ocxl: Fix page fault handler in case of fault on dying process

If a process exits without doing proper cleanup, there's a window
where an opencapi device can try to access the memory of the dying
process and may trigger a page fault. That's an expected scenario and
the ocxl driver holds a reference on the mm_struct of the process
until the opencapi device is notified of the process exiting.
However, if mm_users is already at 0, i.e. the address space of the
process has already been destroyed, the driver shouldn't try resolving
the page fault, as it will fail, but it can also try accessing already
freed data.

It is fixed by only calling the bottom half of the page fault handler
if mm_users is greater than 0 and get a reference on mm_users instead
of mm_count. Otherwise, we can safely return a translation fault to
the device, as its associated memory context is being removed. The
opencapi device will be properly cleaned up shortly after when closing
the file descriptors.

Fixes: 5ef3166e ("ocxl: Driver code for 'generic' opencapi devices")
Cc: stable@vger.kernel.org # v4.16+
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-By: Alastair D'Silva <alastair@d-silva.org>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/misc/ocxl/link.c

@@ -136,7 +136,7 @@ static void xsl_fault_handler_bh(struct work_struct *fault_work)
 	int rc;
 
 	/*
-	 * We need to release a reference on the mm whenever exiting this
+	 * We must release a reference on mm_users whenever exiting this
 	 * function (taken in the memory fault interrupt handler)
 	 */
 	rc = copro_handle_mm_fault(fault->pe_data.mm, fault->dar, fault->dsisr,
@@ -172,7 +172,7 @@ static void xsl_fault_handler_bh(struct work_struct *fault_work)
 	}
 	r = RESTART;
 ack:
-	mmdrop(fault->pe_data.mm);
+	mmput(fault->pe_data.mm);
 	ack_irq(spa, r);
 }
 
@@ -184,6 +184,7 @@ static irqreturn_t xsl_fault_handler(int irq, void *data)
 	struct pe_data *pe_data;
 	struct ocxl_process_element *pe;
 	int lpid, pid, tid;
+	bool schedule = false;
 
 	read_irq(spa, &dsisr, &dar, &pe_handle);
 	trace_ocxl_fault(spa->spa_mem, pe_handle, dsisr, dar, -1);
@@ -226,14 +227,19 @@ static irqreturn_t xsl_fault_handler(int irq, void *data)
 	}
 	WARN_ON(pe_data->mm->context.id != pid);
 
+	if (mmget_not_zero(pe_data->mm)) {
 			spa->xsl_fault.pe = pe_handle;
 			spa->xsl_fault.dar = dar;
 			spa->xsl_fault.dsisr = dsisr;
 			spa->xsl_fault.pe_data = *pe_data;
-	mmgrab(pe_data->mm); /* mm count is released by bottom half */
-
+			schedule = true;
+			/* mm_users count released by bottom half */
+	}
 	rcu_read_unlock();
+	if (schedule)
 		schedule_work(&spa->xsl_fault.fault_work);
+	else
+		ack_irq(spa, ADDRESS_ERROR);
 	return IRQ_HANDLED;
 }