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Subject[PATCH] per-process securebits
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Hash: SHA1
Here is the patch to add per-process securebits.

Its all code that lives inside the capability LSM and the new securebits
implementation is only active if CONFIG_SECURITY_FILE_CAPABILITIES is
enabled (it doesn't make much sense to support this feature without
filesystem capabilities).

The patch assumes the CAP_SETPCAP fix of last week, but is otherwise on
top of 2.6.24-rc8-mm1.

Cheers

Andrew
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From: Andrew G. Morgan <morgan@kernel.org>
Date: Thu, 31 Jan 2008 23:08:53 -0800
Subject: [PATCH] Implement per-process securebits

[This patch represents a no-op unless CONFIG_SECURITY_FILE_CAPABILITIES
is enabled at configure time.]

Filesystem capability support makes it possible to do away with
(set)uid-0 based privilege and use capabilities instead. That is, with
filesystem support for capabilities but without this present patch,
it is (conceptually) possible to manage a system with capabilities
alone and never need to obtain privilege via (set)uid-0.

Of course, conceptually isn't quite the same as currently possible
since few user applications, certainly not enough to run a viable
system, are currently prepared to leverage capabilities to exercise
privilege. Further, many applications exist that may never get
upgraded in this way, and the kernel will continue to want to support
their setuid-0 base privilege needs.

Where pure-capability applications evolve and replace setuid-0
binaries, it is desirable that there be a mechanisms by which they
can contain their privilege. In addition to leveraging the per-process
bounding and inheritable sets, this should include suppressing the
privilege of the uid-0 superuser from the process' tree of children.

The feature added by this patch can be leveraged to suppress the
privilege associated with (set)uid-0. This suppression requires
CAP_SETPCAP to initiate, and only immediately affects the 'current'
process (it is inherited through fork()/exec()). This
reimplementation differs significantly from the historical support for
securebits which was system-wide, unwieldy and which has ultimately
withered to a dead relic in the source of the modern kernel.

With this patch applied a process, that is capable(CAP_SETPCAP), can
now drop all legacy privilege (through uid=0) for itself and all
subsequently fork()'d/exec()'d children with:

prctl(PR_SET_SECUREBITS, 0x2f);

Applying the following patch to progs/capsh.c from libcap-2.05
adds support for this new prctl interface to capsh.c:

http://www.kernel.org/pub/linux/libs/security/linux-privs/libcap2/support-for-prctl-based-securebits.patch

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
---
include/linux/capability.h | 3 +-
include/linux/init_task.h | 3 +-
include/linux/prctl.h | 9 +++-
include/linux/sched.h | 3 +-
include/linux/securebits.h | 25 ++++++++---
include/linux/security.h | 14 +++---
kernel/sys.c | 25 +----------
security/capability.c | 1 +
security/commoncap.c | 103 ++++++++++++++++++++++++++++++++++++++++----
security/dummy.c | 2 +-
security/security.c | 4 +-
security/selinux/hooks.c | 5 +-
12 files changed, 139 insertions(+), 58 deletions(-)
diff --git a/include/linux/capability.h b/include/linux/capability.h
index 7d50ff6..eaab759 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -155,6 +155,7 @@ typedef struct kernel_cap_struct {
* Add any capability from current's capability bounding set
* to the current process' inheritable set
* Allow taking bits out of capability bounding set
+ * Allow modification of the securebits for a process
*/

#define CAP_SETPCAP 8
@@ -490,8 +491,6 @@ extern const kernel_cap_t __cap_init_eff_set;
int capable(int cap);
int __capable(struct task_struct *t, int cap);

-extern long cap_prctl_drop(unsigned long cap);
-
#endif /* __KERNEL__ */

#endif /* !_LINUX_CAPABILITY_H */
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index b0fa0f2..81f5582 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -9,6 +9,7 @@
#include <linux/ipc.h>
#include <linux/pid_namespace.h>
#include <linux/user_namespace.h>
+#include <linux/securebits.h>
#include <net/net_namespace.h>

