bpf: Prevent writable memory-mapping of read-only ringbuf pages

Only the very first page of BPF ringbuf that contains consumer position
counter is supposed to be mapped as writeable by user-space. Producer
position is read-only and can be modified only by the kernel code. BPF ringbuf
data pages are read-only as well and are not meant to be modified by
user-code to maintain integrity of per-record headers.

This patch allows to map only consumer position page as writeable and
everything else is restricted to be read-only. remap_vmalloc_range()
internally adds VM_DONTEXPAND, so all the established memory mappings can't be
extended, which prevents any future violations through mremap()'ing.

Fixes: 457f443 ("bpf: Implement BPF ring buffer and verifier support for it")
Reported-by: Ryota Shiga (Flatt Security)
Reported-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/ringbuf.c

@@ -221,25 +221,20 @@ static int ringbuf_map_get_next_key(struct bpf_map *map, void *key,
 	return -ENOTSUPP;
 }
 
-static size_t bpf_ringbuf_mmap_page_cnt(const struct bpf_ringbuf *rb)
-{
-	size_t data_pages = (rb->mask + 1) >> PAGE_SHIFT;
-
-	/* consumer page + producer page + 2 x data pages */
-	return RINGBUF_POS_PAGES + 2 * data_pages;
-}
-
 static int ringbuf_map_mmap(struct bpf_map *map, struct vm_area_struct *vma)
 {
 	struct bpf_ringbuf_map *rb_map;
-	size_t mmap_sz;
 
 	rb_map = container_of(map, struct bpf_ringbuf_map, map);
-	mmap_sz = bpf_ringbuf_mmap_page_cnt(rb_map->rb) << PAGE_SHIFT;
-
-	if (vma->vm_pgoff * PAGE_SIZE + (vma->vm_end - vma->vm_start) > mmap_sz)
-		return -EINVAL;
 
+	if (vma->vm_flags & VM_WRITE) {
+		/* allow writable mapping for the consumer_pos only */
+		if (vma->vm_pgoff != 0 || vma->vm_end - vma->vm_start != PAGE_SIZE)
+			return -EPERM;
+	} else {
+		vma->vm_flags &= ~VM_MAYWRITE;
+	}
+	/* remap_vmalloc_range() checks size and offset constraints */
 	return remap_vmalloc_range(vma, rb_map->rb,
 				   vma->vm_pgoff + RINGBUF_PGOFF);
 }