init/aws-nitro: support partial read/write ethernet packet forwarding

Ensure the total requested number of bytes is forwarded to/from the host
vsock providing the network access and the guest TAP device routing the
application network traffic.

To write this code I used Cursor and the claude-4.6-opus-high model.

Signed-off-by: Jake Correnti <jakecorrenti+github@proton.me>

Author: jakecorrenti

init/aws-nitro/device/net_tap_afvsock.c

@@ -31,6 +31,64 @@
 #define TUN_DEV_MAJOR 10
 #define TUN_DEV_MINOR 200
 
+/*
+ * The Extended Ethernet Frame header is 14 bytes, representing the Destination
+ * Address (6 bytes), Source Address (6 bytes) and the Ethertype (2 bytes).
+ */
+#define ETH_HEADER_LEN 14
+
+#define PROXY_HEADER_LEN 4
+
+/*
+ * Read exactly n bytes into the buffer, retrying on partial reads.
+ * Returns n on success, 0 on clean EOF, or -1 on error.
+ */
+static ssize_t read_exact(int fd, void *buf, size_t n)
+{
+    size_t total = 0;
+
+    while (total < n) {
+        ssize_t r = read(fd, (char *)buf + total, n - total);
+        if (r < 0) {
+            if (errno == EINTR)
+                continue;
+            return -1;
+        } else if (r == 0) {
+            if (total > 0) {
+                errno = EIO;
+                return -1;
+            }
+            return 0;
+        }
+        total += r;
+    }
+    return (ssize_t)total;
+}
+
+/*
+ * Write exactly n bytes from the buffer to the fd, retrying on partial writes.
+ * Returns n on success, or -1 on error.
+ */
+static ssize_t write_all(int fd, const void *buf, size_t n)
+{
+    size_t total = 0;
+
+    while (total < n) {
+        ssize_t w = write(fd, (const char *)buf + total, n - total);
+        if (w <= 0) {
+            if (w < 0 && errno == EINTR)
+                continue;
+
+            if (w == 0)
+                errno = EIO;
+
+            return -1;
+        }
+        total += w;
+    }
+    return (ssize_t)total;
+}
+
 /*
  * Forward ethernet packets to/from the host vsock providing network access and
  * the guest TAP device routing application network traffic.
@@ -53,7 +111,7 @@ static int tap_vsock_forward(int tun_fd, int vsock_fd, int shutdown_fd,
     sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
     if (sock_fd < 0) {
         perror("creating INET socket to get TAP MTU");
-        return -errno;
+        return -1;
     }
 
     memset(&ifr, 0, sizeof(struct ifreq));
@@ -63,22 +121,24 @@ static int tap_vsock_forward(int tun_fd, int vsock_fd, int shutdown_fd,
     if (ret < 0) {
         close(sock_fd);
         perror("fetch MTU of TAP device");
-        exit(-errno);
+        return -1;
     }
 
     close(sock_fd);
 
-    buf = (unsigned char *)malloc(ifr.ifr_mtu);
+    uint32_t eth_frame_size = ifr.ifr_mtu + ETH_HEADER_LEN;
+    buf = (unsigned char *)malloc(eth_frame_size);
     if (buf == NULL) {
         perror("allocate buffer for TAP/vsock communication");
-        exit(-1);
+        return -1;
     }
 
-    // Forward the MTU to the host for it to allocate a corresponding buffer.
-    ret = write(vsock_fd, (void *)&ifr.ifr_mtu, sizeof(int));
-    if (ret < sizeof(int)) {
-        perror("write TAP device MTU to host");
-        exit(-errno);
+    // Forward the max ethernet frame size to the host for it to allocate a
+    // corresponding buffer.
+    if (write_all(vsock_fd, &eth_frame_size, sizeof(eth_frame_size)) < 0) {
+        perror("write max ethernet frame size to host");
+        free(buf);
+        return -1;
     }
 
     pfds[0].fd = vsock_fd;
@@ -97,25 +157,81 @@ static int tap_vsock_forward(int tun_fd, int vsock_fd, int shutdown_fd,
         event_found = false;
         // Event on vsock. Read the frame and write it to the TAP device.
         if (pfds[0].revents & POLLIN) {
-            nread = read(vsock_fd, &sz, 4);
-            if (nread != 4)
-                exit(0);
+            nread = read_exact(vsock_fd, &sz, PROXY_HEADER_LEN);
+            if (nread == 0) {
+                // vsock connection closed cleanly
+                break;
+            } else if (nread < 0) {
+                perror("unable to read the proxy header from vsock");
+                free(buf);
+                return -1;
+            }
 
