systemd: make home directory readable by systemd-coredump

In https://github.com/systemd/systemd/commit/924453c22599cc246746a0233b2f52a27ade0819
ProtectHome was set to true for systemd-coredump in order to reduce risk, since an attacker could craft a malicious binary in order to compromise systemd-coredump.
At that point the object analysis was done in the main systemd-coredump process.
Because of this systemd-coredump is unable to product symbolicated call-stacks for binaries running under /home ("n/a" is shown instead of function names).

However, later in https://github.com/systemd/systemd/commit/61aea456c12c54f49c4a76259af130e576130ce9

 systemd-coredump was changed to do the object analysis in a forked process,
covering those security concerns.

Let's set ProtectHome to read-only so that systemd-coredump produces symbolicated call-stacks for processes running under /home.

Note: it still does not work in /tmp (because of PrivateTmp=yes) and in /root (for unknown reasons).

Before the change (with minidebuginfo enabled):

    root@qemux86-64:~# /home/sleep 1000 &
    [1] 426
    root@qemux86-64:~# kill -11 $(pidof sleep)
    root@qemux86-64:~# coredumpctl info
               PID: 426 (sleep)
               UID: 0 (root)
               GID: 0 (root)
            Signal: 11 (SEGV)
         Timestamp: Fri 2024-09-06 17:25:18 UTC (3s ago)
      Command Line: /home/sleep 1000
        Executable: /home/sleep
     Control Group: /system.slice/system-serial\x2dgetty.slice/serial-getty@ttyS0.service
              Unit: serial-getty@ttyS0.service
             Slice: system-serial\x2dgetty.slice
           Boot ID: 44ef4ddfaad249ceaa29d1e9f330d3b5
        Machine ID: fb279f18f2c849c59768754c7a274ee3
          Hostname: qemux86-64
           Storage: /var/lib/systemd/coredump/core.sleep.0.44ef4ddfaad249ceaa29d1e9f330d3b5.426.1725643518000000.zst (present)
      Size on Disk: 16.5K
           Message: Process 426 (sleep) of user 0 dumped core.

                    Stack trace of thread 426:
                    #0  0x00007f365f3849a7 clock_nanosleep (libc.so.6 + 0xd49a7)
                    #1  0x00007f365f38f667 __nanosleep (libc.so.6 + 0xdf667)
                    #2  0x0000561fee703737 n/a (/home/sleep + 0x7737)
                    #3  0x000000003a6227c5 n/a (n/a + 0x0)
                    ELF object binary architecture: AMD x86-64
    [1]+  Segmentation fault      (core dumped) /home/sleep 1000

After the change (with minidebuginfo enabled):

    root@qemux86-64:~# /home/sleep 1000 &
    [1] 450
    root@qemux86-64:~# kill -11 $(pidof sleep)
    root@qemux86-64:~# coredumpctl info
               PID: 450 (sleep)
               UID: 0 (root)
               GID: 0 (root)
            Signal: 11 (SEGV)
         Timestamp: Fri 2024-09-06 17:30:12 UTC (4s ago)
      Command Line: /home/sleep 1000
        Executable: /home/sleep
     Control Group: /system.slice/system-serial\x2dgetty.slice/serial-getty@ttyS0.service
              Unit: serial-getty@ttyS0.service
             Slice: system-serial\x2dgetty.slice
           Boot ID: 44ef4ddfaad249ceaa29d1e9f330d3b5
        Machine ID: fb279f18f2c849c59768754c7a274ee3
          Hostname: qemux86-64
           Storage: /var/lib/systemd/coredump/core.sleep.0.44ef4ddfaad249ceaa29d1e9f330d3b5.450.1725643812000000.zst (present)
      Size on Disk: 16.5K
           Message: Process 450 (sleep) of user 0 dumped core.

                    Stack trace of thread 450:
                    #0  0x00007f795dd689a7 clock_nanosleep (libc.so.6 + 0xd49a7)
                    #1  0x00007f795dd73667 __nanosleep (libc.so.6 + 0xdf667)
                    #2  0x0000561965c9d737 rpl_nanosleep (sleep + 0x7737)
                    #3  0x0000561965c9d0c1 xnanosleep (sleep + 0x70c1)
                    #4  0x0000561965c985c8 main (sleep + 0x25c8)
                    #5  0x00007f795dcba01b __libc_start_call_main (libc.so.6 + 0x2601b)
                    #6  0x00007f795dcba0d9 __libc_start_main (libc.so.6 + 0x260d9)
                    #7  0x0000561965c98685 _start (sleep + 0x2685)
                    ELF object binary architecture: AMD x86-64
    [1]+  Segmentation fault      (core dumped) /home/sleep 1000

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>