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Message-ID: <1534b975275b78d61d851eb86faa226fd9be5c7a.camel@intel.com>
Subject: Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support
From:   Kai Huang <kai.huang@intel.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Wander Lairson Costa <wander@redhat.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        khalid.elmously@canonical.com, philip.cox@canonical.com,
        linux-kernel@vger.kernel.org
Date:   Thu, 05 May 2022 22:50:09 +1200
In-Reply-To: <40ccd0f0-35a1-5aa7-9e51-25ab196d79e5@linux.intel.com>
References: <20220501183500.2242828-1-sathyanarayanan.kuppuswamy@linux.intel.com>
         <20220501183500.2242828-4-sathyanarayanan.kuppuswamy@linux.intel.com>
         <243e918c523320ba3d216cbe22d24fe5ce33f370.camel@intel.com>
         <20220503012721.ok7fbvxmnvsr6qny@box.shutemov.name>
         <58d07b2d-cef5-17ed-9c57-e12fe5665e04@intel.com>
         <40ccd0f0-35a1-5aa7-9e51-25ab196d79e5@linux.intel.com>
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> +       /* Submit GetQuote Request */
> +       ret = tdx_get_quote_hypercall(buf);
> +       if (ret) {
> +               pr_err("GetQuote hypercall failed, status:%lx\n", ret);
> +               ret = -EIO;
> +               goto free_entry;
> +       }
> +
> +       /* Add current quote entry to quote_list */
> +       add_quote_entry(entry);
> +
> +       /* Wait for attestation completion */
> +       ret = wait_for_completion_interruptible(&entry->compl);
> +       if (ret < 0) {
> +               ret = -EIO;
> +               goto del_entry;
> +       }

This is misuse of wait_for_completion_interruptible(). 

xxx_interruptible() essentially means this operation can be interrupted by
signal.  Using xxx_interruptible() in driver IOCTL essentially means when it
returns due to signal, the IOCTL should return -EINTR to let userspace know that
your application received some signal needs handling, and this IOCTL isn't
finished and you should retry.  So here we should return -EINTR (and cleanup all
staff has been done) when wait_for_completion_interruptible() returns -
ERESTARTSYS (in fact, it returns only -ERESTARTSYS or 0).

Since normally userspace application just ignore signals, and in this particular
case, asking userspace to retry just makes things more complicated to handle, I
think you can just use wait_for_completion_killable(), which only returns when
the application receives signal that it is going to be killed.

> +
> +       /* Copy output data back to user buffer */
> +       if (copy_to_user((void __user *)quote_req.buf, buf->vmaddr, 
> quote_req.len))
> +               ret = -EFAULT;
> +
> +del_entry:
> +       del_quote_entry(entry);
> +free_entry:
> +       free_quote_entry(entry);

As I (and Isaku) mentioned before, when wait_for_completion_killable() returns
with error, you cannot just convert the buffer to private and free it.  The VMM
is still owning it (IN_FLIGHT).

One way to handle is you can put those buffers that are still owned by VMM to a
new list, and have some kernel thread to periodically check buffer's status and
free those are already released by VMM.  I haven't thought thoroughly, so maybe
there's better way to handle, though.

-- 
Thanks,
-Kai