diff --git a/read-cache.c b/read-cache.c
index 9cff715d6b..cc67dd1d5d 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1525,7 +1525,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	if (mmap_size < sizeof(struct cache_header) + 20)
 		die("index file smaller than expected");
 
-	mmap = xmmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+	mmap = xmmap(NULL, mmap_size, PROT_READ, MAP_PRIVATE, fd, 0);
 	if (mmap == MAP_FAILED)
 		die_errno("unable to map index file");
 	close(fd);