Message-ID: <20071003223028.GA2544@sergelap.austin.ibm.com>
Date:	Wed, 3 Oct 2007 17:30:28 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	lkml <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Andrew Morgan <morgan@...nel.org>,
	linux-security-module@...r.kernel.org
Subject: [PATCH 2/2 -mm] capabilities: introduce per-process capability
	bounding set (v4)

>From d93ecb90d82f9e2b7f48c74f5e6ed97cac3683c7 Mon Sep 17 00:00:00 2001
From: Serge Hallyn <serue@...ibm.com>
Date: Fri, 28 Sep 2007 10:33:56 -0500
Subject: [PATCH 2/2 -mm] capabilities: introduce per-process capability bounding set (v4)

The capability bounding set is a set beyond which capabilities
cannot grow.  Currently cap_bset is per-system.  It can be
manipulated through sysctl, but only init can add capabilities.
Root can remove capabilities.  By default it includes all caps
except CAP_SETPCAP.

This patch makes the bounding set per-process.  It is inherited
at fork from parent.  Noone can add elements.  CAP_SYS_ADMIN is
required to remove them.  The reason for that is to stop an
unprivileged user from removing key capabilities, then running
a setuid root binary (or one with file capabilities) which
assumes it got all, not some, of the capabilities it needs
and fails unsafely partway through.  Perhaps a new capability
should be introduced rather than using CAP_SYS_ADMIN.  Or,
since I don't have a *concrete* example of an exploit, perhaps
any user should be able to drop capabilities from his capbset.

One example use of this is to start a safer container.  For
instance, until device namespaces or per-container device
whitelists are introduced, it is probably wise to take CAP_MKNOD
away from a container.

The following hacky test program will get and set the bounding
 set.  For instance

	./bset get
		(lists capabilities in bset)
	./bset strset cap_sys_admin
		(starts shell with new bset)
		(use capset, setuid binary, or binary with
		file capabilities to try to increase caps)

===========================================================
bset.c:
===========================================================

unsigned long newval;
int cmd_getbcap;

char *captable[] = {
	"cap_dac_override",
	"cap_dac_read_search",
	"cap_fowner",
	"cap_fsetid",
	"cap_kill",
	"cap_setgid",
	"cap_setuid",
	"cap_setpcap",
	"cap_linux_immutable",
	"cap_net_bind_service",
	"cap_net_broadcast",
	"cap_net_admin",
	"cap_net_raw",
	"cap_ipc_lock",
	"cap_ipc_owner",
	"cap_sys_module",
	"cap_sys_rawio",
	"cap_sys_chroot",
	"cap_sys_ptrace",
	"cap_sys_pacct",
	"cap_sys_admin",
	"cap_sys_boot",
	"cap_sys_nice",
	"cap_sys_resource",
	"cap_sys_time",
	"cap_sys_tty_config",
	"cap_mknod",
	"cap_lease",
	"cap_audit_write",
	"cap_audit_control",
	"cap_setfcap"};

char *inttocap(unsigned long v)
{
	char *str = NULL;
	int i;

	str = malloc(1);
	str[0] = '\0';
	for (i=0; i<31; i++) {
		if (v & (1 << (i+1))) {
			char *tmp = captable[i];
			str = realloc(str, strlen(str)+2+strlen(tmp));
			sprintf(str+strlen(str), ",%s", tmp);
		}
	}
	return str;
}

int getbcap(void)
{
	unsigned long bcap;
	int ret;
	unsigned int ver;

	ret = prctl(PR_GET_CAPBSET, &ver, &bcap);
	if (ret == -1)
		perror("prctl");
	if (ver != _LINUX_CAPABILITY_VERSION)
		printf("wrong capability version: %lu not %lu\n",
					ver, _LINUX_CAPABILITY_VERSION);
	printf("prctl get_bcap returned %lu (ret %d)\n", bcap, ret);
	printf("that is %s\n", inttocap(bcap));
	return ret;
}

int setbcap(unsigned long val)
{
	int ret;

	ret = prctl(PR_SET_CAPBSET, _LINUX_CAPABILITY_VERSION, val);
	return ret;
}

int usage(char *me)
{
	printf("Usage: %s get\n", me);
	printf("       %s set <newval>\n", me);
	printf("       %s strset capability_string\n", me);
	printf("         capability_string is for instance:\n");
	printf("         cap_sys_admin,cap_mknod,cap_dac_override\n");
	return 1;
}

