bpf: Use VM_MAP instead of VM_ALLOC for ringbuf

After commit 2fd3fb0be1d1 ("kasan, vmalloc: unpoison VM_ALLOC pages
after mapping"), non-VM_ALLOC mappings will be marked as accessible
in __get_vm_area_node() when KASAN is enabled. But now the flag for
ringbuf area is VM_ALLOC, so KASAN will complain out-of-bound access
after vmap() returns. Because the ringbuf area is created by mapping
allocated pages, so use VM_MAP instead.

After the change, info in /proc/vmallocinfo also changes from
  [start]-[end]   24576 ringbuf_map_alloc+0x171/0x290 vmalloc user
to
  [start]-[end]   24576 ringbuf_map_alloc+0x171/0x290 vmap user

Fixes: 457f44363a ("bpf: Implement BPF ring buffer and verifier support for it")
Reported-by: syzbot+5ad567a418794b9b5983@syzkaller.appspotmail.com
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220202060158.6260-1-houtao1@huawei.com

kernel/bpf/ringbuf.c

@@ -104,7 +104,7 @@ static struct bpf_ringbuf *bpf_ringbuf_area_alloc(size_t data_sz, int numa_node)
 	}
 
 	rb = vmap(pages, nr_meta_pages + 2 * nr_data_pages,
-		  VM_ALLOC | VM_USERMAP, PAGE_KERNEL);
+		  VM_MAP | VM_USERMAP, PAGE_KERNEL);
 	if (rb) {
 		kmemleak_not_leak(pages);
 		rb->pages = pages;