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Message-ID: <20170522140306.GA3907@infradead.org>
Date: Mon, 22 May 2017 07:03:06 -0700
From: Christoph Hellwig <hch@...radead.org>
To: 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,
Casey Schaufler <casey@...aufler-ca.com>,
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 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?
Yeah, stackable LSM make that a little harder, but they should not be
enable by default anyway. But even with those we can still chain
them together with a list with external linkage.
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