misc: misc_minor_alloc to use ida for all dynamic/misc dynamic minors

misc_minor_alloc was allocating id using ida for minor only in case of
MISC_DYNAMIC_MINOR but misc_minor_free was always freeing ids
using ida_free causing a mismatch and following warn:
> > WARNING: CPU: 0 PID: 159 at lib/idr.c:525 ida_free+0x3e0/0x41f
> > ida_free called for id=127 which is not allocated.
> > <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
...
> > [<60941eb4>] ida_free+0x3e0/0x41f
> > [<605ac993>] misc_minor_free+0x3e/0xbc
> > [<605acb82>] misc_deregister+0x171/0x1b3

misc_minor_alloc is changed to allocate id from ida for all minors
falling in the range of dynamic/ misc dynamic minors

Fixes: ab76079 ("char: misc: Increase the maximum number of dynamic misc devices to 1048448")
Signed-off-by: Vimal Agrawal <vimal.agrawal@sophos.com>
Reviewed-by: Dirk VanDerMerwe <dirk.vandermerwe@sophos.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20241021133812.23703-1-vimal.agrawal@sophos.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/misc.c

@@ -63,16 +63,30 @@ static DEFINE_MUTEX(misc_mtx);
 #define DYNAMIC_MINORS 128 /* like dynamic majors */
 static DEFINE_IDA(misc_minors_ida);
 
-static int misc_minor_alloc(void)
+static int misc_minor_alloc(int minor)
 {
-	int ret;
-
-	ret = ida_alloc_max(&misc_minors_ida, DYNAMIC_MINORS - 1, GFP_KERNEL);
-	if (ret >= 0) {
-		ret = DYNAMIC_MINORS - ret - 1;
+	int ret = 0;
+
+	if (minor == MISC_DYNAMIC_MINOR) {
+		/* allocate free id */
+		ret = ida_alloc_max(&misc_minors_ida, DYNAMIC_MINORS - 1, GFP_KERNEL);
+		if (ret >= 0) {
+			ret = DYNAMIC_MINORS - ret - 1;
+		} else {
+			ret = ida_alloc_range(&misc_minors_ida, MISC_DYNAMIC_MINOR + 1,
+					      MINORMASK, GFP_KERNEL);
+		}
 	} else {
-		ret = ida_alloc_range(&misc_minors_ida, MISC_DYNAMIC_MINOR + 1,
-				      MINORMASK, GFP_KERNEL);
+		/* specific minor, check if it is in dynamic or misc dynamic range  */
+		if (minor < DYNAMIC_MINORS) {
+			minor = DYNAMIC_MINORS - minor - 1;
+			ret = ida_alloc_range(&misc_minors_ida, minor, minor, GFP_KERNEL);
+		} else if (minor > MISC_DYNAMIC_MINOR) {
+			ret = ida_alloc_range(&misc_minors_ida, minor, minor, GFP_KERNEL);
+		} else {
+			/* case of non-dynamic minors, no need to allocate id */
+			ret = 0;
+		}
 	}
 	return ret;
 }
@@ -219,7 +233,7 @@ int misc_register(struct miscdevice *misc)
 	mutex_lock(&misc_mtx);
 
 	if (is_dynamic) {
-		int i = misc_minor_alloc();
+		int i = misc_minor_alloc(misc->minor);
 
 		if (i < 0) {
 			err = -EBUSY;
@@ -228,13 +242,20 @@ int misc_register(struct miscdevice *misc)
 		misc->minor = i;
 	} else {
 		struct miscdevice *c;
+		int i;
 
 		list_for_each_entry(c, &misc_list, list) {
 			if (c->minor == misc->minor) {
 				err = -EBUSY;
 				goto out;
 			}
 		}
+
+		i = misc_minor_alloc(misc->minor);
+		if (i < 0) {
+			err = -EBUSY;
+			goto out;
+		}
 	}
 
 	dev = MKDEV(MISC_MAJOR, misc->minor);