Spectrum OS's host has no network access. Updates must be downloaded by VMs. The downloads are placed into a bind-mounted directory. The VM can write whatever it wants into that directory. This includes symlinks that subsequent code might open, which would create a path traversal vulnerability. It also includes paths with names containing containing terminal escape sequences, newlines, or other nastiness. Furthermore, the directory should not have any subdirectories either. Add a simple C program that checks for such ugliness and indicates (via its exit code) if the VM misbehaved. It also ensures that both SHA256SUMS and SHA256SUMS.gpg are present. Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com> --- host/rootfs/Makefile | 6 +- lib/kcmdline-utils.mk | 6 ++ tools/default.nix | 1 + tools/meson.build | 1 + tools/updates-dir-check/meson.build | 4 ++ tools/updates-dir-check/updates-dir-check.c | 94 +++++++++++++++++++++++++++++ 6 files changed, 110 insertions(+), 2 deletions(-) diff --git a/host/rootfs/Makefile b/host/rootfs/Makefile index 00d125774bb7b98736d0928c69cb307740cee034..15752286f5924291768f0655a12b90c702730520 100644 --- a/host/rootfs/Makefile +++ b/host/rootfs/Makefile @@ -62,6 +62,9 @@ build/fifo: build/empty: mkdir -p $@ +build/etc: + mkdir -p $@ + # s6-rc-compile's input is a directory, but that doesn't play nice # with Make, because it won't know to update if some file in the # directory is changed, or a file is created or removed in a @@ -69,8 +72,7 @@ build/empty: # including files that aren't intended to be part of the input, like # temporary editor files or .license files. So for all these reasons, # only explicitly listed files are made available to s6-rc-compile. -build/etc/s6-rc: $(S6_RC_FILES) file-list.mk - mkdir -p $$(dirname $@) +build/etc/s6-rc: $(S6_RC_FILES) file-list.mk build/etc rm -rf $@ set -uo pipefail && dir=$$(mktemp -d) && \ { tar -c $(S6_RC_FILES) | tar -C $$dir -x --strip-components 3; } && \ diff --git a/lib/kcmdline-utils.mk b/lib/kcmdline-utils.mk new file mode 100644 index 0000000000000000000000000000000000000000..fa228552e583f15fc77a746985060ad5d04cdf2c --- /dev/null +++ b/lib/kcmdline-utils.mk @@ -0,0 +1,6 @@ +# SPDX-License-Identifier: EUPL-1.2+ +# SPDX-FileCopyrightText: 2021-2024 Alyssa Ross <hi@alyssa.is> +READ_ROOTHASH = { \ + set -eufo pipefail; \ + read -r version < ../../version; \ + LC_ALL=C expr "x$$version" : '^\(x0\|x[1-9][0-9]*\)\(\.\(0\|[1-9][0-9]*\)\)\{2\}$$' >/dev/null; } diff --git a/tools/default.nix b/tools/default.nix index ca165b5ec8ae1a63b75af4a34f33e320b262ba7b..e644f4e710e56f32de27ea10047cba3cffd0ecdf 100644 --- a/tools/default.nix +++ b/tools/default.nix @@ -78,6 +78,7 @@ stdenv.mkDerivation (finalAttrs: { ./start-vmm ./subprojects ./sd-notify-adapter + ./updates-dir-check ] ++ lib.optionals driverSupport [ ./xdp-forwarder ])); diff --git a/tools/meson.build b/tools/meson.build index 5d0ae81042fd3d77646594500f32cb1d48a6af0c..7da3bb451a5f1a797bc7e50a67c44dbd37ba60bf 100644 --- a/tools/meson.build +++ b/tools/meson.build @@ -28,6 +28,7 @@ if get_option('host') subdir('lsvm') subdir('start-vmm') subdir('sd-notify-adapter') + subdir('updates-dir-check') endif if get_option('app') diff --git a/tools/updates-dir-check/meson.build b/tools/updates-dir-check/meson.build