[PATCH] selinux: add executable stack check

This patch adds an execstack permission check that controls the ability to
make the main process stack executable so that attempts to make the stack
executable can still be prevented even if the process is allowed the
existing execmem permission in order to e.g. perform runtime code
generation. Note that this does not yet address thread stacks. Note also
that unlike the execmem check, the execstack check is only applied on
mprotect calls, not mmap calls, as the current security_file_mmap hook is
not passed the necessary information presently.

The original author of the code that makes the distinction of the stack
region, is Ingo Molnar, who wrote it within his patch for
/proc/<pid>/maps markers.
(http://marc.theaimsgroup.com/?l=linux-kernel&m=110719881508591&w=2)

The patches also can be found at:
http://pearls.tuxedo-es.org/patches/selinux/policy-execstack.patch
http://pearls.tuxedo-es.org/patches/selinux/kernel-execstack.patch

policy-execstack.patch is the patch that needs to be applied to the policy in
order to support the execstack permission and exclude it
from general_domain_access within macros/core_macros.te.

kernel-execstack.patch adds such permission to the SELinux code within
the kernel and adds the proper permission check to the selinux_file_mprotect() hook.

Signed-off-by: Lorenzo Hernandez Garcia-Hierro <[e-mail]>
Acked-by: James Morris <[e-mail]>
Acked-by: Stephen Smalley <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

security/selinux/hooks.c

···
 		if (rc)
 			return rc;
 	}
+	if (!vma->vm_file && (prot & PROT_EXEC) &&
+		vma->vm_start <= vma->vm_mm->start_stack &&
+		vma->vm_end >= vma->vm_mm->start_stack) {
+		/* Attempt to make the process stack executable.
+		 * This has an additional execstack check.
+		 */
+		rc = task_has_perm(current, current, PROCESS__EXECSTACK);
+		if (rc)
+			return rc;
+	}
 #endif
 
 	return file_map_prot_check(vma->vm_file, prot, vma->vm_flags&VM_SHARED);

security/selinux/include/av_perm_to_string.h

···
    S_(SECCLASS_PROCESS, PROCESS__DYNTRANSITION, "dyntransition")
    S_(SECCLASS_PROCESS, PROCESS__SETCURRENT, "setcurrent")
    S_(SECCLASS_PROCESS, PROCESS__EXECMEM, "execmem")
+   S_(SECCLASS_PROCESS, PROCESS__EXECSTACK, "execstack")
    S_(SECCLASS_MSGQ, MSGQ__ENQUEUE, "enqueue")
    S_(SECCLASS_MSG, MSG__SEND, "send")
    S_(SECCLASS_MSG, MSG__RECEIVE, "receive")

security/selinux/include/av_permissions.h

···
 #define PROCESS__DYNTRANSITION                    0x00800000UL
 #define PROCESS__SETCURRENT                       0x01000000UL
 #define PROCESS__EXECMEM                          0x02000000UL
+#define PROCESS__EXECSTACK                        0x04000000UL
 
 #define IPC__CREATE                               0x00000001UL
 #define IPC__DESTROY                              0x00000002UL