Re: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE_PTR to point at unmapped space)

[Christoph Lameter <cl@linux-foundation.org>]

On Wed, 3 Jun 2009, Eric Paris wrote:

> The 'right'est fix is as Alan suggested, duplicate the code
>
> from security/capability.c::cap_file_mmap()
> to include/linux/security.h::securitry_file_mmap()

Thats easy to do but isnt it a bit weird now to configure mmap_min_addr?
A security model may give it a different interpretation?
What about round_hint_to_min()?


Use mmap_min_addr indepedently of security models

This patch removes the dependency of mmap_min_addr on CONFIG_SECURITY.
It also sets a default mmap_min_addr of 4096.

mmapping of addresses below 4096 will only be possible for processes
with CAP_SYS_RAWIO.


Signed-off-by: Christoph Lameter <cl@linux-foundation.org>

---
 include/linux/mm.h       |    2 --
 include/linux/security.h |    2 ++
 kernel/sysctl.c          |    2 --
 mm/Kconfig               |   19 +++++++++++++++++++
 mm/mmap.c                |    3 +++
 security/Kconfig         |   20 --------------------
 security/security.c      |    3 ---
 7 files changed, 24 insertions(+), 27 deletions(-)

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h	2009-06-03 15:00:54.000000000 -0500
+++ linux-2.6/include/linux/mm.h	2009-06-03 15:00:56.000000000 -0500
@@ -580,12 +580,10 @@ static inline void set_page_links(struct
  */
 static inline unsigned long round_hint_to_min(unsigned long hint)
 {
-#ifdef CONFIG_SECURITY
 	hint &= PAGE_MASK;
 	if (((void *)hint != NULL) &&
 	    (hint < mmap_min_addr))
 		return PAGE_ALIGN(mmap_min_addr);
-#endif
 	return hint;
 }

Index: linux-2.6/kernel/sysctl.c
===================================================================
--- linux-2.6.orig/kernel/sysctl.c	2009-06-03 15:00:54.000000000 -0500
+++ linux-2.6/kernel/sysctl.c	2009-06-03 15:00:56.000000000 -0500
@@ -1225,7 +1225,6 @@ static struct ctl_table vm_table[] = {
 		.strategy	= &sysctl_jiffies,
 	},
 #endif
-#ifdef CONFIG_SECURITY
 	{
 		.ctl_name	= CTL_UNNUMBERED,
 		.procname	= "mmap_min_addr",
@@ -1234,7 +1233,6 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_doulongvec_minmax,
 	},
-#endif
 #ifdef CONFIG_NUMA
 	{
 		.ctl_name	= CTL_UNNUMBERED,
Index: linux-2.6/mm/mmap.c
===================================================================
--- linux-2.6.orig/mm/mmap.c	2009-06-03 15:00:54.000000000 -0500
+++ linux-2.6/mm/mmap.c	2009-06-03 15:01:18.000000000 -0500
@@ -87,6 +87,9 @@ int sysctl_overcommit_ratio = 50;	/* def
 int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
 struct percpu_counter vm_committed_as;

+/* amount of vm to protect from userspace access */
+unsigned long mmap_min_addr = CONFIG_DEFAULT_MMAP_MIN_ADDR;
+
 /*
  * Check that a process has enough memory to allocate a new virtual
  * mapping. 0 means there is enough memory for the allocation to
Index: linux-2.6/security/security.c
===================================================================
--- linux-2.6.orig/security/security.c	2009-06-03 15:00:54.000000000 -0500
+++ linux-2.6/security/security.c	2009-06-03 15:00:56.000000000 -0500
@@ -26,9 +26,6 @@ extern void security_fixup_ops(struct se

 struct security_operations *security_ops;	/* Initialized to NULL */

-/* amount of vm to protect from userspace access */
-unsigned long mmap_min_addr = CONFIG_SECURITY_DEFAULT_MMAP_MIN_ADDR;
-
 static inline int verify(struct security_operations *ops)
 {
 	/* verify the security_operations structure exists */
Index: linux-2.6/mm/Kconfig
===================================================================
--- linux-2.6.orig/mm/Kconfig	2009-06-03 15:00:54.000000000 -0500
+++ linux-2.6/mm/Kconfig	2009-06-03 15:00:56.000000000 -0500
@@ -226,6 +226,25 @@ config HAVE_MLOCKED_PAGE_BIT
 config MMU_NOTIFIER
 	bool

+config DEFAULT_MMAP_MIN_ADDR
+        int "Low address space to protect from user allocation"
+        default 4096
+        help
+	  This is the portion of low virtual memory which should be protected
+	  from userspace allocation.  Keeping a user from writing to low pages
+	  can help reduce the impact of kernel NULL pointer bugs.
+
+	  For most ia64, ppc64 and x86 users with lots of address space
+	  a value of 65536 is reasonable and should cause no problems.
+	  On arm and other archs it should not be higher than 32768.
+	  Programs which use vm86 functionality would either need additional
+	  permissions from either the LSM or the capabilities module or have
+	  this protection disabled.
+
+	  This value can be changed after boot using the
+	  /proc/sys/vm/mmap_min_addr tunable.
+
+
 config NOMMU_INITIAL_TRIM_EXCESS
 	int "Turn on mmap() excess space trimming before booting"
 	depends on !MMU
Index: linux-2.6/security/Kconfig
===================================================================
--- linux-2.6.orig/security/Kconfig	2009-06-03 15:00:54.000000000 -0500
+++ linux-2.6/security/Kconfig	2009-06-03 15:00:56.000000000 -0500
@@ -113,26 +113,6 @@ config SECURITY_ROOTPLUG

 	  If you are unsure how to answer this question, answer N.

-config SECURITY_DEFAULT_MMAP_MIN_ADDR
-        int "Low address space to protect from user allocation"
-        depends on SECURITY
-        default 0
-        help
-	  This is the portion of low virtual memory which should be protected
-	  from userspace allocation.  Keeping a user from writing to low pages
-	  can help reduce the impact of kernel NULL pointer bugs.
-
-	  For most ia64, ppc64 and x86 users with lots of address space
-	  a value of 65536 is reasonable and should cause no problems.
-	  On arm and other archs it should not be higher than 32768.
-	  Programs which use vm86 functionality would either need additional
-	  permissions from either the LSM or the capabilities module or have
-	  this protection disabled.
-
-	  This value can be changed after boot using the
-	  /proc/sys/vm/mmap_min_addr tunable.
-
-
 source security/selinux/Kconfig
 source security/smack/Kconfig
 source security/tomoyo/Kconfig
Index: linux-2.6/include/linux/security.h
===================================================================
--- linux-2.6.orig/include/linux/security.h	2009-06-03 15:01:28.000000000 -0500
+++ linux-2.6/include/linux/security.h	2009-06-03 15:01:42.000000000 -0500
@@ -2197,6 +2197,8 @@ static inline int security_file_mmap(str
 				     unsigned long addr,
 				     unsigned long addr_only)
 {
+	if ((addr < mmap_min_addr) && !capable(CAP_SYS_RAWIO))
+		return -EACCES;
 	return 0;
 }

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