svn commit: r197537 - head/sys/vm

[Simon L. Nielsen]

Author: simon
Date: Sun Sep 27 14:49:51 2009
New Revision: 197537
URL: http://svn.freebsd.org/changeset/base/197537

Log:
  Do not allow mmap with the MAP_FIXED argument to map at address zero.
  This is done to make it harder to exploit kernel NULL pointer security
  vulnerabilities.  While this of course does not fix vulnerabilities,
  it does mitigate their impact.
  
  Note that this may break some applications, most likely emulators or
  similar, which for one reason or another require mapping memory at
  zero.
  
  This restriction can be disabled with the security.bsd.mmap_zero
  sysctl variable.
  
  Discussed with:	rwatson, bz
  Tested by:	bz (Wine), simon (VirtualBox)
  Submitted by:	jhb

Modified:
  head/sys/vm/vm_mmap.c

Modified: head/sys/vm/vm_mmap.c
==============================================================================
--- head/sys/vm/vm_mmap.c	Sun Sep 27 14:00:16 2009	(r197536)
+++ head/sys/vm/vm_mmap.c	Sun Sep 27 14:49:51 2009	(r197537)
@@ -97,6 +97,14 @@ SYSCTL_INT(_vm, OID_AUTO, max_proc_mmap,
     "Maximum number of memory-mapped files per process");
 
 /*
+ * 'mmap_zero' determines whether or not MAP_FIXED mmap() requests for
+ * virtual address zero are permitted.
+ */
+static int mmap_zero;
+SYSCTL_INT(_security_bsd, OID_AUTO, mmap_zero, CTLFLAG_RW, &mmap_zero, 0,
+    "Processes may map an object at virtual address zero");
+
+/*
  * Set the maximum number of vm_map_entry structures per process.  Roughly
  * speaking vm_map_entry structures are tiny, so allowing them to eat 1/100
  * of our KVM malloc space still results in generous limits.  We want a
@@ -229,7 +237,8 @@ mmap(td, uap)
 	pos = uap->pos;
 
 	fp = NULL;
-	/* make sure mapping fits into numeric range etc */
+
+	/* Make sure mapping fits into numeric range, etc. */
 	if ((uap->len == 0 && !SV_CURPROC_FLAG(SV_AOUT) &&
 	     curproc->p_osrel >= 800104) ||
 	    ((flags & MAP_ANON) && uap->fd != -1))
@@ -267,6 +276,14 @@ mmap(td, uap)
 		addr -= pageoff;
 		if (addr & PAGE_MASK)
 			return (EINVAL);
+
+		/*
+		 * Mapping to address zero is only permitted if
+		 * mmap_zero is enabled.
+		 */
+		if (addr == 0 && !mmap_zero)
+			return (EINVAL);
+
 		/* Address range must be all in user VM space. */
 		if (addr < vm_map_min(&vms->vm_map) ||
 		    addr + size > vm_map_max(&vms->vm_map))