Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR allocation when SNP is enabled

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Hello Boris,

On 10/13/2022 10:15 AM, Borislav Petkov wrote:
On Mon, Jun 20, 2022 at 11:05:01PM +0000, Ashish Kalra wrote:
+static void snp_leak_pages(unsigned long pfn, unsigned int npages)

That function name looks wrong.

+{
+	WARN(1, "psc failed, pfn 0x%lx pages %d (leaking)\n", pfn, npages);
+	while (npages--) {
+		memory_failure(pfn, 0);
		^^^^^^^^^^^^^^^^^^^^^^

Why?

The page was in FW state and we couldn't transition it back to HV/Shared
state, any access to this page can cause RMP #PF.


  * This function is called by the low level machine check code
  * of an architecture when it detects hardware memory corruption
  * of a page. It tries its best to recover, which includes
  * dropping pages, killing processes etc.

I don't think you wanna do that.

It looks like you want to prevent the page from being used again but not
mark it as PG_hwpoison and whatnot. PG_reserved perhaps?

 * PG_reserved is set for special pages. The "struct page" of such a
 * page should in general not be touched (e.g. set dirty) except by its
 * owner.

If it is "still" accessed/touched then it can cause RMP #PF.
On the other hand,

 * PG_hwpoison... Accessing is
 * not safe since it may cause another machine check. Don't touch!

That sounds exactly the state we want these page(s) to be in ?

Another possibility is PG_error.


+		dump_rmpentry(pfn);
+		pfn++;
+	}
+}
+
+static int snp_reclaim_pages(unsigned long pfn, unsigned int npages, bool locked)
+{
+	struct sev_data_snp_page_reclaim data;
+	int ret, err, i, n = 0;
+
+	for (i = 0; i < npages; i++) {
+		memset(&data, 0, sizeof(data));
+		data.paddr = pfn << PAGE_SHIFT;

Oh wow, this is just silly. A struct for a single u64. Just use a

	u64 paddr;
Ok.

directly. But we had this topic already...

+
+		if (locked)

Ew, that's never a good design - conditional locking.

There is a previous suggestion to change `sev_cmd_mutex` to some sort of nesting lock type to clean up this if (locked) code, though AFAIK, there is no support for nesting lock types.


+			ret = __sev_do_cmd_locked(SEV_CMD_SNP_PAGE_RECLAIM, &data, &err);
+		else
+			ret = sev_do_cmd(SEV_CMD_SNP_PAGE_RECLAIM, &data, &err);

<---- newline here.

+		if (ret)
+			goto cleanup;
+
+		ret = rmp_make_shared(pfn, PG_LEVEL_4K);
+		if (ret)
+			goto cleanup;
+
+		pfn++;
+		n++;
+	}
+
+	return 0;
+
+cleanup:
+	/*
+	 * If failed to reclaim the page then page is no longer safe to
+	 * be released, leak it.
+	 */
+	snp_leak_pages(pfn, npages - n);

So this looks real weird: we go and reclaim pages, we hit an error
during reclaiming a page X somewhere in-between and then we go and mark
the *remaining* pages as not to be used?!

Why?

Why not only that *one* page which failed and then we continue with the
rest?!

I agree and will change to this approach.


+	return ret;
+}
+
+static inline int rmp_make_firmware(unsigned long pfn, int level)
+{
+	return rmp_make_private(pfn, 0, level, 0, true);
+}

That's a silly wrapper used only once. Just do at the callsite:

	/* Mark this page as belonging to firmware */
	rc = rmp_make_private(pfn, 0, level, 0, true);

Ok.

+
+static int snp_set_rmp_state(unsigned long paddr, unsigned int npages, bool to_fw, bool locked,
+			     bool need_reclaim)

Tangential to the above, this is just nuts with those bool arguments.
Just look at the callsites: do you understand what they do?

	snp_set_rmp_state(paddr, npages, true, locked, false);

what does that do? You need to go up to the definition of the function,
count the arguments and see what that "true" arg stands for.

I totally agree, this is simply unreadable.

And this has been mentioned previously too ...
This function can do a lot and when I read the call sites its hard to
see what its doing since we have a combination of arguments which tell
us what behavior is happening ...


What you should do instead is, have separate helpers which do only one
thing:

	rmp_mark_pages_firmware();
	rmp_mark_pages_shared();
	rmp_mark_pages_...

and then have the *callers* issue snp_reclaim_pages() when needed. So you'd have

	rmp_mark_pages_firmware();
	rmp_mark_pages_shared()

and __snp_free_firmware_pages() would do

	rmp_mark_pages_shared();
	snp_reclaim_pages();

Actually, this only needs to call snp_reclaim_pages().

and so on.

And then if you need locking, the callers can decide which sev_do_cmd
variant to issue.

And then if you have common code fragments which you can unify into a
bigger helper function, *then* you can do that.

Instead of multiplexing it this way. Which makes it really hard to
follow what the code does.


Sure i will do this cleanup.


+	unsigned long pfn = __sme_clr(paddr) >> PAGE_SHIFT; /* Cbit maybe set in the paddr */

No side comments pls.

+	int rc, n = 0, i;
+
+	for (i = 0; i < npages; i++) {
+		if (to_fw)
+			rc = rmp_make_firmware(pfn, PG_LEVEL_4K);
+		else
+			rc = need_reclaim ? snp_reclaim_pages(pfn, 1, locked) :
+					    rmp_make_shared(pfn, PG_LEVEL_4K);
+		if (rc)
+			goto cleanup;
+
+		pfn++;
+		n++;
+	}
+
+	return 0;
+
+cleanup:
+	/* Try unrolling the firmware state changes */
+	if (to_fw) {
+		/*
+		 * Reclaim the pages which were already changed to the
+		 * firmware state.
+		 */
+		snp_reclaim_pages(paddr >> PAGE_SHIFT, n, locked);
+
+		return rc;
+	}
+
+	/*
+	 * If failed to change the page state to shared, then its not safe
+	 * to release the page back to the system, leak it.
+	 */
+	snp_leak_pages(pfn, npages - n);
+
+	return rc;
+}

...

+void snp_free_firmware_page(void *addr)
+{
+	if (!addr)
+		return;
+
+	__snp_free_firmware_pages(virt_to_page(addr), 0, false);
+}
+EXPORT_SYMBOL(snp_free_firmware_page);

EXPORT_SYMBOL_GPL() ofc.


Thanks,
Ashish




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