About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / prctl / no_new_privs.txt




Custom Search

Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	The execve system call can grant a newly-started program privileges that
2	its parent did not have.  The most obvious examples are setuid/setgid
3	programs and file capabilities.  To prevent the parent program from
4	gaining these privileges as well, the kernel and user code must be
5	careful to prevent the parent from doing anything that could subvert the
6	child.  For example:
7	
8	 - The dynamic loader handles LD_* environment variables differently if
9	   a program is setuid.
10	
11	 - chroot is disallowed to unprivileged processes, since it would allow
12	   /etc/passwd to be replaced from the point of view of a process that
13	   inherited chroot.
14	
15	 - The exec code has special handling for ptrace.
16	
17	These are all ad-hoc fixes.  The no_new_privs bit (since Linux 3.5) is a
18	new, generic mechanism to make it safe for a process to modify its
19	execution environment in a manner that persists across execve.  Any task
20	can set no_new_privs.  Once the bit is set, it is inherited across fork,
21	clone, and execve and cannot be unset.  With no_new_privs set, execve
22	promises not to grant the privilege to do anything that could not have
23	been done without the execve call.  For example, the setuid and setgid
24	bits will no longer change the uid or gid; file capabilities will not
25	add to the permitted set, and LSMs will not relax constraints after
26	execve.
27	
28	To set no_new_privs, use prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0).
29	
30	Be careful, though: LSMs might also not tighten constraints on exec
31	in no_new_privs mode.  (This means that setting up a general-purpose
32	service launcher to set no_new_privs before execing daemons may
33	interfere with LSM-based sandboxing.)
34	
35	Note that no_new_privs does not prevent privilege changes that do not
36	involve execve.  An appropriately privileged task can still call
37	setuid(2) and receive SCM_RIGHTS datagrams.
38	
39	There are two main use cases for no_new_privs so far:
40	
41	 - Filters installed for the seccomp mode 2 sandbox persist across
42	   execve and can change the behavior of newly-executed programs.
43	   Unprivileged users are therefore only allowed to install such filters
44	   if no_new_privs is set.
45	
46	 - By itself, no_new_privs can be used to reduce the attack surface
47	   available to an unprivileged user.  If everything running with a
48	   given uid has no_new_privs set, then that uid will be unable to
49	   escalate its privileges by directly attacking setuid, setgid, and
50	   fcap-using binaries; it will need to compromise something without the
51	   no_new_privs bit set first.
52	
53	In the future, other potentially dangerous kernel features could become
54	available to unprivileged tasks if no_new_privs is set.  In principle,
55	several options to unshare(2) and clone(2) would be safe when
56	no_new_privs is set, and no_new_privs + chroot is considerable less
57	dangerous than chroot by itself.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.