linux-kernel - Re: [PATCH] cxl/memdev: Avoid mailbox functionality on device memory CXL devices

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Message-ID: <ogv7evtgf5rcljd4ev7rxx6buqg7y3kwlqing2tfshh2ac5zf6@juclnksnv4ki>
Date:   Mon, 31 Jul 2023 19:46:01 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cxl/memdev: Avoid mailbox functionality on device memory
 CXL devices

On Fri, 28 Jul 2023, Dan Williams wrote:

>Ira Weiny wrote:
>> Using the proposed type-2 cxl-test device[1] the following
>> splat was observed:
>>
>>   BUG: kernel NULL pointer dereference, address: 0000000000000278
>>   [...]
>>   RIP: 0010:devm_cxl_add_memdev+0x1de/0x2c0 [cxl_core]
>
>It would be useful to decode this to a line number, the rest of this
>call trace is not adding much.
>
>>   [...]
>>   Call Trace:
>>    <TASK>
>>    ? __die+0x1f/0x70
>>    ? page_fault_oops+0x149/0x420
>>    ? fixup_exception+0x22/0x310
>>    ? kernelmode_fixup_or_oops+0x84/0x110
>>    ? exc_page_fault+0x6d/0x150
>>    ? asm_exc_page_fault+0x22/0x30
>>    ? devm_cxl_add_memdev+0x1de/0x2c0 [cxl_core]
>>    cxl_mock_mem_probe+0x632/0x870 [cxl_mock_mem]
>>    platform_probe+0x40/0x90
>>    really_probe+0x19e/0x3e0
>>    ? __pfx___driver_attach+0x10/0x10
>>    __driver_probe_device+0x78/0x160
>>    driver_probe_device+0x1f/0x90
>>    __driver_attach+0xce/0x1c0
>>    bus_for_each_dev+0x63/0xa0
>>    bus_add_driver+0x112/0x210
>>    driver_register+0x55/0x100
>>    ? __pfx_cxl_mock_mem_driver_init+0x10/0x10 [cxl_mock_mem]
>>    [...]
>>
>> Commit f6b8ab32e3ec made the mailbox functionality optional.  However,
>> some mailbox functionality was merged after that patch.  Therefore some
>> mailbox functionality can be accessed on a device which did not set up
>> the mailbox.
>
>cxl_memdev_security_init() definitely needs to move out of
>devm_cxl_add_memdev() and after that I do not think @mds NULL checks
>need to be sprinkled everywhere. In other words something is wrong at a
>higher level if we get into some of these helper functions without the
>memory device state.

Right, so we can move it directly into cxl_pci_probe() - just as with other
mbox based functionality. This leaves me wondering, however, what to do about
the cxl_memdev_security_shutdown() counterpart. As with the below diff, leaving
it as is and just adding a mds nil check might still be considering a layering
violation in that it would be asymmetrical wrt to the init; but this is tightly
coupled with cxl_memdev_unregister().

Ira does the below fix the crash?

Thanks,
Davidlohr

----8<-------
diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
index 14b547c07f54..4d1bf80c0e54 100644
--- a/drivers/cxl/core/memdev.c
+++ b/drivers/cxl/core/memdev.c
@@ -561,7 +561,7 @@ static void cxl_memdev_security_shutdown(struct device *dev)
  	struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
  	struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlmd->cxlds);
  
-	if (mds->security.poll)
+	if (mds && mds->security.poll)
  		cancel_delayed_work_sync(&mds->security.poll_dwork);
  }
  
@@ -1009,11 +1009,11 @@ static void put_sanitize(void *data)
  	sysfs_put(mds->security.sanitize_node);
  }
  
-static int cxl_memdev_security_init(struct cxl_memdev *cxlmd)
+int cxl_memdev_security_state_init(struct cxl_memdev_state *mds)
  {
-	struct cxl_dev_state *cxlds = cxlmd->cxlds;
-	struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlds);
-	struct device *dev = &cxlmd->dev;
+
+	struct cxl_dev_state *cxlds = &mds->cxlds;
+	struct device *dev = &cxlds->cxlmd->dev;
  	struct kernfs_node *sec;
  
  	sec = sysfs_get_dirent(dev->kobj.sd, "security");
@@ -1029,7 +1029,8 @@ static int cxl_memdev_security_init(struct cxl_memdev *cxlmd)
  	}
  
  	return devm_add_action_or_reset(cxlds->dev, put_sanitize, mds);
- }
+}
+EXPORT_SYMBOL_NS_GPL(cxl_memdev_security_state_init, CXL);
  
  struct cxl_memdev *devm_cxl_add_memdev(struct cxl_dev_state *cxlds)
  {
@@ -1059,10 +1060,6 @@ struct cxl_memdev *devm_cxl_add_memdev(struct cxl_dev_state *cxlds)
  	if (rc)
  		goto err;
  
-	rc = cxl_memdev_security_init(cxlmd);
-	if (rc)
-		goto err;
-
  	rc = devm_add_action_or_reset(cxlds->dev, cxl_memdev_unregister, cxlmd);
  	if (rc)
  		return ERR_PTR(rc);
diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
index f86afef90c91..441270770519 100644
--- a/drivers/cxl/cxlmem.h
+++ b/drivers/cxl/cxlmem.h
@@ -884,6 +884,7 @@ static inline void cxl_mem_active_dec(void)
  #endif
  
  int cxl_mem_sanitize(struct cxl_memdev_state *mds, u16 cmd);
+int cxl_memdev_security_state_init(struct cxl_memdev_state *mds);
  
  struct cxl_hdm {
  	struct cxl_component_regs regs;
diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
index 1cb1494c28fe..5242dbf0044d 100644
--- a/drivers/cxl/pci.c
+++ b/drivers/cxl/pci.c
@@ -887,6 +887,10 @@ static int cxl_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
  	if (IS_ERR(cxlmd))
  		return PTR_ERR(cxlmd);
  
+	rc = cxl_memdev_security_state_init(mds);
+	if (rc)
+		return rc;
+
  	rc = cxl_memdev_setup_fw_upload(mds);
  	if (rc)
  		return rc;