#define INIT_FDTABLE \
@@ -170,7 +171,7 @@ extern struct group_info init_groups;
.cap_inheritable = CAP_INIT_INH_SET, \
.cap_permitted = CAP_FULL_SET, \
.cap_bset = CAP_INIT_BSET, \
- .keep_capabilities = 0, \
+ .securebits = SECUREBITS_DEFAULT, \
.user = INIT_USER, \
.comm = "swapper", \
.thread = INIT_THREAD, \
diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index 3800639..b6c15cc 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -16,7 +16,8 @@
# 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 */
+/* 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

@@ -63,8 +64,12 @@
#define PR_GET_SECCOMP 21
#define PR_SET_SECCOMP 22

-/* Get/set the capability bounding set */
+/* Get/set the capability bounding set (as per security/commoncap.c) */
#define PR_CAPBSET_READ 23
#define PR_CAPBSET_DROP 24

+/* Get/set securebits (as per security/commoncap.c) */
+#define PR_GET_SECUREBITS 25
+#define PR_SET_SECUREBITS 26
+
#endif /* _LINUX_PRCTL_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 198659b..063f575 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -68,7 +68,6 @@ struct sched_param {
#include <linux/smp.h>
#include <linux/sem.h>
#include <linux/signal.h>
-#include <linux/securebits.h>
#include <linux/fs_struct.h>
#include <linux/compiler.h>
#include <linux/completion.h>
@@ -1095,7 +1094,7 @@ struct task_struct {
gid_t gid,egid,sgid,fsgid;
struct group_info *group_info;
kernel_cap_t cap_effective, cap_inheritable, cap_permitted, cap_bset;
- unsigned keep_capabilities:1;
+ unsigned securebits;
struct user_struct *user;
#ifdef CONFIG_KEYS
struct key *request_key_auth; /* assumed request_key authority */
diff --git a/include/linux/securebits.h b/include/linux/securebits.h
index 5b06178..c1f19db 100644
--- a/include/linux/securebits.h
+++ b/include/linux/securebits.h
@@ -3,28 +3,39 @@

#define SECUREBITS_DEFAULT 0x00000000

-extern unsigned securebits;
-
/* When set UID 0 has no special privileges. When unset, we support
inheritance of root-permissions and suid-root executable under
compatibility mode. We raise the effective and inheritable bitmasks
*of the executable file* if the effective uid of the new process is
0. If the real uid is 0, we raise the inheritable bitmask of the
executable file. */
-#define SECURE_NOROOT 0
+#define SECURE_NOROOT 0
+#define SECURE_NOROOT_LOCKED 1 /* make bit-0 immutable */

/* When set, setuid to/from uid 0 does not trigger capability-"fixes"
to be compatible with old programs relying on set*uid to loose
privileges. When unset, setuid doesn't change privileges. */
-#define SECURE_NO_SETUID_FIXUP 2
+#define SECURE_NO_SETUID_FIXUP 2
+#define SECURE_NO_SETUID_FIXUP_LOCKED 3 /* make bit-2 immutable */
+
+/* When set, a process can retain its capabilities even after
+ transitioning to a non-root user (the set-uid fixup suppressed by
+ bit 2). Bit-4 is cleared when a process calls exec(); setting both
+ bit 4 and 5 will create a barrier through exec that no exec()'d
+ child can use this feature again. */
+#define SECURE_KEEP_CAPS 4
+#define SECURE_KEEP_CAPS_LOCKED 5 /* make bit-4 immutable */

/* Each securesetting is implemented using two bits. One bit specify
whether the setting is on or off. The other bit specify whether the
setting is fixed or not. A setting which is fixed cannot be changed
from user-level. */
+#define issecure_mask(X) (1 << (X))
+#define issecure(X) (issecure_mask(X) & current->securebits)

