libmount: be more restrictive#4393
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Non-root users should only specify mountpoints when calling umount. Device names, tags (LABEL=, UUID=, etc.) and loop device resolution are disabled for unprivileged users to avoid processing untrusted input through tag resolution code paths before permission checks. In umount.c, reject tag arguments and disable swapmatch for restricted users. In libmount, skip source and loopdev swapmatch lookups for restricted contexts as defense in depth. Signed-off-by: Karel Zak <kzak@redhat.com>
Add blkid_evaluate_tag2() that accepts flags to control tag evaluation behavior. The BLKID_EVALUATE_NOPROBE flag disables low-level device scanning (evaluate_by_scan), while keeping udev symlink-based evaluation (evaluate_by_udev) functional. The original blkid_evaluate_tag() is now a thin wrapper around the new function with flags=0. Signed-off-by: Karel Zak <kzak@redhat.com>
Add noprobe flag to struct libmnt_cache that disables low-level device probing via libblkid. When the mount context is restricted (non-root user), the flag is set automatically on cache creation and on externally provided caches. With noprobe enabled: - read_from_blkid() is skipped (device I/O blocked) - mnt_resolve_tag() uses blkid_evaluate_tag2() with BLKID_EVALUATE_NOPROBE, so tags are resolved only via udev /dev/disk/by-* symlinks without device scanning This minimizes the libblkid attack surface for suid mount/umount binaries. Signed-off-by: Karel Zak <kzak@redhat.com>
Non-root tag resolution (LABEL=, UUID=) in mount/umount depends on udev support in libmount. Without it, suid mount/umount cannot resolve tags for unprivileged users since direct device probing is disabled for security reasons. Signed-off-by: Karel Zak <kzak@redhat.com>
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