[PATCH] capabilites: allow the application of capability limits to usermode helpers

[PATCH] capabilites: allow the application of capability limits to usermode helpers

[Eric Paris]

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_SET_PCAP 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_SET_PCAP and CAP_SYS_MODULE
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>
---

 include/linux/kmod.h |    3 ++
 kernel/kmod.c        |  100 ++++++++++++++++++++++++++++++++++++++++++++++++++
 kernel/sysctl.c      |    6 +++
 3 files changed, 109 insertions(+), 0 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 6efd7a7..79bb98d 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -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
 
@@ -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);
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 9cd0591..3571d1e 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -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>
@@ -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
 
 /*
@@ -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);
@@ -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);
@@ -418,6 +440,84 @@ unlock:
 }
 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_SET_PCAP) ||
+		      !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");
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c0bb324..965134b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -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>
@@ -616,6 +617,11 @@ static struct ctl_table kern_table[] = {
 		.child		= random_table,
 	},
 	{
+		.procname	= "usermodehelper",
+		.mode		= 0555,
+		.child		= usermodehelper_table,
+	},
+	{
 		.procname	= "overflowuid",
 		.data		= &overflowuid,
 		.maxlen		= sizeof(int),

Re: [PATCH] capabilites: allow the application of capability limits to usermode helpers

[Andrew G. Morgan]

Acked-by: Andrew G. Morgan <morgan@kernel.org>

Cheers

Andrew

On Wed, Mar 30, 2011 at 10:04 AM, Serge E. Hallyn
<serge.hallyn@ubuntu.com> wrote:
> Quoting Eric Paris (eparis@redhat.com):
>> 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: Serge E. Hallyn <serge.hallyn@canonical.com>
>
> thanks,
> -serge
>
>> Acked-by: David Howells <dhowells@redhat.com>
>> ---
>>
>>  include/linux/kmod.h |    3 ++
>>  kernel/kmod.c        |  100 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>  kernel/sysctl.c      |    6 +++
>>  3 files changed, 109 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/kmod.h b/include/linux/kmod.h
>> index 6efd7a7..79bb98d 100644
>> --- a/include/linux/kmod.h
>> +++ b/include/linux/kmod.h
>> @@ -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
>>
>> @@ -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);
>> diff --git a/kernel/kmod.c b/kernel/kmod.c
>> index 9cd0591..06fdea2 100644
>> --- a/kernel/kmod.c
>> +++ b/kernel/kmod.c
>> @@ -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>
>> @@ -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
>>
>>  /*
>> @@ -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);
>> @@ -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);
>> @@ -418,6 +440,84 @@ unlock:
>>  }
>>  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");
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index c0bb324..965134b 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -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>
>> @@ -616,6 +617,11 @@ static struct ctl_table kern_table[] = {
>>               .child          = random_table,
>>       },
>>       {
>> +             .procname       = "usermodehelper",
>> +             .mode           = 0555,
>> +             .child          = usermodehelper_table,
>> +     },
>> +     {
>>               .procname       = "overflowuid",
>>               .data           = &overflowuid,
>>               .maxlen         = sizeof(int),
>>
>
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Re: [PATCH] capabilites: allow the application of capability limits to usermode helpers

[Serge E. Hallyn]

[-- Attachment #1: Type: text/plain, Size: 6809 bytes --]

Quoting Eric Paris (eparis@redhat.com):
> 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: Serge E. Hallyn <serge.hallyn@canonical.com>

thanks,
-serge

> Acked-by: David Howells <dhowells@redhat.com>
> ---
> 
>  include/linux/kmod.h |    3 ++
>  kernel/kmod.c        |  100 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sysctl.c      |    6 +++
>  3 files changed, 109 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/kmod.h b/include/linux/kmod.h
> index 6efd7a7..79bb98d 100644
> --- a/include/linux/kmod.h
> +++ b/include/linux/kmod.h
> @@ -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
>  
> @@ -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);
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index 9cd0591..06fdea2 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -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>
> @@ -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
>  
>  /*
> @@ -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);
> @@ -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);
> @@ -418,6 +440,84 @@ unlock:
>  }
>  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");
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index c0bb324..965134b 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -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>
> @@ -616,6 +617,11 @@ static struct ctl_table kern_table[] = {
>  		.child		= random_table,
>  	},
>  	{
> +		.procname	= "usermodehelper",
> +		.mode		= 0555,
> +		.child		= usermodehelper_table,
> +	},
> +	{
>  		.procname	= "overflowuid",
>  		.data		= &overflowuid,
>  		.maxlen		= sizeof(int),
> 

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 490 bytes --]

[PATCH] capabilites: allow the application of capability limits to usermode helpers

[Eric Paris]

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>
---

 include/linux/kmod.h |    3 ++
 kernel/kmod.c        |  100 ++++++++++++++++++++++++++++++++++++++++++++++++++
 kernel/sysctl.c      |    6 +++
 3 files changed, 109 insertions(+), 0 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 6efd7a7..79bb98d 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -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
 
@@ -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);
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 9cd0591..06fdea2 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -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>
@@ -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
 
 /*
@@ -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);
@@ -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);
@@ -418,6 +440,84 @@ unlock:
 }
 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");
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c0bb324..965134b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -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>
@@ -616,6 +617,11 @@ static struct ctl_table kern_table[] = {
 		.child		= random_table,
 	},
 	{
+		.procname	= "usermodehelper",
+		.mode		= 0555,
+		.child		= usermodehelper_table,
+	},
+	{
 		.procname	= "overflowuid",
 		.data		= &overflowuid,
 		.maxlen		= sizeof(int),