-#define issecure(X) ( (1 << (X+1)) & SECUREBITS_DEFAULT ? \
- (1 << (X)) & SECUREBITS_DEFAULT : \
- (1 << (X)) & securebits )
+#define SECURE_ALL_BITS (issecure_mask(SECURE_NOROOT) | \
+ issecure_mask(SECURE_NO_SETUID_FIXUP) | \
+ issecure_mask(SECURE_KEEP_CAPS))
+#define SECURE_ALL_LOCKS (SECURE_ALL_BITS << 1)

#endif /* !_LINUX_SECUREBITS_H */
diff --git a/include/linux/security.h b/include/linux/security.h
index 7b3e2b6..c550079 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -40,8 +40,6 @@
#define ROOTCONTEXT_MNT 0x04
#define DEFCONTEXT_MNT 0x08

-extern unsigned securebits;
-
struct ctl_table;

/*
@@ -64,6 +62,8 @@ extern int cap_inode_killpriv(struct dentry *dentry);
extern int cap_task_post_setuid (uid_t old_ruid, uid_t old_euid, uid_t old_suid, int flags);
extern void cap_task_reparent_to_init (struct task_struct *p);
extern int cap_task_kill(struct task_struct *p, struct siginfo *info, int sig, u32 secid);
+extern int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
+ unsigned long arg4, unsigned long arg5, int *rc_p);
extern int cap_task_setscheduler (struct task_struct *p, int policy, struct sched_param *lp);
extern int cap_task_setioprio (struct task_struct *p, int ioprio);
extern int cap_task_setnice (struct task_struct *p, int nice);
@@ -684,7 +684,9 @@ struct request_sock;
* @arg3 contains a argument.
* @arg4 contains a argument.
* @arg5 contains a argument.
- * Return 0 if permission is granted.
+ * @rc_p contains a pointer to communicate back the forced return code
+ * Return 0 if permission is granted, and non-zero if the security module
+ * has taken responsibility (setting *rc_p) for the prctl call.
* @task_reparent_to_init:
* Set the security attributes in @p->security for a kernel thread that
* is being reparented to the init task.
@@ -1346,7 +1348,7 @@ struct security_operations {
int (*task_wait) (struct task_struct * p);
int (*task_prctl) (int option, unsigned long arg2,
unsigned long arg3, unsigned long arg4,
- unsigned long arg5);
+ unsigned long arg5, int *rc_p);
void (*task_reparent_to_init) (struct task_struct * p);
void (*task_to_inode)(struct task_struct *p, struct inode *inode);

@@ -1600,7 +1602,7 @@ int security_task_kill(struct task_struct *p, struct siginfo *info,
int sig, u32 secid);
int security_task_wait(struct task_struct *p);
int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
- unsigned long arg4, unsigned long arg5);
+ unsigned long arg4, unsigned long arg5, int *rc_p);
void security_task_reparent_to_init(struct task_struct *p);
void security_task_to_inode(struct task_struct *p, struct inode *inode);
int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
@@ -2149,7 +2151,7 @@ static inline int security_task_wait (struct task_struct *p)
static inline int security_task_prctl (int option, unsigned long arg2,
unsigned long arg3,
unsigned long arg4,
- unsigned long arg5)
+ unsigned long arg5, int *rc_p)
{
return 0;
}
diff --git a/kernel/sys.c b/kernel/sys.c
index 5a61f80..d350d09 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1631,8 +1631,7 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
{
long error;

- error = security_task_prctl(option, arg2, arg3, arg4, arg5);
- if (error)
+ if (security_task_prctl(option, arg2, arg3, arg4, arg5, &error))
return error;

switch (option) {
@@ -1685,17 +1684,6 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
error = -EINVAL;
break;

- case PR_GET_KEEPCAPS:
- if (current->keep_capabilities)
- error = 1;
- break;
- case PR_SET_KEEPCAPS:
- if (arg2 != 0 && arg2 != 1) {
- error = -EINVAL;
- break;
- }
- current->keep_capabilities = arg2;
- break;
case PR_SET_NAME: {
struct task_struct *me = current;
unsigned char ncomm[sizeof(me->comm)];
@@ -1730,17 +1718,6 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
error = prctl_set_seccomp(arg2);
break;

