linux-kernel - Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support

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Date:   Thu, 05 May 2022 22:50:09 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        Wander Lairson Costa <wander@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support

> +       /* 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