tcp: add TCP_ZEROCOPY_RECEIVE support for zerocopy receive

When adding tcp mmap() implementation, I forgot that socket lock
had to be taken before current->mm->mmap_sem. syzbot eventually caught
the bug.

Since we can not lock the socket in tcp mmap() handler we have to
split the operation in two phases.

1) mmap() on a tcp socket simply reserves VMA space, and nothing else.
  This operation does not involve any TCP locking.

2) getsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) implements
 the transfert of pages from skbs to one VMA.
  This operation only uses down_read(&current->mm->mmap_sem) after
  holding TCP lock, thus solving the lockdep issue.

This new implementation was suggested by Andy Lutomirski with great details.

Benefits are :

- Better scalability, in case multiple threads reuse VMAS
   (without mmap()/munmap() calls) since mmap_sem wont be write locked.

- Better error recovery.
   The previous mmap() model had to provide the expected size of the
   mapping. If for some reason one part could not be mapped (partial MSS),
   the whole operation had to be aborted.
   With the tcp_zerocopy_receive struct, kernel can report how
   many bytes were successfuly mapped, and how many bytes should
   be read to skip the problematic sequence.

- No more memory allocation to hold an array of page pointers.
  16 MB mappings needed 32 KB for this array, potentially using vmalloc() :/

- skbs are freed while mmap_sem has been released

Following patch makes the change in tcp_mmap tool to demonstrate
one possible use of mmap() and setsockopt(... TCP_ZEROCOPY_RECEIVE ...)

Note that memcg might require additional changes.

Fixes: 93ab6cc6 ("tcp: implement mmap() for zero copy receive")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Cc: linux-mm@kvack.org
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tcp.h

@@ -122,6 +122,7 @@ enum {
 #define TCP_MD5SIG_EXT		32	/* TCP MD5 Signature with extensions */
 #define TCP_FASTOPEN_KEY	33	/* Set the key for Fast Open (cookie) */
 #define TCP_FASTOPEN_NO_COOKIE	34	/* Enable TFO without a TFO cookie */
+#define TCP_ZEROCOPY_RECEIVE	35
 
 struct tcp_repair_opt {
 	__u32	opt_code;
@@ -276,4 +277,11 @@ struct tcp_diag_md5sig {
 	__u8	tcpm_key[TCP_MD5SIG_MAXKEYLEN];
 };
 
+/* setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) */
+
+struct tcp_zerocopy_receive {
+	__u64 address;		/* in: address of mapping */
+	__u32 length;		/* in/out: number of bytes to map/mapped */
+	__u32 recv_skip_hint;	/* out: amount of bytes to skip */
+};
 #endif /* _UAPI_LINUX_TCP_H */

net/ipv4/af_inet.c

@@ -994,7 +994,9 @@ const struct proto_ops inet_stream_ops = {
 	.getsockopt	   = sock_common_getsockopt,
 	.sendmsg	   = inet_sendmsg,
 	.recvmsg	   = inet_recvmsg,
+#ifdef CONFIG_MMU
 	.mmap		   = tcp_mmap,
+#endif
 	.sendpage	   = inet_sendpage,
 	.splice_read	   = tcp_splice_read,
 	.read_sock	   = tcp_read_sock,

net/ipv4/tcp.c

@@ -1726,73 +1726,74 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
 }
 EXPORT_SYMBOL(tcp_set_rcvlowat);
 