- case PR_CAPBSET_READ:
- if (!cap_valid(arg2))
- return -EINVAL;
- return !!cap_raised(current->cap_bset, arg2);
- case PR_CAPBSET_DROP:
-#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
- return cap_prctl_drop(arg2);
-#else
- return -EINVAL;
-#endif
-
default:
error = -EINVAL;
break;
diff --git a/security/capability.c b/security/capability.c
index 9e99f36..8340655 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -45,6 +45,7 @@ static struct security_operations capability_ops = {
.task_setioprio = cap_task_setioprio,
.task_setnice = cap_task_setnice,
.task_post_setuid = cap_task_post_setuid,
+ .task_prctl = cap_task_prctl,
.task_reparent_to_init = cap_task_reparent_to_init,

.syslog = cap_syslog,
diff --git a/security/commoncap.c b/security/commoncap.c
index 5aba826..9b87182 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -24,11 +24,8 @@
#include <linux/hugetlb.h>
#include <linux/mount.h>
#include <linux/sched.h>
-
-/* Global security state */
-
-unsigned securebits = SECUREBITS_DEFAULT; /* systemwide security settings */
-EXPORT_SYMBOL(securebits);
+#include <linux/prctl.h>
+#include <linux/securebits.h>

int cap_netlink_send(struct sock *sk, struct sk_buff *skb)
{
@@ -368,7 +365,7 @@ void cap_bprm_apply_creds (struct linux_binprm *bprm, int unsafe)

/* AUD: Audit candidate if current->cap_effective is set */

- current->keep_capabilities = 0;
+ current->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
}

int cap_bprm_secureexec (struct linux_binprm *bprm)
@@ -448,7 +445,7 @@ static inline void cap_emulate_setxuid (int old_ruid, int old_euid,
{
if ((old_ruid == 0 || old_euid == 0 || old_suid == 0) &&
(current->uid != 0 && current->euid != 0 && current->suid != 0) &&
- !current->keep_capabilities) {
+ !issecure(SECURE_KEEP_CAPS)) {
cap_clear (current->cap_permitted);
cap_clear (current->cap_effective);
}
@@ -582,7 +579,7 @@ int cap_task_kill(struct task_struct *p, struct siginfo *info,
* this task could get inconsistent info. There can be no
* racing writer bc a task can only change its own caps.
*/
-long cap_prctl_drop(unsigned long cap)
+static long cap_prctl_drop(unsigned long cap)
{
if (!capable(CAP_SETPCAP))
return -EPERM;
@@ -591,6 +588,7 @@ long cap_prctl_drop(unsigned long cap)
cap_lower(current->cap_bset, cap);
return 0;
}
+
#else
int cap_task_setscheduler (struct task_struct *p, int policy,
struct sched_param *lp)
@@ -612,12 +610,99 @@ int cap_task_kill(struct task_struct *p, struct siginfo *info,
}
#endif

