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Date: Mon, 22 May 2017 08:09:12 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: Christoph Hellwig <hch@...radead.org>,
Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: linux-security-module@...r.kernel.org, linux-mm@...ck.org,
kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org,
Greg KH <gregkh@...uxfoundation.org>, Igor Stoppa <igor.stoppa@...wei.com>,
James Morris <james.l.morris@...cle.com>, Kees Cook <keescook@...omium.org>,
Paul Moore <paul@...l-moore.com>, Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: [PATCH] LSM: Make security_hook_heads a local variable.
On 5/22/2017 7:03 AM, Christoph Hellwig wrote:
> On Sun, May 21, 2017 at 08:14:05PM +0900, Tetsuo Handa wrote:
>> A sealable memory allocator patch was proposed at
>> http://lkml.kernel.org/r/20170519103811.2183-1-igor.stoppa@huawei.com ,
>> and is waiting for a follow-on patch showing how any of the kernel
>> can be changed to use this new subsystem. So, here it is for LSM hooks.
>>
>> The LSM hooks ("struct security_hook_heads security_hook_heads" and
>> "struct security_hook_list ...[]") will benefit from this allocator via
>> protection using set_memory_ro()/set_memory_rw(), and it will remove
>> CONFIG_SECURITY_WRITABLE_HOOKS config option.
>>
>> This means that these structures will be allocated at run time using
>> smalloc(), and therefore the address of these structures will be
>> determined at run time rather than compile time.
>>
>> But currently, LSM_HOOK_INIT() macro depends on the address of
>> security_hook_heads being known at compile time. But we already
>> initialize security_hook_heads as an array of "struct list_head".
>>
>> Therefore, let's use index number (or relative offset from the head
>> of security_hook_heads) instead of absolute address of
>> security_hook_heads so that LSM_HOOK_INIT() macro does not need to
>> know absolute address of security_hook_heads. Then, security_add_hooks()
>> will be able to allocate and copy "struct security_hook_list ...[]" using
>> smalloc().
>>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Paul Moore <paul@...l-moore.com>
>> Cc: Stephen Smalley <sds@...ho.nsa.gov>
>> Cc: Casey Schaufler <casey@...aufler-ca.com>
>> Cc: James Morris <james.l.morris@...cle.com>
>> Cc: Igor Stoppa <igor.stoppa@...wei.com>
>> Cc: Greg KH <gregkh@...uxfoundation.org>
>> ---
>> include/linux/lsm_hooks.h | 6 +++---
>> security/security.c | 10 ++++++++--
>> 2 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>> index 080f34e..865c11d 100644
>> --- a/include/linux/lsm_hooks.h
>> +++ b/include/linux/lsm_hooks.h
>> @@ -1884,8 +1884,8 @@ struct security_hook_heads {
>> */
>> struct security_hook_list {
>> struct list_head list;
>> - struct list_head *head;
>> union security_list_options hook;
>> + const unsigned int idx;
>> char *lsm;
>> };
>>
>> @@ -1896,9 +1896,9 @@ struct security_hook_list {
>> * text involved.
>> */
>> #define LSM_HOOK_INIT(HEAD, HOOK) \
>> - { .head = &security_hook_heads.HEAD, .hook = { .HEAD = HOOK } }
>> + { .idx = offsetof(struct security_hook_heads, HEAD) / \
>> + sizeof(struct list_head), .hook = { .HEAD = HOOK } }
>>
>> -extern struct security_hook_heads security_hook_heads;
>> extern char *lsm_names;
>>
>> extern void security_add_hooks(struct security_hook_list *hooks, int count,
>> diff --git a/security/security.c b/security/security.c
>> index 54b1e39..d6883ce 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -33,7 +33,7 @@
>> /* Maximum number of letters for an LSM name string */
>> #define SECURITY_NAME_MAX 10
>>
>> -struct security_hook_heads security_hook_heads __lsm_ro_after_init;
>> +static struct security_hook_heads security_hook_heads __lsm_ro_after_init;
>> char *lsm_names;
>> /* Boot-time LSM user choice */
>> static __initdata char chosen_lsm[SECURITY_NAME_MAX + 1] =
>> @@ -152,10 +152,16 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
>> char *lsm)
>> {
>> int i;
>> + struct list_head *list = (struct list_head *) &security_hook_heads;
> Eww, struct casts. This whole security_hook_heads scheme stink,
> even with the slight improvements from Tetsuo. It has everything we
> shouldn't do - function pointers in structures that are not hard
> read-only, structure casts, etc.
>
> What's the reason why can't just have good old const function tables?
The set of hooks used by most security modules are sparse.
> Yeah, stackable LSM make that a little harder, but they should not be
> enable by default anyway.
With the number of security modules queued up behind full stacking
I can't say that I agree with your assertion.
> But even with those we can still chain
> them together with a list with external linkage.
I gave up that approach in 2012. Too many unnecessary calls to
null functions, and massive function vectors with a tiny number
of non-null entries. From a data structure standpoint, it was
just wrong. The list scheme is exactly right for the task at
hand.
> --
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