x86/sgx: Fix deadlock in SGX NUMA node search

When the current node doesn't have an EPC section configured by firmware
and all other EPC sections are used up, CPU can get stuck inside the
while loop that looks for an available EPC page from remote nodes
indefinitely, leading to a soft lockup. Note how nid_of_current will
never be equal to nid in that while loop because nid_of_current is not
set in sgx_numa_mask.

Also worth mentioning is that it's perfectly fine for the firmware not
to setup an EPC section on a node. While setting up an EPC section on
each node can enhance performance, it is not a requirement for
functionality.

Rework the loop to start and end on *a* node that has SGX memory. This
avoids the deadlock looking for the current SGX-lacking node to show up
in the loop when it never will.

Fixes: 901ddbb ("x86/sgx: Add a basic NUMA allocation scheme to sgx_alloc_epc_page()")
Reported-by: "Molina Sabido, Gerardo" <[email protected]>
Signed-off-by: Aaron Lu <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Kai Huang <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Tested-by: Zhimin Luo <[email protected]>
Link: https://lore.kernel.org/all/20240905080855.1699814-2-aaron.lu%40intel.com

Author: Aaron Lu

arch/x86/kernel/cpu/sgx/main.c

@@ -475,24 +475,25 @@ struct sgx_epc_page *__sgx_alloc_epc_page(void)
 {
 	struct sgx_epc_page *page;
 	int nid_of_current = numa_node_id();
-	int nid = nid_of_current;
+	int nid_start, nid;
 
-	if (node_isset(nid_of_current, sgx_numa_mask)) {
-		page = __sgx_alloc_epc_page_from_node(nid_of_current);
-		if (page)
-			return page;
-	}
-
-	/* Fall back to the non-local NUMA nodes: */
-	while (true) {
-		nid = next_node_in(nid, sgx_numa_mask);
-		if (nid == nid_of_current)
-			break;
+	/*
+	 * Try local node first. If it doesn't have an EPC section,
+	 * fall back to the non-local NUMA nodes.
+	 */
+	if (node_isset(nid_of_current, sgx_numa_mask))
+		nid_start = nid_of_current;
+	else
+		nid_start = next_node_in(nid_of_current, sgx_numa_mask);
 
+	nid = nid_start;
+	do {
 		page = __sgx_alloc_epc_page_from_node(nid);
 		if (page)
 			return page;
-	}
+
+		nid = next_node_in(nid, sgx_numa_mask);
+	} while (nid != nid_start);
 
 	return ERR_PTR(-ENOMEM);
 }