+int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
+ unsigned long arg4, unsigned long arg5, int *rc_p)
+{
+ int error = 0;
+
+ switch (option) {
+ case PR_CAPBSET_READ:
+ if (!cap_valid(arg2))
+ error = -EINVAL;
+ else
+ error = !!cap_raised(current->cap_bset, arg2);
+ break;
+#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
+ case PR_CAPBSET_DROP:
+ error = cap_prctl_drop(arg2);
+ break;
+
+ /*
+ * The next four prctl's remain to assist with transitioning a
+ * system from legacy UID=0 based privilege (when filesystem
+ * capabilities are not in use) to a system using filesystem
+ * capabilities only - as the POSIX.1e draft intended.
+ *
+ * Note:
+ *
+ * PR_SET_SECUREBITS =
+ * issecure_mask(SECURE_KEEP_CAPS_LOCKED)
+ * | issecure_mask(SECURE_NOROOT)
+ * | issecure_mask(SECURE_NOROOT_LOCKED)
+ * | issecure_mask(SECURE_NO_SETUID_FIXUP)
+ * | issecure_mask(SECURE_NO_SETUID_FIXUP_LOCKED)
+ *
+ * will ensure that the current process and all of its
+ * children will be locked into a pure
+ * capability-based-privilege environment.
+ */
+ case PR_SET_SECUREBITS:
+ if ((((current->securebits & SECURE_ALL_LOCKS) >> 1)
+ & (current->securebits ^ arg2)) /*[1]*/
+ || ((current->securebits & SECURE_ALL_LOCKS
+ & ~arg2)) /*[2]*/
+ || (arg2 & ~(SECURE_ALL_LOCKS | SECURE_ALL_BITS)) /*[3]*/
+ || (cap_capable(current, CAP_SETPCAP) != 0)) { /*[4]*/
+ /*
+ * [1] no changing of bits that are locked
+ * [2] no unlocking of locks
+ * [3] no setting of unsupported bits
+ * [4] doing anything requires privilege (go read about
+ * the "sendmail capabilities bug")
+ */
+ error = -EPERM; /* cannot change a locked bit */
+ } else {
+ current->securebits = arg2;
+ }
+ break;
+ case PR_GET_SECUREBITS:
+ error = current->securebits;
+ break;
+
+#endif /* def CONFIG_SECURITY_FILE_CAPABILITIES */
+
+ case PR_GET_KEEPCAPS:
+ if (issecure(SECURE_KEEP_CAPS))
+ error = 1;
+ break;
+ case PR_SET_KEEPCAPS:
+ if (arg2 > 1) /* Note, we rely on arg2 being unsigned here */
+ error = -EINVAL;
+ else if (issecure(SECURE_KEEP_CAPS_LOCKED))
+ error = -EPERM;
+ else if (arg2)
+ current->securebits |= issecure_mask(SECURE_KEEP_CAPS);
+ else
+ current->securebits &=
+ ~issecure_mask(SECURE_KEEP_CAPS);
+ break;
+
+ default:
+ /* No functionality available - continue with default */
+ return 0;
+ }
+
+ /* Functionality provided */
+ *rc_p = error;
+ return 1;
+}
+
void cap_task_reparent_to_init (struct task_struct *p)
{
cap_set_init_eff(p->cap_effective);
cap_clear(p->cap_inheritable);
cap_set_full(p->cap_permitted);
- p->keep_capabilities = 0;
+ p->securebits = SECUREBITS_DEFAULT;
return;
}

diff --git a/security/dummy.c b/security/dummy.c
index 649326b..c9e6d9f 100644
--- a/security/dummy.c
+++ b/security/dummy.c
@@ -595,7 +595,7 @@ static int dummy_task_kill (struct task_struct *p, struct siginfo *info,
}

static int dummy_task_prctl (int option, unsigned long arg2, unsigned long arg3,
- unsigned long arg4, unsigned long arg5)
+ unsigned long arg4, unsigned long arg5, int *rc_p)
{
return 0;
}
diff --git a/security/security.c b/security/security.c
index b6c57a6..c3cc14e 100644
--- a/security/security.c
+++ b/security/security.c
@@ -683,9 +683,9 @@ int security_task_wait(struct task_struct *p)
}

int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
- unsigned long arg4, unsigned long arg5)
+ unsigned long arg4, unsigned long arg5, int *rc_p)
{
- return security_ops->task_prctl(option, arg2, arg3, arg4, arg5);
+ return security_ops->task_prctl(option, arg2, arg3, arg4, arg5, rc_p);
}

void security_task_reparent_to_init(struct task_struct *p)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index f1e3281..3c88858 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3209,12 +3209,13 @@ static int selinux_task_prctl(int option,
unsigned long arg2,
unsigned long arg3,
unsigned long arg4,
- unsigned long arg5)
+ unsigned long arg5,
+ int *rc_p)
{
/* The current prctl operations do not appear to require
any SELinux controls since they merely observe or modify
the state of the current process. */
- return 0;
+ return secondary_ops->task_prctl(option, arg2, arg3, arg4, arg5, rc_p);
}

static int selinux_task_wait(struct task_struct *p)
--
1.5.3.7
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