bpf: Fix bug in mmap() implementation for BPF array map

mmap() subsystem allows user-space application to memory-map region with
initial page offset. This wasn't taken into account in initial implementation
of BPF array memory-mapping. This would result in wrong pages, not taking into
account requested page shift, being memory-mmaped into user-space. This patch
fixes this gap and adds a test for such scenario.

Fixes: fc970227 ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200512235925.3817805-1-andriin@fb.com

kernel/bpf/arraymap.c

@@ -486,7 +486,12 @@ static int array_map_mmap(struct bpf_map *map, struct vm_area_struct *vma)
 	if (!(map->map_flags & BPF_F_MMAPABLE))
 		return -EINVAL;
 
-	return remap_vmalloc_range(vma, array_map_vmalloc_addr(array), pgoff);
+	if (vma->vm_pgoff * PAGE_SIZE + (vma->vm_end - vma->vm_start) >
+	    PAGE_ALIGN((u64)array->map.max_entries * array->elem_size))
+		return -EINVAL;
+
+	return remap_vmalloc_range(vma, array_map_vmalloc_addr(array),
+				   vma->vm_pgoff + pgoff);
 }
 
 const struct bpf_map_ops array_map_ops = {

tools/testing/selftests/bpf/prog_tests/mmap.c

@@ -217,6 +217,14 @@ void test_mmap(void)
 
 	munmap(tmp2, 4 * page_size);
 
+	/* map all 4 pages, but with pg_off=1 page, should fail */
+	tmp1 = mmap(NULL, 4 * page_size, PROT_READ, MAP_SHARED | MAP_FIXED,
+		    data_map_fd, page_size /* initial page shift */);
+	if (CHECK(tmp1 != MAP_FAILED, "adv_mmap7", "unexpected success")) {
+		munmap(tmp1, 4 * page_size);
+		goto cleanup;
+	}
+
 	tmp1 = mmap(NULL, map_sz, PROT_READ, MAP_SHARED, data_map_fd, 0);
 	if (CHECK(tmp1 == MAP_FAILED, "last_mmap", "failed %d\n", errno))
 		goto cleanup;