[ZBX-23453] vfs.dev.discovery causes SELinux errors Created: 2023 Sep 22  Updated: 2023 Sep 25

Status: Confirmed
Project: ZABBIX BUGS AND ISSUES
Component/s: Agent (G)
Affects Version/s: 6.0.21
Fix Version/s: None

Type: Problem report Priority: Major
Reporter: Kento Takahashi Assignee: Jurijs Klopovskis
Resolution: Unresolved Votes: 2
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified


 Description   

Steps to reproduce:

  1. Install zabbix-agent 6.0.21 on AlmaLinux 8
  2. Start zabbix-agent service
  3. Monitor vfs.dev.discovery item

Result:
SELinux errors are output to /var/log/audit/audit.log.

type=AVC msg=audit(1695357845.405:128): avc:  denied  { getattr } for  pid=1428 comm="zabbix_agentd" path="/run/initctl" dev="tmpfs" ino=19850 scontext=system_u:system_r:zabbix_agent_t:s0 tcontext=system_u:object_r:initctl_t:s0 tclass=fifo_file permissive=0
type=AVC msg=audit(1695357845.405:129): avc:  denied  { getattr } for  pid=1428 comm="zabbix_agentd" path="/run/systemd/journal/dev-log" dev="tmpfs" ino=11844 scontext=system_u:system_r:zabbix_agent_t:s0 tcontext=system_u:object_r:devlog_t:s0 tclass=sock_file permissive=0
type=AVC msg=audit(1695357845.405:130): avc:  denied  { getattr } for  pid=1428 comm="zabbix_agentd" path="/proc/kcore" dev="proc" ino=4026532029 scontext=system_u:system_r:zabbix_agent_t:s0 tcontext=system_u:object_r:proc_kcore_t:s0 tclass=file permissive=0

Expected:
no SELinux errors
P.S.
The same problem is reported in Fedora. https://bugzilla.redhat.com/show_bug.cgi?id=2170630

 Comments   
Comment by Kento Takahashi [ 2023 Sep 22 ]

I found the cause after some investigation.
/run/initctl, /run/systemd/journal/dev-log and /proc/kcore have symlinks under /dev.

# ls -lZ /dev/initctl
lrwxrwxrwx. 1 root root system_u:object_r:device_t:s0 12 Sep 22 13:38 /dev/initctl -> /run/initctl
# ls -lZ /dev/log
lrwxrwxrwx. 1 root root system_u:object_r:devlog_t:s0 28 Sep 22 13:38 /dev/log -> /run/systemd/journal/dev-log
# ls -lZ /dev/core
lrwxrwxrwx. 1 root root system_u:object_r:device_t:s0 11 Sep 22 13:38 /dev/core -> /proc/kcore

Here is the code in vfs.dev.discovery item.

// src/libs/zbxsysinfo/linux/diskio.c

int     VFS_DEV_DISCOVERY(AGENT_REQUEST *request, AGENT_RESULT *result)
{
(snip.)
        if (NULL != (dir = opendir(ZBX_DEV_PFX)))
        {
(snip.)
                while (NULL != (entries = readdir(dir)))
                {
                        zbx_snprintf(tmp, sizeof(tmp), ZBX_DEV_PFX "%s", entries->d_name);

                        if (0 == zbx_stat(tmp, &stat_buf) && 0 != S_ISBLK(stat_buf.st_mode))
                        {

At first, open directory /dev by opendir(). Next, get directory entries by readdir(). Then, do stat() for each entry.
Because stat() is used here, not lstat(), zabbix-agent accesses /run/initctl, /run/systemd/journal/dev-log and /proc/kcore.
This causes SELinux errors.

I think it would be no problem to replace stat() with lstat() because it is no need to follow symlinks here.

                                        if (0 == lstat(tmp, &lstat_buf))
                                        {
(snip.)
                                                        if (0 != S_ISLNK(lstat_buf.st_mode))
                                                                dev_bypass = 1;
                                                        else
                                                                zbx_snprintf(tmp, sizeof(tmp), "rom");
(snip.)
                                if (0 == dev_bypass)
                                {
                                        zbx_json_addobject(&j, NULL);
                                        zbx_json_addstring(&j, "{#DEVNAME}", entries->d_name, ZBX_JSON_TYPE_STRING);
                                        zbx_json_addstring(&j, "{#DEVTYPE}", 1 == devtype_found ? tmp + offset : "",
                                                        ZBX_JSON_TYPE_STRING);
                                        zbx_json_close(&j);
                                }

/dev files are checked by lstat() if it is symlink after first stat() and symlinks are not included in item response.
So I think first stat() do not need to follow symlinks.

I tried to make a patch and confirmed that it fixes the SELinux errors.
Please check it.

--- src/libs/zbxsysinfo/linux/diskio.c.bak	2023-09-22 15:54:43.132604806 +0900
+++ src/libs/zbxsysinfo/linux/diskio.c	2023-09-22 15:55:01.896519054 +0900
@@ -324,7 +324,7 @@
 		{
 			zbx_snprintf(tmp, sizeof(tmp), ZBX_DEV_PFX "%s", entries->d_name);
 
-			if (0 == zbx_stat(tmp, &stat_buf) && 0 != S_ISBLK(stat_buf.st_mode))
+			if (0 == lstat(tmp, &stat_buf) && 0 != S_ISBLK(stat_buf.st_mode))
 			{
 				int	offset = 0;
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