grant bpfloader explicit membership in some groups
(this is instead of relying on the CAP_CHOWN capability it has)
The goal is to allow bpfloader to open maps/programs it creates,
so that it can reuse them. By virtue of CAP_CHOWN it can create,
pin, then give away ownership, and no longer be able to bpf_obj_get()
the pinned map or program (to reuse it at a later time).
This could be considered a partial (more targetted) workaround
for the lack of bpfloader CAP_DAC_OVERRIDE (or CAP_DAC_READ_SEARCH).
But for obvious reasons jeffv@ doesn't really want to grant that.
In some sense this doesn't actually really grant any privs on a writeable
filesystem, as CHOWN already allows stealing ownership...
However explicit membership is much easier to reason about,
and does not require playing:
- stat (to get current uid/gid/mode)
- chown (set uid to root, ie. self -- works due to CAP_CHOWN)
- chmod (grant user read if missing)
- bpf_obj_get (this now succeeds -- does not require capabilities)
- chmod (restore mode)
- chown (restore uid -- works due to CAP_CHOWN)
games in order to open pinned bpf maps/programs we'd normally be unable
to open due to unix uid/gid/mode restrictions.
Yes, I've verified the above 'magic' actually works with current privs,
provided we grant the missing 'getattr' selinux priv to allow the stat() call.
(obviously without it we can still gain access, we just can't undo things)
Currently /sys/fs/bpf maps and program ownership on a tip-of-tree T device looks like:
$ adb shell getprop ro.build.fingerprint
google/oriole/oriole:13/TP1A.220624.007/8785063:userdebug/dev-keys
$ adb shell ls -l /sys/fs/bpf/* | egrep '^-' | cut -d' ' -f3-4 | sort | uniq -c
count uid gid examples
5 root graphics platform: gpu_mem.o & gpu_work.o
5 root net_admin tethering apex T+: netd.o skfilter_..._xtbpf & schedact_ingress_account programs
10 root net_bw_acct tethering apex T+: netd.o maps
24 root network_stack tethering apex S+: offload.o & test.o
1 root root tethering apex T+: netd.o cgroupsock_inet_create program
38 root system platform & tethering apex T+: time_in_state.o, block.o, clatd.o, dscp_policy.o, netd.o cgroupskb_(e|in)gress_stats
And additionally due to the utter lack of a 'groups' line in bpfloader.rc,
the default bpfloader gid is of course 'root'.
This suggests we should use:
groups root graphics network_stack net_admin net_bw_acct system
(but only really mainline updatable stuff matters, so we could limit
this to just networking and strip out 'graphics'...)
A glance through:
system/core/libcutils/include/private/android_filesystem_config.h
Finds the following groups which might be of interest to bpfloader & mainline networking:
* root
* system
* graphics
dhcp
vpn
mdnsr
clat
dns
dns_tether
* network_stack
inet
net_raw
* net_admin
net_bw_stats
* net_bw_acct
[stars mark the one's we've already identified previously]
Networking mainline code runs in 3 processes: netd, system_server and network_stack.
Based on looking at a live oriole device, these processes have the following
uid/gid/groups/capabilities:
netd - uid:0[root] gid:0[root] + 3005[net_admin]
Cap: 00000000000074ef=cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_kill,cap_setgid,cap_setuid,cap_net_bind_service,cap_net_admin,cap_net_raw,cap_ipc_lock
networkstack.process - uid:1073[network_stack] gid:1073[network_stack] + 1073[network_stack]
3002[net_bt] 3003[inet] 3004[net_raw] 3005[net_admin] 3006[net_bw_stats] 3007[net_bw_acct] 9997[everybody]
Cap: 0000000000003c00=cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw
system_server - uid:1000[system] gid:1000[system] + 1001[radio] 1002[bluetooth] 1003[graphics]
1004[input] 1005[audio] 1006[camera] 1007[log] 1008[compass] 1009[mount] 1010[wifi]
1018[usb] 1021[gps] 1023[media_rw] 1024[mtp] 1032[package_info] 1065[reserved_disk]
3001[net_bt_admin] 3002[net_bt] 3003[inet] 3005[net_admin] 3006[net_bt_stats] 3007[net_bw_acct]
3009[readproc] 3010[wakeloc] 3011[uhid] 3012[readtracefs]
Cap: 0000001806897c20=cap_kill,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_sys_module,cap_sys_ptrace,cap_sys_nice,cap_sys_time,cap_sys_tty_config,cap_wake_alarm,cap_block_suspend
Since netd has DAC_OVERRIDE, it really doesn't matter from a group analysis perspective
(side note: it probably should have a lot more groups than it actually does...)
Either way, both *root & *net_admin are already in the above list.
For the network stack process the obvious groups are:
*network_stack, net_raw, *net_admin, net_bw_stats, *net_bw_acct
which means we should add:
net_raw, net_bw_stats
to the above list.
(I'm assuming 'inet' & 'everybody' are too generic groups to be of use for bpf,
and that we don't [yet] care about bluetooth (net_bt) being able to use bpf directly)
For the system server the choice is harder, but I'd tend to pick:
*system, *graphics, *net_admin, *net_bw_acct
(Again ignoring non-networking stuff, and assuming radio/bluetooth/wifi bpf
use will come at some later point in time.)
This gives us decent coverage of the 3 processes (and combinations there-of):
netd process -> group root
network stack process -> group network_stack
system server process -> group system
both network stack and system server -> group net_bw_acct
Note that due to DAC_OVERRIDE netd always has unix access no matter what,
and needs to be limited via selinux contexts instead.
Additionally 'net_admin' is used for xt_bpf iptables programs due to need
for netutils_wrappers support and it is also usable by all 3 processes.
This means we can fully explain all groups that currently show up as in use.
Adding net_raw & net_bw_stats is possibly not needed, but also won't hurt,
and might be useful in the future.
We could also argue that we should add:
dhcp, vpn, mdnsr, clat, dns, dns_tether & inet
But since none of our mainline code running processes are currently
members of those groups (besides netd due to DAC_OVERRIDE), there doesn't
seem to be much benefit (this can't be changed with mainline pushes).
I assume new stuff which would need these groups will actually only be loaded
on U+ bpfloader, which will have a less hacky solution for this problem anyway.
Note: on U+ bpfloader we should probably fix this by simply caching
all bpf map/prog filedescriptors in a path->fd hashmap, and thus
avoid the need to ever reopen anything. This is a far more invasive change,
but once done we should be able to revert this change.
For safety we'll also want to make sure we abort() if we detect cases
that cannot be safely handled by S bpfloader, an example would be
maps with uid != root in tethering location.
Bug: 218408035
Bug: 237716689
Test: TreeHugger, manual testing
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I742868b1a6819547fcd7a3573946a2fc479a21a5
2 files changed
