[PATCH 11/36] cmd/libsnap-confine-private: Defend against hardlink attacks

[PATCH 11/36] cmd/libsnap-confine-private: Defend against hardlink attacks

When snap-confine goes to execute other helper binaries (snap-update-ns
etc) via sc_open_snapd_tool(), these other binaries are located relative to
the currently executing snap-confine process via /proc/self/exe. Since it
is possible for regular users to hardlink setuid binaries when
fs.protected_hardlinks is 0, it is possible to hardlink snap-confine to
another location and then place an attacker controlled binary in place of
snap-update-ns and have this executed as root by snap-confine. Protect
against this by checking that snap-confine is located either within
/usr/lib/snapd or within the core or snapd snaps as expected.

This resolves CVE-2021-44730.

Signed-off-by: Alex Murray <alex.murray@canonical.com>
Gbp-Pq: Topic cve202144730
Gbp-Pq: Name 0011-cmd-libsnap-confine-private-Defend-against-hardlink-.patch