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Message-ID: <87k266hacq.fsf@rustcorp.com.au>
Date: Thu, 27 Apr 2017 11:37:17 +0930
From: Rusty Russell <rusty@...tcorp.com.au>
To: Djalal Harouni <tixxdz@...il.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andy Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, "Serge E. Hallyn" <serge@...lyn.com>, kernel-hardening@...ts.openwall.com, LSM List <linux-security-module@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, Dongsu Park <dpark@...teo.net>, Casey Schaufler <casey@...aufler-ca.com>, James Morris <james.l.morris@...cle.com>, Paul Moore <paul@...l-moore.com>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jonathan Corbet <corbet@....net>, Jessica Yu <jeyu@...hat.com>, Arnaldo Carvalho de Melo <acme@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, Ingo Molnar <mingo@...nel.org>, Zendyani <zendyani@...il.com>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction
Djalal Harouni <tixxdz@...il.com> writes:
> Hi Rusty,
>
> On Mon, Apr 24, 2017 at 6:29 AM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>> Djalal Harouni <tixxdz@...il.com> 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.
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