On 11/2/25 07:18, Alyssa Ross wrote: > Demi Marie Obenour writes: > >> On 11/1/25 08:17, Alyssa Ross wrote: >>> Demi Marie Obenour writes: >>> >>>> On 10/29/25 08:01, Alyssa Ross wrote: >>>>> 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 >>>>>> 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 >>>>>> --- >>>>>> 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. Got it, thanks! >>>>>> + 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? I'm not familiar with s6 at all, but I think it is at least worth investigating. Also, all else equal it is best to reject invalid untrusted input as early as possible. -- Sincerely, Demi Marie Obenour (she/her/hers)