2016-10-26 20:32:15

by Topi Miettinen

Subject: selinux: should execmem disable shmat(..., SHM_EXEC)?

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

2016-10-27 14:28:45

by Stephen Smalley

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.