linux-kernel - Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for tdx_enc_status

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Message-ID: <20230523223750.botogigv6ht7p2zg@box.shutemov.name>
Date:   Wed, 24 May 2023 01:37:50 +0300
From:   kirill.shutemov@...ux.intel.com
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Dexuan Cui <decui@...rosoft.com>, ak@...ux.intel.com,
        arnd@...db.de, bp@...en8.de, brijesh.singh@....com,
        dan.j.williams@...el.com, dave.hansen@...ux.intel.com,
        haiyangz@...rosoft.com, hpa@...or.com, jane.chu@...cle.com,
        kys@...rosoft.com, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, luto@...nel.org, mingo@...hat.com,
        peterz@...radead.org, rostedt@...dmis.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        tglx@...utronix.de, tony.luck@...el.com, wei.liu@...nel.org,
        x86@...nel.org, mikelley@...rosoft.com,
        linux-kernel@...r.kernel.org, Tianyu.Lan@...rosoft.com
Subject: Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for
 tdx_enc_status_changed()

On Tue, May 23, 2023 at 01:39:11PM -0700, Dave Hansen wrote:
> On 5/4/23 15:53, Dexuan Cui wrote:
> > When a TDX guest runs on Hyper-V, the hv_netvsc driver's netvsc_init_buf()
> > allocates buffers using vzalloc(), and needs to share the buffers with the
> > host OS by calling set_memory_decrypted(), which is not working for
> > vmalloc() yet. Add the support by handling the pages one by one.
> 
> I think this sets a bad precedent.
> 
> There are consequences for converting pages between shared and private.
> Doing it on a vmalloc() mapping is guaranteed to fracture the underlying
> EPT/SEPT mappings.
> 
> How does this work with load_unaligned_zeropad()?  Couldn't it be
> running around poking at one of these vmalloc()'d pages via the direct
> map during a shared->private conversion before the page has been accepted?

Alias processing in __change_page_attr_set_clr() will change direct
mapping if you call it on vmalloc()ed memory. I think we are safe wrt
load_unaligned_zeropad() here.
-- 
  Kiryl Shutsemau / Kirill A. Shutemov