linux-kernel - Re: [PATCH v2] LSM: Convert security_hook_heads into explicit array of struct list

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Message-ID: <CAGXu5jK2azpWLOg=EGEOhPySN52_JpMcXoWaSkNSZhxF1_dNFA@mail.gmail.com>
Date:   Sun, 28 May 2017 14:19:22 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     linux-security-module <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Christoph Hellwig <hch@...radead.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        James Morris <james.l.morris@...cle.com>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: [PATCH v2] LSM: Convert security_hook_heads into explicit array
 of struct list_head

On Sun, May 28, 2017 at 1:29 PM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Commit 3dfc9b02864b19f4 ("LSM: Initialize security_hook_heads upon
> registration.") treats "struct security_hook_heads" as an implicit array
> of "struct list_head" so that we can eliminate code for static
> initialization. Although we haven't encountered compilers which do not
> treat sizeof(security_hook_heads) != sizeof(struct list_head) *
> (sizeof(security_hook_heads) / sizeof(struct list_head)), Casey does not
> like the assumption that a structure of N elements can be assumed to be
> the same as an array of N elements.
>
> Now that Kees found that randstruct complains about such casting
>
>   security/security.c: In function 'security_init':
>   security/security.c:59:20: note: found mismatched op0 struct pointer types: 'struct list_head' and 'struct security_hook_heads'
>
>     struct list_head *list = (struct list_head *) &security_hook_heads;
>
> and Christoph thinks that we should fix it rather than make randstruct
> whitelist it, this patch fixes it.
>
> It would be possible to revert commit 3dfc9b02864b19f4, but this patch
> converts security_hook_heads into an explicit array of struct list_head
> by introducing an enum, due to reasons explained below.
>
> Igor proposed a sealable memory allocator, and the LSM hooks
> ("struct security_hook_heads security_hook_heads" and
> "struct security_hook_list ...[]") will benefit from that allocator via
> protection using set_memory_ro()/set_memory_rw(), and that allocator
> will remove CONFIG_SECURITY_WRITABLE_HOOKS config option. Thus, we will
> likely be moving to that direction.
>
> This means that these structures will be allocated at run time using
> that allocator, 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. If we use an enum
> so that LSM_HOOK_INIT() macro does not need to know absolute address of
> security_hook_heads, it will help us to use that allocator for LSM hooks.
>
> As a result of introducing an enum, security_hook_heads becomes a local
> variable. In order to pass 80 columns check by scripts/checkpatch.pl ,
> rename security_hook_heads to hook_heads.
>
> 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: Christoph Hellwig <hch@...radead.org>

Looks good to me; thanks for persisting! :)

Acked-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook
Pixel Security