Demi Marie Obenour writes: > 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. As early as possible would be in virtiofsd, not ad-hoc for this one service here.