-/* When user wants to mmap X pages, we first need to perform the mapping
- * before freeing any skbs in receive queue, otherwise user would be unable
- * to fallback to standard recvmsg(). This happens if some data in the
- * requested block is not exactly fitting in a page.
- *
- * We only support order-0 pages for the moment.
- * mmap() on TCP is very strict, there is no point
- * trying to accommodate with pathological layouts.
- */
+#ifdef CONFIG_MMU
+static const struct vm_operations_struct tcp_vm_ops = {
+};
+
 int tcp_mmap(struct file *file, struct socket *sock,
 	     struct vm_area_struct *vma)
 {
-	unsigned long size = vma->vm_end - vma->vm_start;
-	unsigned int nr_pages = size >> PAGE_SHIFT;
-	struct page **pages_array = NULL;
-	u32 seq, len, offset, nr = 0;
-	struct sock *sk = sock->sk;
-	const skb_frag_t *frags;
+	if (vma->vm_flags & (VM_WRITE | VM_EXEC))
+		return -EPERM;
+	vma->vm_flags &= ~(VM_MAYWRITE | VM_MAYEXEC);
+
+	/* Instruct vm_insert_page() to not down_read(mmap_sem) */
+	vma->vm_flags |= VM_MIXEDMAP;
+
+	vma->vm_ops = &tcp_vm_ops;
+	return 0;
+}
+EXPORT_SYMBOL(tcp_mmap);
+
+static int tcp_zerocopy_receive(struct sock *sk,
+				struct tcp_zerocopy_receive *zc)
+{
+	unsigned long address = (unsigned long)zc->address;
+	const skb_frag_t *frags = NULL;
+	u32 length = 0, seq, offset;
+	struct vm_area_struct *vma;
+	struct sk_buff *skb = NULL;
 	struct tcp_sock *tp;
-	struct sk_buff *skb;
 	int ret;
 
-	if (vma->vm_pgoff || !nr_pages)
+	if (address & (PAGE_SIZE - 1) || address != zc->address)
 		return -EINVAL;
 
-	if (vma->vm_flags & VM_WRITE)
-		return -EPERM;
-	/* TODO: Maybe the following is not needed if pages are COW */
-	vma->vm_flags &= ~VM_MAYWRITE;
-
-	lock_sock(sk);
-
-	ret = -ENOTCONN;
 	if (sk->sk_state == TCP_LISTEN)
-		goto out;
+		return -ENOTCONN;
 
 	sock_rps_record_flow(sk);
 
-	if (tcp_inq(sk) < size) {
-		ret = sock_flag(sk, SOCK_DONE) ? -EIO : -EAGAIN;
+	down_read(&current->mm->mmap_sem);
+
+	ret = -EINVAL;
+	vma = find_vma(current->mm, address);
+	if (!vma || vma->vm_start > address || vma->vm_ops != &tcp_vm_ops)
 		goto out;
-	}
+	zc->length = min_t(unsigned long, zc->length, vma->vm_end - address);
+
 	tp = tcp_sk(sk);
 	seq = tp->copied_seq;
-	/* Abort if urgent data is in the area */
-	if (unlikely(tp->urg_data)) {
-		u32 urg_offset = tp->urg_seq - seq;
+	zc->length = min_t(u32, zc->length, tcp_inq(sk));
+	zc->length &= ~(PAGE_SIZE - 1);
 
