Tony Finch wrote:
> Ludovic Gasc <gml...@gmail.com> wrote:
> >
> > 1. The list of minimal capabilities needed for bind to run correctly:
> > http://man7.org/linux/man-pages/man7/capabilities.7.html
>
> named already drops capabilities - have a look at the code around here:
> https://source.isc.org/cgi-bin/gitweb.cgi?p=bind9.git;a=blob;f=bin/named/unix/os.c;hb=v9_11_2#l234
>
> Note that it's a bit clever - the privileges are dropped in two stages,
> right at the start, and after the server has been configured.
I checked just now to see what that code actually ends up doing, and on
my system I ended up with:
$ grep -h ^Cap /proc/$(pidof named)/**/status | sort | uniq -c
6 CapAmb: 0000000000000000
6 CapBnd: 0000003fffffffff
6 CapEff: 0000000001000400
6 CapInh: 0000000000000000
6 CapPrm: 0000000001000400
$
That decodes to:
- The effective and permitted capabilities sets were reduced to
CAP_NET_BIND_SERVICE and CAP_SYS_RESOURCE.
- The ambient and inheritable capabilities sets were cleared.
- The capability bounding set was left completely open-ended.
It's not clear why CAP_SYS_RESOURCE needs to be retained past startup:
/*
* XXX We might want to add CAP_SYS_RESOURCE, though it's not
* clear it would work right given the way linuxthreads work.
* XXXDCL But since we need to be able to set the maximum number
* of files, the stack size, data size, and core dump size to
* support named.conf options, this is now being added to test.
*/
SET_CAP(CAP_SYS_RESOURCE);
See commits 5e4b7294d88ab58371d8c98e05ea80086dcb67cd,
108490a7f8529aff50a0ac7897580b59a73d9845. "[T]o test"?
CAP_SYS_RESOURCE is documented as permitting:
CAP_SYS_RESOURCE
* Use reserved space on ext2 filesystems;
* make ioctl(2) calls controlling ext3 journaling;
* override disk quota limits;
* increase resource limits (see setrlimit(2));
* override RLIMIT_NPROC resource limit;
* override maximum number of consoles on console allocation;
* override maximum number of keymaps;
* allow more than 64hz interrupts from the real-time clock;
* raise msg_qbytes limit for a System V message queue above the
limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
* allow the RLIMIT_NOFILE resource limit on the number of "in-
flight" file descriptors to be bypassed when passing file
descriptors to another process via a UNIX domain socket (see
unix(7));
* override the /proc/sys/fs/pipe-size-max limit when setting the
capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
* use F_SETPIPE_SZ to increase the capacity of a pipe above the
limit specified by /proc/sys/fs/pipe-max-size;
* override /proc/sys/fs/mqueue/queues_max limit when creating
POSIX message queues (see mq_overview(7));
* employ the prctl(2) PR_SET_MM operation;
* set /proc/[pid]/oom_score_adj to a value lower than the value
last set by a process with CAP_SYS_RESOURCE.
I would guess that retaining CAP_NET_BIND_SERVICE and CAP_SYS_RESOURCE
during the process runtime permits open-ended reloading of the config at
runtime (e.g., binding to a new IP address on port 53 without needing to
restart the daemon). So even though BIND drops some capabilities, it's
still running with elevated privileges compared to a traditional
non-root user.
systemd permits a nice pattern for network daemons that want to run as
an unprivileged user, but bind to a privileged port (and without using
socket activation), without starting the process as root. Basically, you
put something like this in the unit file:
[Service]
User=…
Group=…
CapabilityBoundingSet=CAP_NET_BIND_SERVICE CAP_SYS_CHROOT CAP_SETPCAP
AmbientCapabilities=CAP_NET_BIND_SERVICE CAP_SYS_CHROOT CAP_SETPCAP
…
Any needed filesystem directories and permissions need to be set up
correctly before hand. The service is started by the init system as the
unprivileged User/Group specified in the unit file, so there's no need
to change UID/GID. CAP_NET_BIND_SERVICE is then used to bind to a
privileged port, CAP_SYS_CHROOT is used to perform the chroot, and
CAP_SETPCAP is used to drop all remaining capabilities from the
capability sets and the capability bounding set, so you end up with a
completely unprivileged process at runtime. (Alternatively you could
keep CAP_NET_BIND_SERVICE and drop CAP_SYS_CHROOT and CAP_SETPCAP, if
you wanted to retain the capability to perform privileged binds at
runtime. Or you could eliminate CAP_SYS_CHROOT and use other systemd
functionality to make parts of the filesystem inaccessible, etc.) This
pattern might be a bit hard to retrofit into BIND at this point, though,
other than by adding more knobs.
--
Robert Edmonds
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