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

[Alyssa Ross <hi@alyssa.is>]

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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

> 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>

> 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?

> +}
> +
> +[[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

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