unsigned long captoint(char *cap)
{
	if (strcmp(cap, "cap_dac_override") == 0)
		return 1;
	else if (strcmp(cap, "cap_dac_read_search") == 0)
		return 2;
	else if (strcmp(cap, "cap_fowner") == 0)
		return 3;
	else if (strcmp(cap, "cap_fsetid") == 0)
		return 4;
	else if (strcmp(cap, "cap_kill") == 0)
		return 5;
	else if (strcmp(cap, "cap_setgid") == 0)
		return 6;
	else if (strcmp(cap, "cap_setuid") == 0)
		return 7;
	else if (strcmp(cap, "cap_setpcap") == 0)
		return 8;
	else if (strcmp(cap, "cap_linux_immutable") == 0)
		return 9;
	else if (strcmp(cap, "cap_net_bind_service") == 0)
		return 10;
	else if (strcmp(cap, "cap_net_broadcast") == 0)
		return 11;
	else if (strcmp(cap, "cap_net_admin") == 0)
		return 12;
	else if (strcmp(cap, "cap_net_raw") == 0)
		return 13;
	else if (strcmp(cap, "cap_ipc_lock") == 0)
		return 14;
	else if (strcmp(cap, "cap_ipc_owner") == 0)
		return 15;
	else if (strcmp(cap, "cap_sys_module") == 0)
		return 16;
	else if (strcmp(cap, "cap_sys_rawio") == 0)
		return 17;
	else if (strcmp(cap, "cap_sys_chroot") == 0)
		return 18;
	else if (strcmp(cap, "cap_sys_ptrace") == 0)
		return 19;
	else if (strcmp(cap, "cap_sys_pacct") == 0)
		return 20;
	else if (strcmp(cap, "cap_sys_admin") == 0)
		return 21;
	else if (strcmp(cap, "cap_sys_boot") == 0)
		return 22;
	else if (strcmp(cap, "cap_sys_nice") == 0)
		return 23;
	else if (strcmp(cap, "cap_sys_resource") == 0)
		return 24;
	else if (strcmp(cap, "cap_sys_time") == 0)
		return 25;
	else if (strcmp(cap, "cap_sys_tty_config") == 0)
		return 26;
	else if (strcmp(cap, "cap_mknod") == 0)
		return 27;
	else if (strcmp(cap, "cap_lease") == 0)
		return 28;
	else if (strcmp(cap, "cap_audit_write") == 0)
		return 29;
	else if (strcmp(cap, "cap_audit_control") == 0)
		return 30;
	else if (strcmp(cap, "cap_setfcap") == 0)
		return 31;
}

unsigned long parse_cap_string(char *capstring)
{
	unsigned long tmp, newval = 0;
	char *token = strtok(capstring, ",");

	while (token) {
		tmp = captoint(token);
		if (tmp < 0)
			return -1;
		newval |= 1<<tmp;
		token = strtok(NULL, ",");
	}
	return newval;
}

int read_args(int argc, char *argv[])
{
	if (strcmp(argv[1], "get") == 0) {
		cmd_getbcap = 1;
		return 0;
	}
	if (strcmp(argv[1], "strset") == 0) {
		newval = parse_cap_string(argv[2]);
		if (newval < 0)
			return newval;
		return 0;
	}
	if (strcmp(argv[1], "set") != 0)
		return 1;
	if (argc != 3)
		return 1;
	newval = strtoul(argv[2], NULL, 10);
	return 0;
}

int main(int argc, char *argv[])
{
	int ret;

	if (argc<2)
		return usage(argv[0]);

	if ((ret=read_args(argc, argv)))
		return ret;

	if (cmd_getbcap)
		return getbcap();

	ret = setbcap(newval);
	if (ret != 0)
		return ret;
	return execl("/bin/bash", "/bin/bash", NULL);
}
===========================================================

Changelog:
	Enforce current-> capabilities are subsets of the
	new bounding set.

	Return -EINVAL if no capabilities.

