Re: [PATCH v3 03/14] tools: Add directory checker for updates

Demi Marie Obenour
2025-11-19T23:59:19Z

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On 11/19/25 09:45, Alyssa Ross wrote:
> Demi Marie Obenour <demiobenour@gmail.com> 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

Nice catch :)

>> 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 <demiobenour@gmail.com>
>> ---
>> 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 <hi@alyssa.is>

Do you want me to send another version, or would you rather fix this
up on commit? All your changes look good.

>> 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 <demiobenour@gmail.com>
>> +#include <assert.h>
>> +#include <errno.h>
>> +#include <stddef.h>
>> +#include <stdlib.h>
>> +#include <string.h>
>> +
>> +#include <fcntl.h>
>> +#include <sys/types.h>
>> +#include <dirent.h>
>> +#include <unistd.h>
>> +
>> +#include <err.h>
>> +
>> +[[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?

I totally forgot that %hhu exists!

>> +}
>> +
>> +[[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.)

Good catch!

>> +{
>> + 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.

It isn't necessarily obvious that any file with this prefix is
a signature.

>> + 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 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.

In this case, only the directory got changed, so I only need to flush
the directory.

>> + 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.

Do you mean opendir()? That works, thanks!

>> + checkdir(fd, check_sig);
>> + }
>> + return 0;
>> +}
>>
>> --
>> 2.52.0

--
Sincerely,
Demi Marie Obenour (she/her/hers)

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