selinux: put the mmap() DAC controls before the MAC controls

It turns out that doing the SELinux MAC checks for mmap() before the
DAC checks was causing users and the SELinux policy folks headaches
as users were seeing a lot of SELinux AVC denials for the
memprotect:mmap_zero permission that would have also been denied by
the normal DAC capability checks (CAP_SYS_RAWIO).

Example:

 # cat mmap_test.c
  #include <stdlib.h>
  #include <stdio.h>
  #include <errno.h>
  #include <sys/mman.h>

  int main(int argc, char *argv[])
  {
        int rc;
        void *mem;

        mem = mmap(0x0, 4096,
                   PROT_READ | PROT_WRITE,
                   MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
        if (mem == MAP_FAILED)
                return errno;
        printf("mem = %p\n", mem);
        munmap(mem, 4096);

        return 0;
  }
 # gcc -g -O0 -o mmap_test mmap_test.c
 # ./mmap_test
 mem = (nil)
 # ausearch -m AVC | grep mmap_zero
 type=AVC msg=audit(...): avc:  denied  { mmap_zero }
   for pid=1025 comm="mmap_test"
   scontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
   tcontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
   tclass=memprotect

This patch corrects things so that when the above example is run by a
user without CAP_SYS_RAWIO the SELinux AVC is no longer generated as
the DAC capability check fails before the SELinux permission check.

Signed-off-by: Paul Moore <pmoore@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>

security/selinux/hooks.c

@@ -3204,24 +3204,20 @@ error:
 
 static int selinux_mmap_addr(unsigned long addr)
 {
-	int rc = 0;
-	u32 sid = current_sid();
-
-	/*
-	 * notice that we are intentionally putting the SELinux check before
-	 * the secondary cap_file_mmap check.  This is such a likely attempt
-	 * at bad behaviour/exploit that we always want to get the AVC, even
-	 * if DAC would have also denied the operation.
-	 */
-	if (addr < CONFIG_LSM_MMAP_MIN_ADDR) {
-		rc = avc_has_perm(sid, sid, SECCLASS_MEMPROTECT,
-				  MEMPROTECT__MMAP_ZERO, NULL);
-		if (rc)
-			return rc;
-	}
+	int rc;
 
 	/* do DAC check on address space usage */
-	return cap_mmap_addr(addr);
+	rc = cap_mmap_addr(addr);
+	if (rc)
+		return rc;
+
+	if (addr < CONFIG_LSM_MMAP_MIN_ADDR) {
+		u32 sid = current_sid();
+		rc = avc_has_perm(sid, sid, SECCLASS_MEMPROTECT,
+				  MEMPROTECT__MMAP_ZERO, NULL);
+	}
+
+	return rc;
 }
 
 static int selinux_mmap_file(struct file *file, unsigned long reqprot,