +    if (!report_buf)
>>> +        return -ENOMEM;
>>> +
>>> +    /*
>>> +     * Allocate space for TDREPORT buffer (1024-byte aligned).
>>> +     * Full page alignment is more than enough.
>>> +     */
>>> +    tdreport_buf = (void *)get_zeroed_page(GFP_KERNEL);
>>
>> Maybe we should add BUILD_BUG_ON(TDX_TDREPORT_LEN > PAGE_SIZE)
> 
> Currently, it is a constant value < PAGE_SIZE. But I can add the
> BUILD_BUG_ON check for it.

That's kinda silly.  If it might ever be bigger than a page, you just do:

	tdreport_buf = alloc_pages();

But, seriously, TDX_TDREPORT_LEN is part of the ABI and can't change.
kmalloc() would work too since TDX_TDREPORT_LEN is a power of 2.