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From: Sathyanarayanan Kuppuswamy  <sathyanarayanan.kuppuswamy@linux.intel.com>
To: Hans de Goede <hdegoede@redhat.com>,
	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, Mark Gross <mgross@linux.intel.com>
Cc: "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>,
	linux-kernel@vger.kernel.org,
	platform-driver-x86@vger.kernel.org
Subject: Re: [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver
Date: Mon, 4 Apr 2022 12:56:35 -0700	[thread overview]
Message-ID: <aece84e1-2c90-2c18-993a-96f8fed7bb46@linux.intel.com> (raw)
In-Reply-To: <8308a830-3096-3f94-4f12-5fd2c290524e@redhat.com>

Hi Hans,

On 4/4/22 3:07 AM, Hans de Goede wrote:
>> +static int __init tdx_attest_init(void)
>> +{
>> +	dma_addr_t handle;
>> +	long ret = 0;
>> +
>> +	mutex_lock(&attestation_lock);
>> +
>> +	ret = misc_register(&tdx_attest_device);
>> +	if (ret) {
>> +		pr_err("misc device registration failed\n");
>> +		mutex_unlock(&attestation_lock);
>> +		return ret;
>> +	}
> Why not do this as the last thing of the probe?

We need misc device reference in dma_alloc_coherent() and
dma_set_coherent_mask() calls. This is the reason for keeping
misc_register() at the beginining of the init function.

> 
> That will avoid the need to unregister this again in all
> the error-exit paths and also fixes a possible deadlock.
> 

Agree. But, unless we create another device locally, I don't
think we can avoid this. Do you prefer this approach?

> Right now you possibly have:
> 
> 1. probe() locks attestation_lock
> 2. probe() registers misc-device
> 3. userspace calls tdx_attest_ioctl
> 4. tdx_attest_ioctl blocks waiting for attestastion_lock
> 5. Something goes wrong in probe, probe calls
>     misc_deregister()
> 6. misc_deregister waits for the ioctl to finish
> 7. deadlock
> 
> I'm not sure about 6, but if 6 does not happen then
> instead we now have tdx_attest_ioctl running
> after the misc_deregister, with tdquote_data and
> tdreport_data as NULL, or pointing to free-ed memory
> leading to various crash scenarios.

Makes sense. But as I have mentioned above, we have reason
for keeping the misc_register() at the begining of the
init function.

One way to avoid this deadlock is to use global initalization
check.

--- a/drivers/platform/x86/intel/tdx/intel_tdx_attest.c
+++ b/drivers/platform/x86/intel/tdx/intel_tdx_attest.c
@@ -48,6 +48,8 @@ static void *tdreport_data;
  /* DMA handle used to allocate and free tdquote DMA buffer */
  dma_addr_t tdquote_dma_handle;

+static bool device_initialized;
+
  static void attestation_callback_handler(void)
  {
         complete(&attestation_done);
@@ -60,6 +62,9 @@ static long tdx_attest_ioctl(struct file *file, 
unsigned int cmd,
         struct tdx_gen_quote tdquote_req;
         long ret = 0;

+       if (!device_initialized)
+               return -ENODEV;
+
         mutex_lock(&attestation_lock);

         switch (cmd) {
@@ -191,6 +196,8 @@ static int __init tdx_attest_init(void)

         mutex_unlock(&attestation_lock);

+       device_initialized = true;
+
         pr_debug("module initialization success\n");

         return 0;

Please let me know your comment on above solution.

> 
> TL;DR: you must always delay registering any
> interfaces for userspace until your code is
> ready to deal with userspace calls.
> 
> Regards,
> 
> Hans
> 
> p.s.
> 
> As I mentioned with v1:
> 
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer

  reply	other threads:[~2022-04-04 21:23 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-30 22:17 [PATCH v2 0/6] Add TDX Guest Attestation support Kuppuswamy Sathyanarayanan
2022-03-30 22:18 ` [PATCH v2 1/6] x86/tdx: Add tdx_mcall_tdreport() API support Kuppuswamy Sathyanarayanan
2022-03-30 22:18 ` [PATCH v2 2/6] x86/tdx: Add tdx_hcall_get_quote() " Kuppuswamy Sathyanarayanan
2022-03-31  1:55   ` Aubrey Li
2022-03-30 22:18 ` [PATCH v2 3/6] x86/tdx: Add SetupEventNotifyInterrupt TDX hypercall support Kuppuswamy Sathyanarayanan
2022-03-30 22:18 ` [PATCH v2 4/6] x86/tdx: Add TDX Guest event notify interrupt vector support Kuppuswamy Sathyanarayanan
2022-03-30 22:18 ` [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver Kuppuswamy Sathyanarayanan
2022-04-04 10:07   ` Hans de Goede
2022-04-04 19:56     ` Sathyanarayanan Kuppuswamy [this message]
2022-04-11 14:38       ` Hans de Goede
2022-04-04 10:09   ` Hans de Goede
2022-04-04 10:11     ` Hans de Goede
2022-03-30 22:18 ` [PATCH v2 6/6] tools/tdx: Add a sample attestation user app Kuppuswamy Sathyanarayanan
  -- strict thread matches above, loose matches on Subject: below --
2021-07-07 20:42 [PATCH v2 0/6] Add TDX Guest Support (Attestation support) Kuppuswamy Sathyanarayanan
2021-07-07 20:42 ` [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver Kuppuswamy Sathyanarayanan
2021-07-08 22:21   ` Andy Lutomirski
2021-07-08 22:35     ` Dave Hansen
2021-07-09  0:38       ` Andi Kleen
2021-07-13  0:33         ` Kuppuswamy, Sathyanarayanan
2021-07-13  0:44           ` Dave Hansen
2021-07-08 23:34     ` Kuppuswamy, Sathyanarayanan
2021-07-08 23:36   ` Dan Williams
2021-07-08 23:57     ` Kuppuswamy, Sathyanarayanan
2021-07-09  0:20       ` Dan Williams
2021-07-09  0:36         ` Andi Kleen
2021-07-09  1:37           ` Dan Williams
2021-07-09  1:44             ` Andi Kleen
2021-07-09  2:04               ` Dan Williams
2021-07-09  2:43                 ` Kuppuswamy, Sathyanarayanan

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