packet: refine ring v3 block size test to hold one frame

TPACKET_V3 stores variable length frames in fixed length blocks.
Blocks must be able to store a block header, optional private space
and at least one minimum sized frame.

Frames, even for a zero snaplen packet, store metadata headers and
optional reserved space.

In the block size bounds check, ensure that the frame of the
chosen configuration fits. This includes sockaddr_ll and optional
tp_reserve.

Syzbot was able to construct a ring with insuffient room for the
sockaddr_ll in the header of a zero-length frame, triggering an
out-of-bounds write in dev_parse_header.

Convert the comparison to less than, as zero is a valid snap len.
This matches the test for minimum tp_frame_size immediately below.

Fixes: f6fb8f10 ("af-packet: TPACKET_V3 flexible buffer implementation.")
Fixes: eb73190f ("net/packet: refine check for priv area size")
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -4226,6 +4226,8 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u,
 	}
 
 	if (req->tp_block_nr) {
+		unsigned int min_frame_size;
+
 		/* Sanity tests and some calculations */
 		err = -EBUSY;
 		if (unlikely(rb->pg_vec))
@@ -4248,12 +4250,12 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u,
 			goto out;
 		if (unlikely(!PAGE_ALIGNED(req->tp_block_size)))
 			goto out;
+		min_frame_size = po->tp_hdrlen + po->tp_reserve;
 		if (po->tp_version >= TPACKET_V3 &&
-		    req->tp_block_size <=
-		    BLK_PLUS_PRIV((u64)req_u->req3.tp_sizeof_priv) + sizeof(struct tpacket3_hdr))
+		    req->tp_block_size <
+		    BLK_PLUS_PRIV((u64)req_u->req3.tp_sizeof_priv) + min_frame_size)
 			goto out;
-		if (unlikely(req->tp_frame_size < po->tp_hdrlen +
-					po->tp_reserve))
+		if (unlikely(req->tp_frame_size < min_frame_size))
 			goto out;
 		if (unlikely(req->tp_frame_size & (TPACKET_ALIGNMENT - 1)))
 			goto out;