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

Alyssa Ross
2025-11-02T12:18:09Z

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> 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.

Hmm, thinking about it some more I suppose there's a problem with find:
there's no way to get it to exit as soon as it finds a matching file,
with a failing error code, so it could end up running way too long.

So the C program is fine, I guess.

>>>> 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.

Doesn't look like it:

https://github.com/mesonbuild/meson/blob/d00f840c573103c2d51aed2b169386f7acfe7026/mesonbuild/compilers/compilers.py#L255-L264

b_ndebug defaults to false.

>>>>> + 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.

What's the difference between systemd-journald's behaviour and the
logging we have?

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