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capabilites: allow the application of capability limits to usermode h…
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…elpers

There is no way to limit the capabilities of usermodehelpers. This problem
reared its head recently when someone complained that any user with
cap_net_admin was able to load arbitrary kernel modules, even though the user
didn't have cap_sys_module.  The reason is because the actual load is done by
a usermode helper and those always have the full cap set.  This patch addes new
sysctls which allow us to bound the permissions of usermode helpers.

/proc/sys/kernel/usermodehelper/bset
/proc/sys/kernel/usermodehelper/inheritable

You must have CAP_SYS_MODULE  and CAP_SETPCAP to change these (changes are
&= ONLY).  When the kernel launches a usermodehelper it will do so with these
as the bset and pI.

-v2:	make globals static
	create spinlock to protect globals

-v3:	require both CAP_SETPCAP and CAP_SYS_MODULE
-v4:	fix the typo s/CAP_SET_PCAP/CAP_SETPCAP/ because I didn't commit
Signed-off-by: Eric Paris <eparis@redhat.com>
No-objection-from: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: David Howells <dhowells@redhat.com>
Acked-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Signed-off-by: James Morris <jmorris@namei.org>
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Eric Paris authored and James Morris committed Apr 4, 2011
1 parent cfc64fd commit 17f60a7
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Showing 3 changed files with 109 additions and 0 deletions.
3 changes: 3 additions & 0 deletions include/linux/kmod.h
Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,7 @@
#include <linux/errno.h>
#include <linux/compiler.h>
#include <linux/workqueue.h>
#include <linux/sysctl.h>

#define KMOD_PATH_LEN 256

Expand Down Expand Up @@ -109,6 +110,8 @@ call_usermodehelper(char *path, char **argv, char **envp, enum umh_wait wait)
NULL, NULL, NULL);
}

extern struct ctl_table usermodehelper_table[];

extern void usermodehelper_init(void);

extern int usermodehelper_disable(void);
Expand Down
100 changes: 100 additions & 0 deletions kernel/kmod.c
Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@
#include <linux/kmod.h>
#include <linux/slab.h>
#include <linux/completion.h>
#include <linux/cred.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/workqueue.h>
Expand All @@ -43,6 +44,13 @@ extern int max_threads;

static struct workqueue_struct *khelper_wq;

#define CAP_BSET (void *)1
#define CAP_PI (void *)2

static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
static DEFINE_SPINLOCK(umh_sysctl_lock);

#ifdef CONFIG_MODULES

/*
Expand Down Expand Up @@ -132,6 +140,7 @@ EXPORT_SYMBOL(__request_module);
static int ____call_usermodehelper(void *data)
{
struct subprocess_info *sub_info = data;
struct cred *new;
int retval;

spin_lock_irq(&current->sighand->siglock);
Expand All @@ -153,6 +162,19 @@ static int ____call_usermodehelper(void *data)
goto fail;
}

retval = -ENOMEM;
new = prepare_kernel_cred(current);
if (!new)
goto fail;

spin_lock(&umh_sysctl_lock);
new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
new->cap_inheritable = cap_intersect(usermodehelper_inheritable,
new->cap_inheritable);
spin_unlock(&umh_sysctl_lock);

commit_creds(new);

retval = kernel_execve(sub_info->path,
(const char *const *)sub_info->argv,
(const char *const *)sub_info->envp);
Expand Down Expand Up @@ -418,6 +440,84 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info,
}
EXPORT_SYMBOL(call_usermodehelper_exec);

static int proc_cap_handler(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos)
{
struct ctl_table t;
unsigned long cap_array[_KERNEL_CAPABILITY_U32S];
kernel_cap_t new_cap;
int err, i;

if (write && (!capable(CAP_SETPCAP) ||
!capable(CAP_SYS_MODULE)))
return -EPERM;

/*
* convert from the global kernel_cap_t to the ulong array to print to
* userspace if this is a read.
*/
spin_lock(&umh_sysctl_lock);
for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++) {
if (table->data == CAP_BSET)
cap_array[i] = usermodehelper_bset.cap[i];
else if (table->data == CAP_PI)
cap_array[i] = usermodehelper_inheritable.cap[i];
else
BUG();
}
spin_unlock(&umh_sysctl_lock);

t = *table;
t.data = &cap_array;

/*
* actually read or write and array of ulongs from userspace. Remember
* these are least significant 32 bits first
*/
err = proc_doulongvec_minmax(&t, write, buffer, lenp, ppos);
if (err < 0)
return err;

/*
* convert from the sysctl array of ulongs to the kernel_cap_t
* internal representation
*/
for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++)
new_cap.cap[i] = cap_array[i];

/*
* Drop everything not in the new_cap (but don't add things)
*/
spin_lock(&umh_sysctl_lock);
if (write) {
if (table->data == CAP_BSET)
usermodehelper_bset = cap_intersect(usermodehelper_bset, new_cap);
if (table->data == CAP_PI)
usermodehelper_inheritable = cap_intersect(usermodehelper_inheritable, new_cap);
}
spin_unlock(&umh_sysctl_lock);

return 0;
}

struct ctl_table usermodehelper_table[] = {
{
.procname = "bset",
.data = CAP_BSET,
.maxlen = _KERNEL_CAPABILITY_U32S * sizeof(unsigned long),
.mode = 0600,
.proc_handler = proc_cap_handler,
},
{
.procname = "inheritable",
.data = CAP_PI,
.maxlen = _KERNEL_CAPABILITY_U32S * sizeof(unsigned long),
.mode = 0600,
.proc_handler = proc_cap_handler,
},
{ }
};

void __init usermodehelper_init(void)
{
khelper_wq = create_singlethread_workqueue("khelper");
Expand Down
6 changes: 6 additions & 0 deletions kernel/sysctl.c
Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@
#include <linux/kprobes.h>
#include <linux/pipe_fs_i.h>
#include <linux/oom.h>
#include <linux/kmod.h>

#include <asm/uaccess.h>
#include <asm/processor.h>
Expand Down Expand Up @@ -615,6 +616,11 @@ static struct ctl_table kern_table[] = {
.mode = 0555,
.child = random_table,
},
{
.procname = "usermodehelper",
.mode = 0555,
.child = usermodehelper_table,
},
{
.procname = "overflowuid",
.data = &overflowuid,
Expand Down

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