On Fri, Jul 27, 2018 at 8:41 PM Mark Salyzyn salyzyn@google.com wrote:
Any system can chose to change the permissions of a sysfs node, default, DAC (and MAC) is just layers of multi-level security (or lack thereof). As well intentioned as a default DAC is in the kernel, leaking kernel addresses is still an attack surface that we want to close tightly.
For instance on Android:
chmod 0755 /sys/kernel/debug/tracingis in the common init.rc file ...
If DAC has been adjusted at runtime to permit access to the node, I would think that if the caller does not have all the credentials/capabilities, we do want the addresses to be abstracted by the kernel.
If you adjust the access controls on debugfs to permit things that aren't possible upstream, you may have to add new access controls to ensure that that doesn't lead to security issues. And, in fact, you did:
walleye:/ # ls -laZ /sys/kernel/debug total 0 drwxr-xr-x 100 root root u:object_r:debugfs:s0 0 2018-07-27 18:08 . drwxr-xr-x 19 root root u:object_r:sysfs:s0 0 1970-06-04 10:30 .. [...] drwxr-xr-x 6 root root u:object_r:debugfs_tracing:s0 0 1970-01-01 01:00 tracing drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 tsens drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 tzdbg drwxr-xr-x 4 root root u:object_r:debugfs_ufs:s0 0 1970-01-01 01:00 ufshcd0 drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 usb drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 usb-pdphy drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 usb_diag drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 vmem -r--r--r-- 1 root root u:object_r:debugfs:s0 0 1970-01-01 01:00 wakeup_sources drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 2018-07-27 18:07 wcd_spi drwxr-xr-x 2 root root u:object_r:debugfs:s0 0 2018-07-27 18:07 wdsp0 drwxr-xr-x 2 root root u:object_r:debugfs_wlan:s0 0 2018-07-27 18:07 wlan0 drwxr-xr-x 3 root root u:object_r:debugfs:s0 0 2018-07-27 18:07 wlan_qdf
Stuff in the debugfs mount is labeled as "debugfs", with a few exceptions. And the SELinux policy locks down access to debugfs:
public/domain.te:neverallow { domain -init -vendor_init -system_server -dumpstate } debugfs:file no_rw_file_perms;
On Fri, Jul 27, 2018 at 11:31 AM, Steven Rostedt rostedt@goodmis.org wrote:
On Fri, 27 Jul 2018 11:13:51 -0700 Nick Desaulniers ndesaulniers@google.com wrote:
I found the internal bug report (reported Jan '17, you'll have to forgive me if my memory of the issue is hazy, or if the fix used at the time wasn't perfect), which was reported against the Nexus 6.
From the report, it was possible to `cat
/sys/kernel/debug/tracing/printk_formats` without being root, which I can't do on my workstations much more modern kernel (Nexus 6 was 3.10). So I guess the question is what governs access to files below /sys/kernel/debug, and why was it missing from those kernels? I assume some check was added, but either not backported to 3.10 stable (or more likely not pulled in to Nexus 6's kernel through stable; Android is now in a much better place for that kind of issue).
As of commit 82aceae4f0d4 ("debugfs: more tightly restrict default mount mode") /sys/kernel/debug has been default mounted as 0700 (root only). But that was introduced in 3.7. Not sure why your 3.10 kernel didn't have that. Perhaps there's another commit that fixed permissions not being inherited?
-- Steve
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