-		ret = -EINVAL;
-		if (urg_offset < size)
-			goto out;
-	}
-	ret = -ENOMEM;
-	pages_array = kvmalloc_array(nr_pages, sizeof(struct page *),
-				     GFP_KERNEL);
-	if (!pages_array)
-		goto out;
+	zap_page_range(vma, address, zc->length);
+
+	zc->recv_skip_hint = 0;
+	ret = 0;
+	while (length + PAGE_SIZE <= zc->length) {
+		if (zc->recv_skip_hint < PAGE_SIZE) {
+			if (skb) {
+				skb = skb->next;
+				offset = seq - TCP_SKB_CB(skb)->seq;
+			} else {
 				skb = tcp_recv_skb(sk, seq, &offset);
-	ret = -EINVAL;
-skb_start:
-	/* We do not support anything not in page frags */
+			}
+
+			zc->recv_skip_hint = skb->len - offset;
 			offset -= skb_headlen(skb);
-	if ((int)offset < 0)
-		goto out;
-	if (skb_has_frag_list(skb))
-		goto out;
-	len = skb->data_len - offset;
+			if ((int)offset < 0 || skb_has_frag_list(skb))
+				break;
 			frags = skb_shinfo(skb)->frags;
 			while (offset) {
 				if (frags->size > offset)
@@ -1800,44 +1801,38 @@ int tcp_mmap(struct file *file, struct socket *sock,
 				offset -= frags->size;
 				frags++;
 			}
-	while (nr < nr_pages) {
-		if (len) {
-			if (len < PAGE_SIZE)
-				goto out;
-			if (frags->size != PAGE_SIZE || frags->page_offset)
-				goto out;
-			pages_array[nr++] = skb_frag_page(frags);
-			frags++;
-			len -= PAGE_SIZE;
-			seq += PAGE_SIZE;
-			continue;
 		}
-		skb = skb->next;
-		offset = seq - TCP_SKB_CB(skb)->seq;
-		goto skb_start;
-	}
-	/* OK, we have a full set of pages ready to be inserted into vma */
-	for (nr = 0; nr < nr_pages; nr++) {
-		ret = vm_insert_page(vma, vma->vm_start + (nr << PAGE_SHIFT),
-				     pages_array[nr]);
+		if (frags->size != PAGE_SIZE || frags->page_offset)
+			break;
+		ret = vm_insert_page(vma, address + length,
+				     skb_frag_page(frags));
 		if (ret)
-			goto out;
+			break;
+		length += PAGE_SIZE;
+		seq += PAGE_SIZE;
+		zc->recv_skip_hint -= PAGE_SIZE;
+		frags++;
 	}
-	/* operation is complete, we can 'consume' all skbs */
+out:
+	up_read(&current->mm->mmap_sem);
+	if (length) {
 		tp->copied_seq = seq;
 		tcp_rcv_space_adjust(sk);
 
 		/* Clean up data we have read: This will do ACK frames. */
 		tcp_recv_skb(sk, seq, &offset);
-	tcp_cleanup_rbuf(sk, size);
-
+		tcp_cleanup_rbuf(sk, length);
 		ret = 0;
-out:
-	release_sock(sk);
-	kvfree(pages_array);
+		if (length == zc->length)
+			zc->recv_skip_hint = 0;
+	} else {
+		if (!zc->recv_skip_hint && sock_flag(sk, SOCK_DONE))
+			ret = -EIO;
+	}
+	zc->length = length;
 	return ret;
 }
-EXPORT_SYMBOL(tcp_mmap);
+#endif
 
 static void tcp_update_recv_tstamps(struct sk_buff *skb,
 				    struct scm_timestamping *tss)
@@ -3472,6 +3467,25 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 		}
 		return 0;
 	}
+#ifdef CONFIG_MMU
+	case TCP_ZEROCOPY_RECEIVE: {
+		struct tcp_zerocopy_receive zc;
+		int err;
+
+		if (get_user(len, optlen))
+			return -EFAULT;
+		if (len != sizeof(zc))
+			return -EINVAL;
+		if (copy_from_user(&zc, optval, len))
+			return -EFAULT;
+		lock_sock(sk);
+		err = tcp_zerocopy_receive(sk, &zc);
+		release_sock(sk);
+		if (!err && copy_to_user(optval, &zc, len))
+			err = -EFAULT;
+		return err;
+	}
+#endif
 	default:
 		return -ENOPROTOOPT;
 	}

net/ipv6/af_inet6.c

@@ -578,7 +578,9 @@ const struct proto_ops inet6_stream_ops = {
 	.getsockopt	   = sock_common_getsockopt,	/* ok		*/
 	.sendmsg	   = inet_sendmsg,		/* ok		*/
 	.recvmsg	   = inet_recvmsg,		/* ok		*/
+#ifdef CONFIG_MMU
 	.mmap		   = tcp_mmap,
+#endif
 	.sendpage	   = inet_sendpage,
 	.sendmsg_locked    = tcp_sendmsg_locked,
 	.sendpage_locked   = tcp_sendpage_locked,