	As suggested by Andrew Morgan, send the capability
	version along with the bset for prctl(PR_SET_CAPBSET)
	and PR_GET_CAPBSET)

Signed-off-by: Serge Hallyn <serue@...ibm.com>
---
 include/linux/capability.h |   24 +++++++++++++++++++++++-
 include/linux/init_task.h  |    1 +
 include/linux/prctl.h      |    4 ++++
 include/linux/sched.h      |    2 +-
 include/linux/security.h   |    5 -----
 include/linux/sysctl.h     |    3 ---
 kernel/fork.c              |    1 +
 kernel/sys.c               |   18 +++++++++++++++++-
 kernel/sysctl.c            |   35 -----------------------------------
 kernel/sysctl_check.c      |    7 -------
 security/commoncap.c       |   37 +++++++++++++++++++++++++++++++++----
 11 files changed, 80 insertions(+), 57 deletions(-)

diff --git a/include/linux/capability.h b/include/linux/capability.h
index 7a8d7ad..d853b0d 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -197,7 +197,6 @@ typedef __u32 kernel_cap_t;
 #define CAP_IPC_OWNER        15
 
 /* Insert and remove kernel modules - modify kernel without limit */
-/* Modify cap_bset */
 #define CAP_SYS_MODULE       16
 
 /* Allow ioperm/iopl access */
@@ -254,6 +253,7 @@ typedef __u32 kernel_cap_t;
    arbitrary SCSI commands */
 /* Allow setting encryption key on loopback filesystem */
 /* Allow setting zone reclaim policy */
+/* Allow taking bits out of capability bounding set */
 
 #define CAP_SYS_ADMIN        21
 
@@ -332,6 +332,17 @@ typedef __u32 kernel_cap_t;
 #define CAP_INIT_EFF_SET    to_cap_t(~0 & ~CAP_TO_MASK(CAP_SETPCAP))
 #define CAP_INIT_INH_SET    to_cap_t(0)
 
+#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
+/*
+ * Because of the reduced scope of CAP_SETPCAP when filesystem
+ * capabilities are in effect, it is safe to allow this capability to
+ * be available in the default configuration.
+ */
+# define CAP_INIT_BSET  CAP_FULL_SET
+#else
+# define CAP_INIT_BSET  CAP_INIT_EFF_SET
+#endif
+
 #define CAP_TO_MASK(x) (1 << (x))
 #define cap_raise(c, flag)   (cap_t(c) |=  CAP_TO_MASK(flag))
 #define cap_lower(c, flag)   (cap_t(c) &= ~CAP_TO_MASK(flag))
@@ -377,6 +388,17 @@ static inline kernel_cap_t cap_invert(kernel_cap_t c)
 int capable(int cap);
 int __capable(struct task_struct *t, int cap);
 
+#ifdef CONFIG_COMMONCAP
+extern int cap_prctl_setbset(unsigned long new_bset);
+extern int cap_prctl_getbset(unsigned long *bset);
+#else
+#include <linux/errno.h>
+static inline int cap_prctl_setbset(unsigned long new_bset)
+{ return -EINVAL; }
+static inline int cap_prctl_getbset(unsigned long *bset)
+{ return -EINVAL; }
+#endif
+
 #endif /* __KERNEL__ */
 
 #endif /* !_LINUX_CAPABILITY_H */
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index cae35b6..5c84d14 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -147,6 +147,7 @@ extern struct group_info init_groups;
 	.cap_effective	= CAP_INIT_EFF_SET,				\
 	.cap_inheritable = CAP_INIT_INH_SET,				\
 	.cap_permitted	= CAP_FULL_SET,					\
+	.cap_bset 	= CAP_INIT_BSET,				\
 	.keep_capabilities = 0,						\
 	.user		= INIT_USER,					\
 	.comm		= "swapper",					\
diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index e2eff90..a7de023 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -63,4 +63,8 @@
 #define PR_GET_SECCOMP	21
 #define PR_SET_SECCOMP	22
 
+/* Get/set the capability bounding set */
+#define PR_GET_CAPBSET 23
+#define PR_SET_CAPBSET 24
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4f21af1..7c21341 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -973,7 +973,7 @@ struct task_struct {
 	uid_t uid,euid,suid,fsuid;
 	gid_t gid,egid,sgid,fsgid;
 	struct group_info *group_info;
-	kernel_cap_t   cap_effective, cap_inheritable, cap_permitted;
+	kernel_cap_t   cap_effective, cap_inheritable, cap_permitted, cap_bset;
 	unsigned keep_capabilities:1;
 	struct user_struct *user;
 #ifdef CONFIG_KEYS
diff --git a/include/linux/security.h b/include/linux/security.h
index ff3f857..e2d2f06 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -34,11 +34,6 @@
 #include <linux/xfrm.h>
 #include <net/flow.h>
 
