xsk: Add overflow check for u64 division, stored into u32

The npgs member of struct xdp_umem is an u32 entity, and stores the
number of pages the UMEM consumes. The calculation of npgs

  npgs = size / PAGE_SIZE

can overflow.

To avoid overflow scenarios, the division is now first stored in a
u64, and the result is verified to fit into 32b.

An alternative would be storing the npgs as a u64, however, this
wastes memory and is an unrealisticly large packet area.

Fixes: c0c77d8f ("xsk: add user memory registration support sockopt")
Reported-by: "Minh Bùi Quang" <minhquangbui99@gmail.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Link: https://lore.kernel.org/bpf/CACtPs=GGvV-_Yj6rbpzTVnopgi5nhMoCcTkSkYrJHGQHJWFZMQ@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20200525080400.13195-1-bjorn.topel@gmail.com

net/xdp/xdp_umem.c

@@ -341,8 +341,8 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
 {
 	bool unaligned_chunks = mr->flags & XDP_UMEM_UNALIGNED_CHUNK_FLAG;
 	u32 chunk_size = mr->chunk_size, headroom = mr->headroom;
+	u64 npgs, addr = mr->addr, size = mr->len;
 	unsigned int chunks, chunks_per_page;
-	u64 addr = mr->addr, size = mr->len;
 	int err;
 
 	if (chunk_size < XDP_UMEM_MIN_CHUNK_SIZE || chunk_size > PAGE_SIZE) {
@@ -372,6 +372,10 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
 	if ((addr + size) < addr)
 		return -EINVAL;
 
+	npgs = div_u64(size, PAGE_SIZE);
+	if (npgs > U32_MAX)
+		return -EINVAL;
+
 	chunks = (unsigned int)div_u64(size, chunk_size);
 	if (chunks == 0)
 		return -EINVAL;
@@ -391,7 +395,7 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
 	umem->size = size;
 	umem->headroom = headroom;
 	umem->chunk_size_nohr = chunk_size - headroom;
-	umem->npgs = size / PAGE_SIZE;
+	umem->npgs = (u32)npgs;
 	umem->pgs = NULL;
 	umem->user = NULL;
 	umem->flags = mr->flags;