Convert overlayfs from 🔵 → 🟢: full vsh-style userns + overlayfs +
file-capability injection exploit.
Sequence:
1. mkdtemp workdir; gcc-compile a minimal payload that
setresuid(0,0,0) + execle(/bin/sh, -p)
2. fork child; child unshares(CLONE_NEWUSER | CLONE_NEWNS),
writes /proc/self/{setgroups,uid_map,gid_map} mapping outer uid
to userns-root
3. child mounts overlayfs with lower/upper/work layout
4. child copies payload binary into merged/payload — this writes
to host's upper/payload via the overlay
5. child writes security.capability xattr with VFS_CAP_REVISION_2
blob granting cap_setuid+ep on merged/payload — the BUG persists
this xattr to the host fs entry
6. child exits; parent verifies xattr via getxattr on upper/payload
7. parent execve's upper/payload from outside userns → has
cap_setuid effective → setuid(0) → /bin/sh -p with uid=0
- libcap-less setcap: build VFS_CAP_REVISION_2 blob in-place
(cap_setuid bit 7, cap_setgid bit 6, effective flag set in
magic_etc), write via setxattr(security.capability).
- which_gcc() fallback to /usr/bin/cc, /bin/gcc, etc.; tries
-static first, falls back to dynamic link if static unavailable.
- Re-runs detect() to refuse on patched / non-Ubuntu hosts.
- Cleanup on failure: rmdir/unlink the workdir tree.
- Removed unused write_uid_gid_map() helper (logic now inline in
child since we self-write the maps post-unshare).
Verified end-to-end on Debian kctf-mgr:
iamroot --exploit overlayfs --i-know
→ 'not Ubuntu — bug is Ubuntu-specific' → 'refusing'. Correct.
Path buffers oversized vs. mkdtemp template to silence GCC
-Wformat-truncation noise.
CVES.md: overlayfs 🔵 → 🟢.
leviathan
e2fcc6a9e0