Demi Marie Obenour writes: > 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 Just one "containing" is fine. :P > 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. systemd-sysupdate can leave > behind temporary files with names starting with '.', so delete them > instead of failing. Linux can lose cache coherency if there is an I/O > error, so call syncfs() on the directory before checking anything. For > the same reason, fsync() the directory if any hidden files were deleted. > > The directory checker also serves another critical function: it checks > if the VM actually downloaded anything. Otherwise, network problems > could cause updates to silently do nothing. Specifically, it checks > that the VM provided a file starting with the prefix "SHA256SUMS.". > These will be the last ones the in-VM updater downloads. An additional > mode is provided to clean out all such files. This will be used to > ensure that before the in-VM updater runs, no such files are present. > Hence, if the VM didn't actually download anything, the user will get a > clear error instead of a false success message or a confusing error. > > Signed-off-by: Demi Marie Obenour > --- > Changes since v2: > > - Purge leftover temporary files rather than returning an error. > > - Split into two modes: one that deletes signature files, and one that > checks that at least one signature file exists. This allows checking > that the VM actually sent something. > --- > tools/default.nix | 1 + > tools/meson.build | 4 ++ > tools/updates-dir-check.c | 133 ++++++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 138 insertions(+) Looking good. Left some style comments, but Reviewed-by: Alyssa Ross > diff --git a/tools/updates-dir-check.c b/tools/updates-dir-check.c > new file mode 100644 > index 0000000000000000000000000000000000000000..07eb059f2718e1ad8ab087fe6509c1437ea3e96c > --- /dev/null > +++ b/tools/updates-dir-check.c > @@ -0,0 +1,133 @@ > +// SPDX-License-Identifier: EUPL-1.2+ > +// SPDX-FileCopyrightText: 2025 Demi Marie Obenour > +#include > +#include > +#include > +#include > +#include > + > +#include > +#include > +#include > +#include > + > +#include > + > +[[noreturn]] static void bad_char(char c, char *msg_component) > +{ > + if (c >= 0x20 && c <= 0x7E) > + errx(EXIT_FAILURE, "Forbidden %s character in filename: '%c'", > + msg_component, (int)c); > + errx(EXIT_FAILURE, > + "Forbidden %s character in filename: byte %d", > + msg_component, (int)(unsigned char)c); Why not %hhu, so you don't need two layers of casts? > +} > + > +[[noreturn]] static void usage(void) > +{ > + errx(EXIT_FAILURE, "Usage: updates-dir-check [cleanup|check] DIRECTORIES..."); > +} > + > +static void checkdir(int fd, bool check_sig) [[gnu::fd_arg_read (1)]] (I'm bad at remembering this too so you'll see other code missing it, but it's good to add.) > +{ > + bool found_sig = false; > + DIR *d = fdopendir(fd); > + if (d == NULL) > + err(EXIT_FAILURE, "fdopendir"); > + // If there is an I/O error while there are dirty pages outstanding, > + // the dirty pages are silently discarded. This means that the contents > + // of the filesystem can change behind userspace's back. Flush all > + // dirty pages in the filesystem with the directory to prevent this. > + if (syncfs(fd) != 0) > + err(EXIT_FAILURE, "syncfs"); > + bool changed = false; > + for (;;) { > + errno = 0; > + struct dirent *entry = readdir(d); > + if (entry == NULL) { > + if (errno) > + err(EXIT_FAILURE, "readdir"); > + break; > + } > + const char *ptr = entry->d_name; > + if (ptr[0] == '.') { > + if (ptr[1] == '\0') > + continue; > + if (ptr[1] == '.' && ptr[2] == '\0') > + continue; > + // systemd-sysupdate uses these for temporary files. > + // It normally cleans them up itself, but if there is an error > + // it does not always clean them up. I'm not sure if it is > + // guaranteed to clean up temporary files from a past run, so > + // delete them instead of returning an error. > + if (unlinkat(fd, ptr, 0)) > + err(EXIT_FAILURE, "Failed to unlink temporary file"); > + changed = true; > + continue; > + } > + char c = ptr[0]; > + if (!((c >= 'A' && c <= 'Z') || > + (c >= 'a' && c <= 'z'))) > + bad_char(c, "initial"); > + while ((c = *++ptr)) { > + if (!((c >= 'A' && c <= 'Z') || > + (c >= 'a' && c <= 'z') || > + (c >= '0' && c <= '9') || > + (c == '_') || > + (c == '-') || > + (c == '.'))) > + bad_char(c, "subsequent"); > + } > + // Empty filenames are rejected as having a bad initial character, > + // and POSIX forbids them from being returned anyway. Therefore, > + // this cannot be out of bounds. > + if (ptr[-1] == '.') > + errx(EXIT_FAILURE, "Filename %s ends with a '.'", entry->d_name); > + if (entry->d_type == DT_UNKNOWN) > + errx(EXIT_FAILURE, "Filesystem didn't report type of file %s", entry->d_name); > + if (entry->d_type != DT_REG) > + errx(EXIT_FAILURE, "Entry contains non-regular file %s", entry->d_name); > + if (strncmp(entry->d_name, "SHA256SUMS.", sizeof("SHA256SUMS.") - 1) == 0) { > + // Found a signature file! This comment seems a bit redundant. > + if (check_sig) > + found_sig = true; > + else { > + if (unlinkat(fd, entry->d_name, 0)) > + err(EXIT_FAILURE, "Unlinking old signature file"); > + changed = true; > + } > + } > + } > + // fsync() the directory if it was changed, to avoid the above > + // cache-incoherency problem. Above where? > + if (changed && fsync(fd)) > + errx(EXIT_FAILURE, "fsync"); > + if (check_sig && !found_sig) { > + warnx("sys.appvm-systemd-sysupdate didn't send a signature file."); > + warnx("There was probably a problem downloading the update."); > + errx(EXIT_FAILURE, "Check its logs for more information."); > + } > + closedir(d); > +} > + > +int main(int argc, char **argv) > +{ > + if (argc != 3) > + usage(); > + > + bool check_sig; > + if (strcmp(argv[1], "cleanup") == 0) > + check_sig = false; > + else if (strcmp(argv[1], "check") == 0) > + check_sig = true; > + else > + usage(); > + > + for (int i = 2; i < argc; ++i) { > + int fd = open(argv[i], O_DIRECTORY|O_RDONLY|O_CLOEXEC); > + if (fd < 0) > + err(EXIT_FAILURE, "open(%s)", argv[i]); Maybe we could just fdopen(argv[1]) inside checkdir()? We don't need any special flags AFAICT. > + checkdir(fd, check_sig); > + } > + return 0; > +} > > -- > 2.52.0