Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction

[Rusty Russell]

Djalal Harouni <tixxdz@xxxxxxxxx> writes:
> Hi Rusty,
>
> On Mon, Apr 24, 2017 at 6:29 AM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
>> Djalal Harouni <tixxdz@xxxxxxxxx> writes:
>>> When value is (1), task must have CAP_SYS_MODULE to be able to trigger a
>>> module auto-load operation, or CAP_NET_ADMIN for modules with a
>>> 'netdev-%s' alias.
>>
>> Sorry, the magic 'netdev-' prefix is a crawling horror.  To do this
>
> Yes I agree, that's the not the best part. I added it for backward
> compatibility since I did notice that some network daemon retain
> CAP_NET_ADMIN for this case. The aim of the patches is to get modules
> autoload covered with CAP_SYS_MODULE and make it more like explicit
> modules loading.
>
>> properly, you need to hand the capability (if any) from the
>> request_module() call.  Probably by adding a new request_module_cap and
>> making request_module() call that, then fixing up the callers.
>
> Hmm, sorry Rusty I'm a bit confused, when you refer to "callers", you
> mean request_module() callers ?

Yes.

> If so, I'm not sure that the best thing here since it may defeat the
> purpose of this feature if we allow to pass capabilities.
>
> Right now we have modules autoload that can be triggered without
> privileges, or with CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_SYS_MODULE...
> and some caps may allow to load other modules to resolve symbols etc.
>
> The public exploits did target CAP_NET_ADMIN, if we offer a way to
> pass in capabilities it will be used it as it is the case now without
> it, not to mention that some capabilities are overloaded, exploits
> will continue for these ones.
>
> The goal is to narrow modules autoload only to CAP_SYS_MODULE or
> disable it completely for process trees. Later you can give
> CAP_SYS_MODULE and you are sure that only /lib/modules/ will be
> autoloaded, no arbitrary loads by using seccomp filter on module
> syscalls, or separate the process in two one with CAP_SYS_MODULE and
> the other with its own capabilities and feature use.
>
> I also don't like that 'netdev-%s' but it is for compatibility for
> specific cases, maybe there are others that we may have to add. But I
> would prefer if we narrow it down to only CAP_SYS_MODULE.

There's one place where this is called, net/core/dev_ioctl.c:

	if (no_module && capable(CAP_NET_ADMIN))
		no_module = request_module("netdev-%s", name);

*That's the place* you want to add the exception, and the cleanest way
is probably to add another arg to __request_module:

(incomplete patch, but you get the idea):

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index c4e441e00db5..2ea82d5d20af 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -33,15 +33,16 @@ extern char modprobe_path[]; /* for sysctl */
 /* modprobe exit status on success, -ve on error.  Return value
  * usually useless though. */
 extern __printf(2, 3)
-int __request_module(bool wait, const char *name, ...);
-#define request_module(mod...) __request_module(true, mod)
-#define request_module_nowait(mod...) __request_module(false, mod)
-#define try_then_request_module(x, mod...) \
-	((x) ?: (__request_module(true, mod), (x)))
+int __request_module(bool wait, int allow_cap, const char *name, ...);
 #else
-static inline int request_module(const char *name, ...) { return -ENOSYS; }
-static inline int request_module_nowait(const char *name, ...) { return -ENOSYS; }
-#define try_then_request_module(x, mod...) (x)
+static inline __printf(2,3)
+int __request_module(bool wait, int allow_cap, const char *name, ...)
+{ return -ENOSYS; }
+#endif
+#define request_module(mod...) __request_module(true, -1, mod)
+#define request_module_nowait(mod...) __request_module(false, -1, mod)
+#define try_then_request_module(x, mod...) \
+	((x) ?: (__request_module(true, -1, mod), (x)))
 #endif
 
 
diff --git a/include/linux/security.h b/include/linux/security.h
index 96899fad7016..9f1217c7cb23 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -889,9 +890,9 @@ static inline int security_kernel_create_files_as(struct cred *cred,
 	return 0;
 }
 
-static inline int security_kernel_module_request(char *kmod_name)
+static inline int security_kernel_module_request(char *kmod_name, int allow_cap)
 {
-	return 0;
+	return cap_kernel_module_request(kmod_name, allow_cap);
 }
 
 static inline int security_kernel_read_file(struct file *file,
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 563f97e2be36..b2d2f525c80b 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -110,6 +110,7 @@ static int call_modprobe(char *module_name, int wait)
 /**
  * __request_module - try to load a kernel module
  * @wait: wait (or not) for the operation to complete
+ * @allow_cap: if positive, always allow modprobe if this capability set.
  * @fmt: printf style format string for the name of the module
  * @...: arguments as specified in the format string
  *
@@ -123,7 +124,8 @@ static int call_modprobe(char *module_name, int wait)
  * If module auto-loading support is disabled then this function
  * becomes a no-operation.
  */
-int __request_module(bool wait, const char *fmt, ...)
+
+int __request_module(bool wait, int allow_cap, const char *fmt, ...)
 {
 	va_list args;
 	char module_name[MODULE_NAME_LEN];
@@ -150,7 +152,7 @@ int __request_module(bool wait, const char *fmt, ...)
 	if (ret >= MODULE_NAME_LEN)
 		return -ENAMETOOLONG;
 
-	ret = security_kernel_module_request(module_name);
+	ret = security_kernel_module_request(module_name, allow_cap);
 	if (ret)
 		return ret;
 
diff --git a/net/core/dev_ioctl.c b/net/core/dev_ioctl.c
index b94b1d293506..e7a7dc28761d 100644
--- a/net/core/dev_ioctl.c
+++ b/net/core/dev_ioctl.c
@@ -367,7 +367,8 @@ void dev_load(struct net *net, const char *name)
 
 	no_module = !dev;
 	if (no_module && capable(CAP_NET_ADMIN))
-		no_module = request_module("netdev-%s", name);
+		no_module = __request_module(true, CAP_NET_ADMIN,
+					     "netdev-%s", name);
 	if (no_module && capable(CAP_SYS_MODULE))
 		request_module("%s", name);
 }

Hope that helps,
Rusty.