linux-kernel - Re: selinux: should execmem disable shmat(..., SHM

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Date:   Thu, 27 Oct 2016 09:01:35 -0400
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     Topi Miettinen <toiwoton@...il.com>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "moderated list:SELINUX SECURITY MODULE" <selinux@...ho.nsa.gov>,
        "open list:SECURITY SUBSYSTEM" 
        <linux-security-module@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: selinux: should execmem disable shmat(..., SHM_EXEC)?

On 10/26/2016 04:31 PM, Topi Miettinen wrote:
> Hi,
> 
> Maybe this is a stupid question and I didn't test this with SELinux, but
> it looks to me that SELinux execmem does not prevent process from
> getting writable and executable memory mappings by using shmat(...,
> SHM_EXEC). Shouldn't this be blocked by execmem, I suppose it is there
> to prevent this kind of memory access?
> 
> Here's a test program:
> #include <sys/ipc.h>
> #include <sys/shm.h>
> 
> int main(void) {
>         int shmid;
>         char *execmem;
>         void (*fn)(void);
> 
>         shmid = shmget(IPC_PRIVATE, 4096, IPC_CREAT | 0777);
>         execmem = shmat(shmid, 0, SHM_EXEC);
>         shmctl(shmid, IPC_RMID, 0);
>         *execmem = 0xc3; // retq
>         fn = (void (*)(void))execmem;
>         fn();
>         shmdt(execmem);
> }
> 
> -Topi
> 

The test program fails with a seg fault and a SELinux avc denial for
execmem permission when run in a domain that lacks execmem permission.
Thanks though for the test; I'll add it to the selinux testsuite to
ensure we don't regress in this area.