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linux-next/include/uapi/linux/prctl.h
Andy Lutomirski 58319057b7 capabilities: ambient capabilities 
Credit where credit is due: this idea comes from Christoph Lameter with
a lot of valuable input from Serge Hallyn.  This patch is heavily based
on Christoph's patch.

===== The status quo =====

On Linux, there are a number of capabilities defined by the kernel.  To
perform various privileged tasks, processes can wield capabilities that
they hold.

Each task has four capability masks: effective (pE), permitted (pP),
inheritable (pI), and a bounding set (X).  When the kernel checks for a
capability, it checks pE.  The other capability masks serve to modify
what capabilities can be in pE.

Any task can remove capabilities from pE, pP, or pI at any time.  If a
task has a capability in pP, it can add that capability to pE and/or pI.
If a task has CAP_SETPCAP, then it can add any capability to pI, and it
can remove capabilities from X.

Tasks are not the only things that can have capabilities; files can also
have capabilities.  A file can have no capabilty information at all [1].
If a file has capability information, then it has a permitted mask (fP)
and an inheritable mask (fI) as well as a single effective bit (fE) [2].
File capabilities modify the capabilities of tasks that execve(2) them.

A task that successfully calls execve has its capabilities modified for
the file ultimately being excecuted (i.e.  the binary itself if that
binary is ELF or for the interpreter if the binary is a script.) [3] In
the capability evolution rules, for each mask Z, pZ represents the old
value and pZ' represents the new value.  The rules are:

  pP' = (X & fP) | (pI & fI)
  pI' = pI
  pE' = (fE ? pP' : 0)
  X is unchanged

For setuid binaries, fP, fI, and fE are modified by a moderately
complicated set of rules that emulate POSIX behavior.  Similarly, if
euid == 0 or ruid == 0, then fP, fI, and fE are modified differently
(primary, fP and fI usually end up being the full set).  For nonroot
users executing binaries with neither setuid nor file caps, fI and fP
are empty and fE is false.

As an extra complication, if you execute a process as nonroot and fE is
set, then the "secure exec" rules are in effect: AT_SECURE gets set,
LD_PRELOAD doesn't work, etc.

This is rather messy.  We've learned that making any changes is
dangerous, though: if a new kernel version allows an unprivileged
program to change its security state in a way that persists cross
execution of a setuid program or a program with file caps, this
persistent state is surprisingly likely to allow setuid or file-capped
programs to be exploited for privilege escalation.

===== The problem =====

Capability inheritance is basically useless.

If you aren't root and you execute an ordinary binary, fI is zero, so
your capabilities have no effect whatsoever on pP'.  This means that you
can't usefully execute a helper process or a shell command with elevated
capabilities if you aren't root.

On current kernels, you can sort of work around this by setting fI to
the full set for most or all non-setuid executable files.  This causes
pP' = pI for nonroot, and inheritance works.  No one does this because
it's a PITA and it isn't even supported on most filesystems.

If you try this, you'll discover that every nonroot program ends up with
secure exec rules, breaking many things.

This is a problem that has bitten many people who have tried to use
capabilities for anything useful.

===== The proposed change =====

This patch adds a fifth capability mask called the ambient mask (pA).
pA does what most people expect pI to do.

pA obeys the invariant that no bit can ever be set in pA if it is not
set in both pP and pI.  Dropping a bit from pP or pI drops that bit from
pA.  This ensures that existing programs that try to drop capabilities
still do so, with a complication.  Because capability inheritance is so
broken, setting KEEPCAPS, using setresuid to switch to nonroot uids, and
then calling execve effectively drops capabilities.  Therefore,
setresuid from root to nonroot conditionally clears pA unless
SECBIT_NO_SETUID_FIXUP is set.  Processes that don't like this can
re-add bits to pA afterwards.

The capability evolution rules are changed:

  pA' = (file caps or setuid or setgid ? 0 : pA)
  pP' = (X & fP) | (pI & fI) | pA'
  pI' = pI
  pE' = (fE ? pP' : pA')
  X is unchanged

If you are nonroot but you have a capability, you can add it to pA.  If
you do so, your children get that capability in pA, pP, and pE.  For
example, you can set pA = CAP_NET_BIND_SERVICE, and your children can
automatically bind low-numbered ports.  Hallelujah!