-/*
- * Bounding set
- */
-extern kernel_cap_t cap_bset;
-
 extern unsigned securebits;
 
 struct ctl_table;
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index e99171f..3771782 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -103,7 +103,6 @@ enum
 	KERN_NODENAME=7,
 	KERN_DOMAINNAME=8,
 
-	KERN_CAP_BSET=14,	/* int: capability bounding set */
 	KERN_PANIC=15,		/* int: panic timeout */
 	KERN_REALROOTDEV=16,	/* real root device to mount after initrd */
 
@@ -968,8 +967,6 @@ extern int proc_dostring(struct ctl_table *, int, struct file *,
 			 void __user *, size_t *, loff_t *);
 extern int proc_dointvec(struct ctl_table *, int, struct file *,
 			 void __user *, size_t *, loff_t *);
-extern int proc_dointvec_bset(struct ctl_table *, int, struct file *,
-			      void __user *, size_t *, loff_t *);
 extern int proc_dointvec_minmax(struct ctl_table *, int, struct file *,
 				void __user *, size_t *, loff_t *);
 extern int proc_dointvec_jiffies(struct ctl_table *, int, struct file *,
diff --git a/kernel/fork.c b/kernel/fork.c
index f85731a..df13a01 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1082,6 +1082,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 #ifdef CONFIG_SECURITY
 	p->security = NULL;
 #endif
+	p->cap_bset = current->cap_bset;
 	p->io_context = NULL;
 	p->audit_context = NULL;
 	cgroup_fork(p);
diff --git a/kernel/sys.c b/kernel/sys.c
index 796299c..cff511f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1739,7 +1739,23 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
 		case PR_SET_SECCOMP:
 			error = prctl_set_seccomp(arg2);
 			break;
-
+		case PR_GET_CAPBSET: {
+			unsigned long bset;
+			error = cap_prctl_getbset(&bset);
+			if (error)
+				return error;
+			error = put_user(_LINUX_CAPABILITY_VERSION,
+					(__u32 __user *)arg2);
+			if (error)
+				break;
+			error = put_user(bset, (unsigned long __user *)arg3);
+			return error;
+		}
+		case PR_SET_CAPBSET:
+			if (arg2 != _LINUX_CAPABILITY_VERSION)
+				return -EINVAL;
+			error = cap_prctl_setbset(arg3);
+			break;
 		default:
 			error = -EINVAL;
 			break;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index b11d22b..7bff6a2 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -363,15 +363,6 @@ static struct ctl_table kern_table[] = {
 		.proc_handler	= &proc_dointvec_taint,
 	},
 #endif
-#ifdef CONFIG_SECURITY_CAPABILITIES
-	{
-		.procname	= "cap-bound",
-		.data		= &cap_bset,
-		.maxlen		= sizeof(kernel_cap_t),
-		.mode		= 0600,
-		.proc_handler	= &proc_dointvec_bset,
-	},
-#endif /* def CONFIG_SECURITY_CAPABILITIES */
 #ifdef CONFIG_BLK_DEV_INITRD
 	{
 		.ctl_name	= KERN_REALROOTDEV,
@@ -1882,26 +1873,6 @@ static int do_proc_dointvec_bset_conv(int *negp, unsigned long *lvalp,
 	return 0;
 }
 
-#ifdef CONFIG_SECURITY_CAPABILITIES
-/*
- *	init may raise the set.
- */
-
-int proc_dointvec_bset(struct ctl_table *table, int write, struct file *filp,
-			void __user *buffer, size_t *lenp, loff_t *ppos)
-{
-	int op;
-
-	if (write && !capable(CAP_SYS_MODULE)) {
-		return -EPERM;
-	}
-
-	op = is_global_init(current) ? OP_SET : OP_AND;
-	return do_proc_dointvec(table,write,filp,buffer,lenp,ppos,
-				do_proc_dointvec_bset_conv,&op);
-}
-#endif /* def CONFIG_SECURITY_CAPABILITIES */
-
 /*
  *	Taint values can only be increased
  */
@@ -2315,12 +2286,6 @@ int proc_dointvec(struct ctl_table *table, int write, struct file *filp,
 	return -ENOSYS;
 }
 
