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. >>>>> + 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?