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Dan Walsh's Blog

Got SELinux?

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DAC_READ_SEARCH/DAC_OVERRIDE - common SELinux issue that people handle badly.
MYTH: ROOT is all powerful.

Root is all powerful is a common misconception by administrators and users of Unix/Linux systems.  Many years ago the Linux kernel tried to break the power of root down into a series of capabilities.  Originally there were 32 capabilities, but recently that grew to 64.  Capabilities allowed programmers to code application in such a way that the ping command can create rawip-sockets or httpd can bind to a port less then 1024 and then drop all of the other capabilities of root.

SELinux also controls the access to all of the capabilities for a process.    A common bugzilla is for a process requiring the DAC_READ_SEARCH or DAC_OVERRIDE capability.  DAC stands for Discretionary Access Control.  DAC Means standard Linux Ownership/permission flags.  Lets look at the power of the capabilities.

more /usr/include/linux/capability.h
/* Override all DAC access, including ACL execute access if
   [_POSIX_ACL] is defined. Excluding DAC access covered by

#define CAP_DAC_OVERRIDE     1

/* Overrides all DAC restrictions regarding read and search on files
   and directories, including ACL restrictions if [_POSIX_ACL] is
   defined. Excluding DAC access covered by CAP_LINUX_IMMUTABLE. */


If you read the descriptions these basically say a process running as UID=0 with DAC_READ_SEARCH can read any file on the system, even if the permission flags would not allow a root process to read it.  Similarly DAC_OVERRIDE, means the process can ignore all permission/ownerships of all files on the system.  Usually when I see AVC messages that require this access, I take a look at the process UID, and almost always I see the process is running as uid=0.

What users often do when they see this access denial is to add the permissions, which is almost always wrong.  These AVC's indicate to me that you have permission flags to tight on a file. Usually a config file.

Imagine the httpd process needs to read /var/lib/myweb/content which is owned by the httpd user and has permissions 600 set on it.

 ls -l /var/lib/myweb/content
-rw-------. 1 apache apache 0 May 12 13:50 /var/lib/myweb/content

If for some reason the httpd process needs to read this file while it is running as UID=0, the system will deny access and generate a DAC_* AVC.  A simple fix would be to change the permission on the file to be 644.

# chmod 644 /var/lib/myweb/content
# ls -l /var/lib/myweb/content
-rw-r--r--. 1 apache apache 0 May 12 13:50 /var/lib/myweb/content

Which would now allow a root process to read the file using the "other" permissions.

Another option would be to change the group to root and change the permissions to 640.

# chmod 640 /var/lib/myweb/content
# chgrp root /var/lib/myweb/content
# ls -l /var/lib/myweb/content
-rw-r-----. 1 apache root 0 May 12 13:50 /var/lib/myweb/content

Now root can read the file based on the group permissions. but others can not read it.  You could also use ACLs to provide access.    Bottom line this is probably not an SELinux issue, and not something you want to loosen SELinux security around.

One problem with SELinux system here is the capabilities AVC message does not tell you which object on the file system blocked the access by default.  The reason for this is performance as I explained in previous blog.

Why doen't SELinux give me the full path in an error message?

If you turn on full auditing and regenerate the AVC, you will get the path of the object with the bad DAC Controls, as I explained in the blog.

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