Unprivileged users can create user namespaces, map themselves to a
nonzero uid, and create both privileged (relative to their namespace)
and unprivileged process trees.  This is currently more or less
impossible.  Hallelujah!

You cannot use pA to try to subvert a setuid, setgid, or file-capped
program: if you execute any such program, pA gets cleared and the
resulting evolution rules are unchanged by this patch.

Users with nonzero pA are unlikely to unintentionally leak that
capability.  If they run programs that try to drop privileges, dropping
privileges will still work.

It's worth noting that the degree of paranoia in this patch could
possibly be reduced without causing serious problems.  Specifically, if
we allowed pA to persist across executing non-pA-aware setuid binaries
and across setresuid, then, naively, the only capabilities that could
leak as a result would be the capabilities in pA, and any attacker
*already* has those capabilities.  This would make me nervous, though --
setuid binaries that tried to privilege-separate might fail to do so,
and putting CAP_DAC_READ_SEARCH or CAP_DAC_OVERRIDE into pA could have
unexpected side effects.  (Whether these unexpected side effects would
be exploitable is an open question.) I've therefore taken the more
paranoid route.  We can revisit this later.

An alternative would be to require PR_SET_NO_NEW_PRIVS before setting
ambient capabilities.  I think that this would be annoying and would
make granting otherwise unprivileged users minor ambient capabilities
(CAP_NET_BIND_SERVICE or CAP_NET_RAW for example) much less useful than
it is with this patch.

===== Footnotes =====

[1] Files that are missing the "security.capability" xattr or that have
unrecognized values for that xattr end up with has_cap set to false.
The code that does that appears to be complicated for no good reason.

[2] The libcap capability mask parsers and formatters are dangerously
misleading and the documentation is flat-out wrong.  fE is *not* a mask;
it's a single bit.  This has probably confused every single person who
has tried to use file capabilities.

[3] Linux very confusingly processes both the script and the interpreter
if applicable, for reasons that elude me.  The results from thinking
about a script's file capabilities and/or setuid bits are mostly
discarded.

Preliminary userspace code is here, but it needs updating:
https://git.kernel.org/cgit/linux/kernel/git/luto/util-linux-playground.git/commit/?h=cap_ambient&id=7f5afbd175d2

Here is a test program that can be used to verify the functionality
(from Christoph):

/*
 * Test program for the ambient capabilities. This program spawns a shell
 * that allows running processes with a defined set of capabilities.
 *
 * (C) 2015 Christoph Lameter <cl@linux.com>
 * Released under: GPL v3 or later.
 *
 *
 * Compile using:
 *
 *	gcc -o ambient_test ambient_test.o -lcap-ng
 *
 * This program must have the following capabilities to run properly:
 * Permissions for CAP_NET_RAW, CAP_NET_ADMIN, CAP_SYS_NICE
 *
 * A command to equip the binary with the right caps is:
 *
 *	setcap cap_net_raw,cap_net_admin,cap_sys_nice+p ambient_test
 *
 *
 * To get a shell with additional caps that can be inherited by other processes:
 *
 *	./ambient_test /bin/bash
 *
 *
 * Verifying that it works:
 *
 * From the bash spawed by ambient_test run
 *
 *	cat /proc/$$/status
 *
 * and have a look at the capabilities.
 */

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <cap-ng.h>
#include <sys/prctl.h>
#include <linux/capability.h>

/*
 * Definitions from the kernel header files. These are going to be removed
 * when the /usr/include files have these defined.
 */
#define PR_CAP_AMBIENT 47
#define PR_CAP_AMBIENT_IS_SET 1
#define PR_CAP_AMBIENT_RAISE 2
#define PR_CAP_AMBIENT_LOWER 3
#define PR_CAP_AMBIENT_CLEAR_ALL 4

static void set_ambient_cap(int cap)
{
	int rc;

	capng_get_caps_process();
	rc = capng_update(CAPNG_ADD, CAPNG_INHERITABLE, cap);
	if (rc) {
		printf("Cannot add inheritable cap\n");
		exit(2);
	}
	capng_apply(CAPNG_SELECT_CAPS);