new file mode 100644 index 0000000000000000000000000000000000000000..e19691d0e35f8a051e897990f0376384b3625c1a --- /dev/null +++ b/tools/updates-dir-check/meson.build @@ -0,0 +1,4 @@ +# SPDX-License-Identifier: EUPL-1.2+ +# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com> + +executable('updates-dir-check', 'updates-dir-check.c', install: true, c_args: ['-D_GNU_SOURCE=1', '-UNDEBUG', '-Wno-error=pedantic']) diff --git a/tools/updates-dir-check/updates-dir-check.c b/tools/updates-dir-check/updates-dir-check.c new file mode 100644 index 0000000000000000000000000000000000000000..94c7d54bec38d6efbd5b8aca257f3ec4ad3fae35 --- /dev/null +++ b/tools/updates-dir-check/updates-dir-check.c @@ -0,0 +1,94 @@ +// SPDX-License-Identifier: EUPL-1.2+ +// SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com> +#include <assert.h> +#include <errno.h> +#include <stddef.h> +#include <string.h> + +#include <sysexits.h> +#include <fcntl.h> +#include <sys/types.h> +#include <dirent.h> + +#include <linux/openat2.h> +#include <sys/syscall.h> +#include <unistd.h> + +#include <err.h> + +static void checkdir(int fd) +{ + DIR *d = fdopendir(fd); + if (d == NULL) + err(1, "fdopendir"); + bool found_sha256sums = false; + bool found_sha256sums_gpg = false; + for (;;) { + errno = 0; + struct dirent *entry = readdir(d); + if (entry == NULL) { + if (errno) + err(1, "readdir"); + break; + } + assert(entry->d_reclen > offsetof(struct dirent, d_name)); + size_t len = strnlen(entry->d_name, entry->d_reclen - offsetof(struct dirent, d_name)); + assert(len < entry->d_reclen - offsetof(struct dirent, d_name)); + assert(len > 0); + if (entry->d_name[0] == '.') + if (len == 1 || (len == 2 && entry->d_name[1] == '.')) + continue; + if (strcmp(entry->d_name, "SHA256SUMS") == 0) { + found_sha256sums = true; + continue; + } + if (strcmp(entry->d_name, "SHA256SUMS.gpg") == 0) { + found_sha256sums_gpg = true; + continue; + } + unsigned char c = (unsigned char)entry->d_name[0]; + if (!((c >= 'A' && c <= 'Z') || + (c >= 'a' && c <= 'z'))) + errx(1, "Filename must begin with an ASCII letter"); + for (size_t i = 1; i < len; ++i) { + c = (unsigned char)entry->d_name[i]; + if (!((c >= 'A' && c <= 'Z') || + (c >= 'a' && c <= 'z') || + (c >= '0' && c <= '9') || + (c == '_') || + (c == '-') || + (c == '.'))) { + if (c >= 0x20 && c <= 0x7E) + errx(1, "Forbidden subsequent character in filename: '%c'", (int)c); + else + errx(1, "Forbidden subsequent character in filename: byte %d", (int)c); + } + } + if (entry->d_name[len - 1] == '.') + errx(1, "Filename must not end with a '.'"); + if (entry->d_type != DT_REG) + errx(1, "Entry contains non-regular file %s", entry->d_name); + } + if (!found_sha256sums) + errx(1, "SHA256SUMS not found"); + if (!found_sha256sums_gpg) + errx(1, "SHA256SUMS.gpg not found"); + closedir(d); +} + +int main(int argc, char **argv) +{ + for (int i = 1; i < argc; ++i) { + // Avoid symlink attacks. + struct open_how how = { + .flags = O_DIRECTORY|O_RDONLY|O_CLOEXEC|O_NOFOLLOW, + .resolve = RESOLVE_NO_SYMLINKS|RESOLVE_NO_MAGICLINKS, + }; + int fd = (int)syscall((long)SYS_openat2, (long)AT_FDCWD, (long)argv[i], + (long)&how, (long)sizeof(how)); + if (fd < 0) + err(1, "open(%s)", argv[i]); + checkdir(fd); + } + return 0; +} -- 2.51.2