[PATCH 5/6] bitmap: bitmap_parselist(): don't wrap around on a huge group size

[shashank]

In the "range:used/group" form bitmap_set_region() walks the range with

	for (start = r->start; start <= r->end; start += r->group_len)

where all values are unsigned int.  bitmap_getnum() accepts any group
size up to UINT_MAX, and bitmap_check_region() only requires it to be
non-zero and at least the used size.  If start + group_len exceeds
UINT_MAX, start wraps around to a small value that is still <= end, and
bits below the start of the range get set:

  "1-1:1/4294967295"    -> bits 0,1 (expected bit 1)
  "15-15:1/4294967281"  -> bits 0,15 (expected bit 15)

The loop still terminates, because start keeps decreasing until it
wraps past zero again, but the resulting mask contains bits outside
the requested range.

This needs a group size within 'start' of UINT_MAX, so it is unlikely
to be hit by accident; all bits set stay below the end of the range,
so there is no out-of-bounds write.

A group larger than the range simply means that the range contains a
single group, so stop the walk when the next group would start beyond
UINT_MAX.

Fixes: 0a5ce0831d04 ("lib/bitmap.c: make bitmap_parselist() thread-safe and much faster")
Assisted-by: LLM
Signed-off-by: shashank <jain.sm@xxxxxxxxx>
---
 lib/bitmap-str.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/lib/bitmap-str.c b/lib/bitmap-str.c
index b58966864657..cafd6388892f 100644
--- a/lib/bitmap-str.c
+++ b/lib/bitmap-str.c
@@ -8,6 +8,7 @@
 #include <linux/hex.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
+#include <linux/overflow.h>
 #include <linux/string.h>
 
 #include "kstrtox.h"
@@ -186,10 +187,12 @@ struct region {
 
 static void bitmap_set_region(const struct region *r, unsigned long *bitmap)
 {
-	unsigned int start;
+	unsigned int start = r->start;
 
-	for (start = r->start; start <= r->end; start += r->group_len)
+	do {
 		bitmap_set(bitmap, start, min(r->end - start + 1, r->off));
+	} while (!check_add_overflow(start, r->group_len, &start) &&
+		 start <= r->end);
 }
 
 static int bitmap_check_region(const struct region *r)
-- 
2.43.0