	/* Note the two 0s at the end. Kernel checks for these */
	if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap, 0, 0)) {
		perror("Cannot set cap");
		exit(1);
	}
}

int main(int argc, char **argv)
{
	int rc;

	set_ambient_cap(CAP_NET_RAW);
	set_ambient_cap(CAP_NET_ADMIN);
	set_ambient_cap(CAP_SYS_NICE);

	printf("Ambient_test forking shell\n");
	if (execv(argv[1], argv + 1))
		perror("Cannot exec");

	return 0;
}

Signed-off-by: Christoph Lameter <cl@linux.com> # Original author
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Aaron Jones <aaronmdjones@gmail.com>
Cc: Ted Ts'o <tytso@mit.edu>
Cc: Andrew G. Morgan <morgan@kernel.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: Markku Savela <msa@moth.iki.fi>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2015-09-04 16:54:41 -07:00

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#ifndef _LINUX_PRCTL_H
#define _LINUX_PRCTL_H
#include <linux/types.h>
/* Values to pass as first argument to prctl() */
#define PR_SET_PDEATHSIG 1 /* Second arg is a signal */
#define PR_GET_PDEATHSIG 2 /* Second arg is a ptr to return the signal */
/* Get/set current->mm->dumpable */
#define PR_GET_DUMPABLE 3
#define PR_SET_DUMPABLE 4
/* Get/set unaligned access control bits (if meaningful) */
#define PR_GET_UNALIGN 5
#define PR_SET_UNALIGN 6
# define PR_UNALIGN_NOPRINT 1 /* silently fix up unaligned user accesses */
# define PR_UNALIGN_SIGBUS 2 /* generate SIGBUS on unaligned user access */
/* Get/set whether or not to drop capabilities on setuid() away from
* uid 0 (as per security/commoncap.c) */
#define PR_GET_KEEPCAPS 7
#define PR_SET_KEEPCAPS 8
/* Get/set floating-point emulation control bits (if meaningful) */
#define PR_GET_FPEMU 9
#define PR_SET_FPEMU 10
# define PR_FPEMU_NOPRINT 1 /* silently emulate fp operations accesses */
# define PR_FPEMU_SIGFPE 2 /* don't emulate fp operations, send SIGFPE instead */
/* Get/set floating-point exception mode (if meaningful) */
#define PR_GET_FPEXC 11
#define PR_SET_FPEXC 12
# define PR_FP_EXC_SW_ENABLE 0x80 /* Use FPEXC for FP exception enables */
# define PR_FP_EXC_DIV 0x010000 /* floating point divide by zero */
# define PR_FP_EXC_OVF 0x020000 /* floating point overflow */
# define PR_FP_EXC_UND 0x040000 /* floating point underflow */
# define PR_FP_EXC_RES 0x080000 /* floating point inexact result */
# define PR_FP_EXC_INV 0x100000 /* floating point invalid operation */
# define PR_FP_EXC_DISABLED 0 /* FP exceptions disabled */
# define PR_FP_EXC_NONRECOV 1 /* async non-recoverable exc. mode */
# define PR_FP_EXC_ASYNC 2 /* async recoverable exception mode */
# define PR_FP_EXC_PRECISE 3 /* precise exception mode */
/* Get/set whether we use statistical process timing or accurate timestamp
* based process timing */
#define PR_GET_TIMING 13
#define PR_SET_TIMING 14
# define PR_TIMING_STATISTICAL 0 /* Normal, traditional,
statistical process timing */
# define PR_TIMING_TIMESTAMP 1 /* Accurate timestamp based
process timing */
#define PR_SET_NAME 15 /* Set process name */
#define PR_GET_NAME 16 /* Get process name */
/* Get/set process endian */
#define PR_GET_ENDIAN 19
#define PR_SET_ENDIAN 20
# define PR_ENDIAN_BIG 0
# define PR_ENDIAN_LITTLE 1 /* True little endian mode */
# define PR_ENDIAN_PPC_LITTLE 2 /* "PowerPC" pseudo little endian */
/* Get/set process seccomp mode */
#define PR_GET_SECCOMP 21
#define PR_SET_SECCOMP 22
/* Get/set the capability bounding set (as per security/commoncap.c) */
#define PR_CAPBSET_READ 23
