diff --git a/kernel/resource.c b/kernel/resource.c
index f150dbbe6f62d31aa07b3fbf3f07a8807fa40991..09c0597840b02dc260c65baafdd1fde0715bcbb4 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1498,8 +1498,15 @@ int iomem_is_exclusive(u64 addr)
 			break;
 		if (p->end < addr)
 			continue;
-		if (p->flags & IORESOURCE_BUSY &&
-		     p->flags & IORESOURCE_EXCLUSIVE) {
+		/*
+		 * A resource is exclusive if IORESOURCE_EXCLUSIVE is set
+		 * or CONFIG_IO_STRICT_DEVMEM is enabled and the
+		 * resource is busy.
+		 */
+		if ((p->flags & IORESOURCE_BUSY) == 0)
+			continue;
+		if (IS_ENABLED(CONFIG_IO_STRICT_DEVMEM)
+				|| p->flags & IORESOURCE_EXCLUSIVE) {
 			err = 1;
 			break;
 		}
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 289dfcbc14eb6a96607b33e7baa9c39fe27cfd27..073496dea848262e6b3324a50b241cf130ae4dbd 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1869,9 +1869,26 @@ config STRICT_DEVMEM
 	  enabled, even in this case there are restrictions on /dev/mem
 	  use due to the cache aliasing requirements.
 
+	  If this option is switched on, and IO_STRICT_DEVMEM=n, the /dev/mem
+	  file only allows userspace access to PCI space and the BIOS code and
+	  data regions.  This is sufficient for dosemu and X and all common
+	  users of /dev/mem.
+
+	  If in doubt, say Y.
+
+config IO_STRICT_DEVMEM
+	bool "Filter I/O access to /dev/mem"
+	depends on STRICT_DEVMEM
+	default STRICT_DEVMEM
+	---help---
+	  If this option is disabled, you allow userspace (root) access to all
+	  io-memory regardless of whether a driver is actively using that
+	  range.  Accidental access to this is obviously disastrous, but
+	  specific access can be used by people debugging kernel drivers.
+
 	  If this option is switched on, the /dev/mem file only allows
-	  userspace access to PCI space and the BIOS code and data regions.
-	  This is sufficient for dosemu and X and all common users of
-	  /dev/mem.
+	  userspace access to *idle* io-memory ranges (see /proc/iomem) This
+	  may break traditional users of /dev/mem (dosemu, legacy X, etc...)
+	  if the driver using a given range cannot be disabled.
 
 	  If in doubt, say Y.