-int proc_dointvec_bset(struct ctl_table *table, int write, struct file *filp,
-			void __user *buffer, size_t *lenp, loff_t *ppos)
-{
-	return -ENOSYS;
-}
-
 int proc_dointvec_minmax(struct ctl_table *table, int write, struct file *filp,
 		    void __user *buffer, size_t *lenp, loff_t *ppos)
 {
diff --git a/kernel/sysctl_check.c b/kernel/sysctl_check.c
index 3c9ef5a..41c7f16 100644
--- a/kernel/sysctl_check.c
+++ b/kernel/sysctl_check.c
@@ -38,10 +38,6 @@ static struct trans_ctl_table trans_kern_table[] = {
 	{ KERN_NODENAME,		"hostname" },
 	{ KERN_DOMAINNAME,		"domainname" },
 
-#ifdef CONFIG_SECURITY_CAPABILITIES
-	{ KERN_CAP_BSET,		"cap-bound" },
-#endif /* def CONFIG_SECURITY_CAPABILITIES */
-
 	{ KERN_PANIC,			"panic" },
 	{ KERN_REALROOTDEV,		"real-root-dev" },
 
@@ -1535,9 +1531,6 @@ int sysctl_check_table(struct ctl_table *table)
 			    (table->strategy == sysctl_ms_jiffies) ||
 			    (table->proc_handler == proc_dostring) ||
 			    (table->proc_handler == proc_dointvec) ||
-#ifdef CONFIG_SECURITY_CAPABILITIES
-			    (table->proc_handler == proc_dointvec_bset) ||
-#endif /* def CONFIG_SECURITY_CAPABILITIES */
 			    (table->proc_handler == proc_dointvec_minmax) ||
 			    (table->proc_handler == proc_dointvec_jiffies) ||
 			    (table->proc_handler == proc_dointvec_userhz_jiffies) ||
diff --git a/security/commoncap.c b/security/commoncap.c
index 43f9027..5be15bb 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -36,9 +36,6 @@
 # define CAP_INIT_BSET  CAP_INIT_EFF_SET
 #endif /* def CONFIG_SECURITY_FILE_CAPABILITIES */
 
-kernel_cap_t cap_bset = CAP_INIT_BSET;    /* systemwide capability bound */
-EXPORT_SYMBOL(cap_bset);
-
 /* Global security state */
 
 unsigned securebits = SECUREBITS_DEFAULT; /* systemwide security settings */
@@ -307,7 +304,8 @@ void cap_bprm_apply_creds (struct linux_binprm *bprm, int unsafe)
 	/* Derived from fs/exec.c:compute_creds. */
 	kernel_cap_t new_permitted, working;
 
-	new_permitted = cap_intersect (bprm->cap_permitted, cap_bset);
+	new_permitted = cap_intersect (bprm->cap_permitted,
+				 current->cap_bset);
 	working = cap_intersect (bprm->cap_inheritable,
 				 current->cap_inheritable);
 	new_permitted = cap_combine (new_permitted, working);
@@ -580,3 +578,34 @@ int cap_vm_enough_memory(struct mm_struct *mm, long pages)
 	return __vm_enough_memory(mm, pages, cap_sys_admin);
 }
 
+/*
+ * cap_prctl_setbset currently requires CAP_SYS_ADMIN.  The reason is
+ * my fear that an ordinary user could selectively take capabilities
+ * out, then run a setuid root binary or binary with file capabilities,
+ * which would perform part of a dangerous action with CAP_SOMECAP1,
+ * then fail to perform the second part of the action because
+ * CAP_SOMECAP2 is not in bset, leaving things in an unsafe state,
+ * i.e a sensitive file owned by the non-root user because CAP_CHOWN
+ * was not allowed.
+ */
+int cap_prctl_setbset(unsigned long new_bset)
+{
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	if (!cap_issubset(new_bset, current->cap_bset))
+		return -EPERM;
+	current->cap_bset = new_bset;
+	current->cap_effective = cap_intersect(current->cap_effective,
+		new_bset);
+	current->cap_permitted = cap_intersect(current->cap_permitted,
+		new_bset);
+	current->cap_inheritable = cap_intersect(current->cap_inheritable,
+		new_bset);
+	return 0;
+}
+
+int cap_prctl_getbset(unsigned long *bset)
+{
+	*bset = current->cap_bset;
+	return 0;
+}
-- 
1.5.1.1.GIT

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