Re: [PATCH 3/7] tools: Add directory checker for updates

Demi Marie Obenour
2025-11-01T18:36:43Z

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On 11/1/25 08:17, Alyssa Ross wrote:
> Demi Marie Obenour <demiobenour@gmail.com> writes:
>
>> On 10/29/25 08:01, 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
>>>> 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(-)
>>>
>>> I still don't really understand why this needs to be a C program instead
>>> of find -H /path/to/dir -not -type f. None of the other checks seem
>>> very necessary?
>>
>> I trust this code more than I trust (especially) the Busybox
>> implementation of find.
>
> This doesn't really make sense to me. All of this is quite trivial find
> behaviour — not the sort of thing that's unlikely to have been widely
> tested. No objection to GNU find though if it helps.

I see: find with a -exec false to return an error if anything matching
is found?

I'm way more familiar with C than with find, which is why I missed this.

>>> How are -Werror=pedantic and -DNDEBUG getting enabled in the first place?
>>
>> I believe Meson sets -DNDEBUG in some cases.
>
> Yes, if the user explicitly asks for it.

I thought it was default for release builds.

>>>> + 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);
>>>> + }
>>>> + }
>>>
>>> Why do we care? Surely we don't expect systemd-sysupdate to put
>>> filenames unescaped into a shell or something.
>>
>> Prevent escape sequence injection into terminals and logs is the
>> main reason. Qubes OS has similar checks in some places, though they
>> are off by default for file copying.
>
> Doing this in a tool that's only used by sysupdate is a very ad-hoc way
> to protect against that. I think if we want to protect against that
> sort of thing it should be done in one place, probably in virtiofsd.

I think sysupdate is more likely to log unsanitized data, especially
as systemd-journald has no problems with it.
--
Sincerely,
Demi Marie Obenour (she/her/hers)

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