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security: Minor improvements to no_new_privs documentation

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The documentation didn't actually mention how to enable no_new_privs.
This also adds a note about possible interactions between
no_new_privs and LSMs (i.e. why teaching systemd to set no_new_privs
is not necessarily a good idea), and it references the new docs
from include/linux/prctl.h.

Suggested-by: Rob Landley <rob@landley.net>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>
This commit is contained in:
Andy Lutomirski 2012-07-05 11:23:24 -07:00 committed by James Morris
parent 26c439d400
commit c540521bba
2 changed files with 9 additions and 0 deletions
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7
Documentation/prctl/no_new_privs.txt
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@ -25,6 +25,13 @@ bits will no longer change the uid or gid; file capabilities will not
add to the permitted set, and LSMs will not relax constraints after
execve.
To set no_new_privs, use prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0).
Be careful, though: LSMs might also not tighten constraints on exec
in no_new_privs mode. (This means that setting up a general-purpose
service launcher to set no_new_privs before execing daemons may
interfere with LSM-based sandboxing.)
Note that no_new_privs does not prevent privilege changes that do not
involve execve. An appropriately privileged task can still call
setuid(2) and receive SCM_RIGHTS datagrams.

2
include/linux/prctl.h
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@ -141,6 +141,8 @@
* Changing LSM security domain is considered a new privilege. So, for example,
* asking selinux for a specific new context (e.g. with runcon) will result
* in execve returning -EPERM.
*
* See Documentation/prctl/no_new_privs.txt for more details.
*/
#define PR_SET_NO_NEW_PRIVS 38
#define PR_GET_NO_NEW_PRIVS 39