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

commit 0909c0ae999c325b9d34c6f4710f40730ae3bc24 upstream.

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.

Change-Id: Id66f2fe0ef85b7529a74065ee794ce240c2db659
Signed-off-by: Paul Moore <pmoore@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>

1 files changed, 8 insertions, 12 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 6e45746704b2..edf2ecc51645 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3269,24 +3269,20 @@ error:
 
 static int selinux_mmap_addr(unsigned long addr)
 {
-	int rc = 0;
-	u32 sid = current_sid();
+	int rc;
+
+	/* do DAC check on address space usage */
+	rc = cap_mmap_addr(addr);
+	if (rc)
+		return rc;
 
-	/*
-	 * 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) {
+		u32 sid = current_sid();
 		rc = avc_has_perm(sid, sid, SECCLASS_MEMPROTECT,
 				  MEMPROTECT__MMAP_ZERO, NULL);
-		if (rc)
-			return rc;
 	}
 
-	/* do DAC check on address space usage */
-	return cap_mmap_addr(addr);
+	return rc;
 }
 
 static int selinux_mmap_file(struct file *file, unsigned long reqprot,