#define PR_CAPBSET_DROP 24
/* Get/set the process' ability to use the timestamp counter instruction */
#define PR_GET_TSC 25
#define PR_SET_TSC 26
# define PR_TSC_ENABLE 1 /* allow the use of the timestamp counter */
# define PR_TSC_SIGSEGV 2 /* throw a SIGSEGV instead of reading the TSC */
/* Get/set securebits (as per security/commoncap.c) */
#define PR_GET_SECUREBITS 27
#define PR_SET_SECUREBITS 28
/*
* Get/set the timerslack as used by poll/select/nanosleep
* A value of 0 means "use default"
*/
#define PR_SET_TIMERSLACK 29
#define PR_GET_TIMERSLACK 30
#define PR_TASK_PERF_EVENTS_DISABLE 31
#define PR_TASK_PERF_EVENTS_ENABLE 32
/*
* Set early/late kill mode for hwpoison memory corruption.
* This influences when the process gets killed on a memory corruption.
*/
#define PR_MCE_KILL 33
# define PR_MCE_KILL_CLEAR 0
# define PR_MCE_KILL_SET 1
# define PR_MCE_KILL_LATE 0
# define PR_MCE_KILL_EARLY 1
# define PR_MCE_KILL_DEFAULT 2
#define PR_MCE_KILL_GET 34
/*
* Tune up process memory map specifics.
*/
#define PR_SET_MM 35
# define PR_SET_MM_START_CODE 1
# define PR_SET_MM_END_CODE 2
# define PR_SET_MM_START_DATA 3
# define PR_SET_MM_END_DATA 4
# define PR_SET_MM_START_STACK 5
# define PR_SET_MM_START_BRK 6
# define PR_SET_MM_BRK 7
# define PR_SET_MM_ARG_START 8
# define PR_SET_MM_ARG_END 9
# define PR_SET_MM_ENV_START 10
# define PR_SET_MM_ENV_END 11
# define PR_SET_MM_AUXV 12
# define PR_SET_MM_EXE_FILE 13
# define PR_SET_MM_MAP 14
# define PR_SET_MM_MAP_SIZE 15
/*
* This structure provides new memory descriptor
* map which mostly modifies /proc/pid/stat[m]
* output for a task. This mostly done in a
* sake of checkpoint/restore functionality.
*/
struct prctl_mm_map {
__u64 start_code; /* code section bounds */
__u64 end_code;
__u64 start_data; /* data section bounds */
__u64 end_data;
__u64 start_brk; /* heap for brk() syscall */
__u64 brk;
__u64 start_stack; /* stack starts at */
__u64 arg_start; /* command line arguments bounds */
__u64 arg_end;
__u64 env_start; /* environment variables bounds */
__u64 env_end;
__u64 *auxv; /* auxiliary vector */
__u32 auxv_size; /* vector size */
__u32 exe_fd; /* /proc/$pid/exe link file */
};
/*
* Set specific pid that is allowed to ptrace the current task.
* A value of 0 mean "no process".
*/
#define PR_SET_PTRACER 0x59616d61
# define PR_SET_PTRACER_ANY ((unsigned long)-1)
#define PR_SET_CHILD_SUBREAPER 36
#define PR_GET_CHILD_SUBREAPER 37
/*
* If no_new_privs is set, then operations that grant new privileges (i.e.
* execve) will either fail or not grant them. This affects suid/sgid,
* file capabilities, and LSMs.
*
* Operations that merely manipulate or drop existing privileges (setresuid,
* capset, etc.) will still work. Drop those privileges if you want them gone.
*
* 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
#define PR_GET_TID_ADDRESS 40
#define PR_SET_THP_DISABLE 41
#define PR_GET_THP_DISABLE 42
/*
* Tell the kernel to start/stop helping userspace manage bounds tables.
*/
#define PR_MPX_ENABLE_MANAGEMENT 43
#define PR_MPX_DISABLE_MANAGEMENT 44
#define PR_SET_FP_MODE 45
#define PR_GET_FP_MODE 46
# define PR_FP_MODE_FR (1 << 0) /* 64b FP registers */
# define PR_FP_MODE_FRE (1 << 1) /* 32b compatibility */
/* Control the ambient capability set */
#define PR_CAP_AMBIENT 47
# define PR_CAP_AMBIENT_IS_SET 1
# define PR_CAP_AMBIENT_RAISE 2
# define PR_CAP_AMBIENT_LOWER 3
# define PR_CAP_AMBIENT_CLEAR_ALL 4
#endif /* _LINUX_PRCTL_H */
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