-            unsigned int len = htonl(sz);
+            unsigned int len = ntohl(sz);
+            if (len > eth_frame_size) {
+                fprintf(stderr,
+                        "ethernet frame size %u exceeds MTU + header size %u\n",
+                        len, eth_frame_size);
+                free(buf);
+                return -1;
+            }
+
+            nread = read_exact(vsock_fd, buf, len);
+            if (nread != (ssize_t)len) {
+                if (nread == 0)
+                    errno = EIO;
+
+                perror("failed to read the ethernet frame from vsock");
+                free(buf);
+                return -1;
+            }
 
-            nread = read(vsock_fd, buf, len);
-            write(tun_fd, buf, nread);
+            // TAP devices are expected to write an entire frame at once and not
+            // do partial writes. Only retry if the syscall is interrupted.
+            ssize_t bytes_written = 0;
+            do {
+                bytes_written = write(tun_fd, buf, nread);
+            } while (bytes_written < 0 && errno == EINTR);
+
+            if (bytes_written != nread) {
+                // the entire frame wasn't written
+                if (bytes_written >= 0)
+                    errno = EIO;
+
+                perror("unable to write the ethernet frame to the TAP device");
+                free(buf);
+                return -1;
+            }
 
             event_found = true;
         }
 
         // Event on the TAP device. Read the frame and write it to the vsock.
         if (pfds[1].revents & POLLIN) {
-            nread = read(tun_fd, buf, ifr.ifr_mtu);
-            if (nread > 0) {
-                sz = htonl(nread);
-                write(vsock_fd, (void *)&sz, 4);
-                write(vsock_fd, buf, nread);
+            // TAP devices are expected to read an entire frame at once and not
+            // do partial reads. Only retry if the syscall is interrupted.
+            do {
+                nread = read(tun_fd, buf, eth_frame_size);
+            } while (nread < 0 && errno == EINTR);
+            if (nread <= 0) {
+                if (nread == 0)
+                    errno = EIO;
+
+                perror("failed to read the ethernet frame from the TAP device");
+                free(buf);
+                return -1;
+            }
+
+            sz = htonl((uint32_t)nread);
+            if (write_all(vsock_fd, (void *)&sz, PROXY_HEADER_LEN) < 0) {
+                perror("unable to write the proxy header to vsock");
+                free(buf);
+                return -1;
+            }
+            if (write_all(vsock_fd, buf, nread) < 0) {
+                perror("unable to write the ethernet frame to vsock");
+                free(buf);
+                return -1;
             }
 
             event_found = true;
@@ -132,6 +248,7 @@ static int tap_vsock_forward(int tun_fd, int vsock_fd, int shutdown_fd,
             break;
     }
 
+    free(buf);
     close(vsock_fd);
     close(tun_fd);
 

src/aws_nitro/src/enclave/proxy/proxies/net.rs

@@ -95,12 +95,12 @@ impl DeviceProxy for NetProxy {
         let (mut vsock, _) = listener.accept().map_err(Error::VsockAccept)?;
 
         /*
-         * Upon initial connection, read the MTU size from the enclave and allocate the buffer
+         * Upon initial connection, read the MTU size + ethernet frame header from the enclave and allocate the buffer
          * accordingly.
          */
         let size = {
             let mut size_buf = [0u8; size_of::<u32>()];
-            let _ = vsock.read(&mut size_buf).map_err(Error::VsockRead)?;
+            vsock.read_exact(&mut size_buf).map_err(Error::VsockRead)?;
 
             u32::from_ne_bytes(size_buf)
         };