linux-kernel - Re: [PATCH] selinux: add __randomize_layout to selinux_audit

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Date:   Wed, 18 May 2022 14:53:35 +0800
From:   Gong Ruiqi <gongruiqi1@...wei.com>
To:     Paul Moore <paul@...l-moore.com>
CC:     Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Kees Cook <keescook@...omium.org>, <selinux@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Wang Weiyang <wangweiyang2@...wei.com>,
        Xiu Jianfeng <xiujianfeng@...wei.com>
Subject: Re: [PATCH] selinux: add __randomize_layout to selinux_audit_data

On 2022/05/18 9:39, Paul Moore wrote:
> On Tue, May 17, 2022 at 9:21 PM GONG, Ruiqi <gongruiqi1@...wei.com> wrote:
>>
>> Randomize the layout of struct selinux_audit_data as suggested in [1],
>> since it contains a pointer to struct selinux_state, an already
>> randomized strucure.
>>
>> [1]: https://github.com/KSPP/linux/issues/188
>>
>> Signed-off-by: GONG, Ruiqi <gongruiqi1@...wei.com>
>> ---
>>  security/selinux/include/avc.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/security/selinux/include/avc.h b/security/selinux/include/avc.h
>> index 2b372f98f2d7..5525b94fd266 100644
>> --- a/security/selinux/include/avc.h
>> +++ b/security/selinux/include/avc.h
>> @@ -53,7 +53,7 @@ struct selinux_audit_data {
>>         u32 denied;
>>         int result;
>>         struct selinux_state *state;
>> -};
>> +} __randomize_layout;
> 
> I'll apologize in advance for the stupid question, but does

Not at all :)

> __randomize_layout result in any problems when the struct is used in a
> trace event?  (see include/trace/events/avc.h)
> 

No, as least it doesn't in the testing I did. I believe we can use the
struct tagged with __randomize_layout as normal except that 1) it should
be initialized with a designated initializer, and 2) pointers to this
type can't be cast to/from pointers to another type. Other operations
like dereferencing members of the struct (as in
include/trace/events/avc.h) shouldn't be a problem.

I did a testing to the patch on a qemu vm by running the selinux
testsuite with tracing events "avc:selinux_audited" enabled. The
testsuite completed successfully and from the tracing log I saw nothing
abnormal with my bare eyes. You can do more testing if you want or you
have other ideas of how to do so ;)