[PATCH 2/4] [tip:x86/mm] Set first MB as RW+NX

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This patch ensures that there are no mixed (RW+X) pages mapped within
first megabyte of kernel space (from PAGE_OFFSET up to beginning of .text).
This is achieved through introduction of new compile-time option
CONFIG_DEBUG_SET_1ST_MB_RWNX which controls how is_kernel_text()
and static_protections() view the memory area in question.

Special accommodations have been made for BIOS32/PCI BIOS services:
according to BIOS32 specification
(http://members.datafast.net.au/dft0802/specs/bios32.pdf), at most two
pages per BIOS32 service should be set executable and no pages need to
be writeable. This patch modifies bios32_service() to set proper page
access permissions at time of service discovery, as described in the
specification. Further, hardcoded protection of memory area between 640k
to 1Mb have been removed from static_protections(), since only pages
mentioned above need to be executable, not whole BIOS region.

The net result of the patch application is as follows:
========== BEFORE ==========
0xc0000000-0xc0100000           1M     RW             GLB x  pte
0xc0100000-0xc04b6000        3800K     ro             GLB x  pte
...

=========== AFTER ===========
0xc0000000-0xc00fb000        1004K     RW             GLB NX pte
0xc00fb000-0xc00fd000           8K     ro             GLB x  pte
0xc00fd000-0xc0100000          12K     RW             GLB NX pte
0xc0100000-0xc04b6000        3800K     ro             GLB x  pte
...

Note that first MB is now RW+NX, except for 8K of RO+X used by BIOS32.

The patch have been developed for Linux 2.6.33-rc5 x86 by Siarhei Liakh
<sliakh.lkml@xxxxxxxxx> and Xuxian Jiang <jiang@xxxxxxxxxxx>.

V1: original patch "Reducing footprint of BIOS32 service mappings"
V2: Set first MB as RW+NX for linux 2.6.33-rc5
V3: minor adjustments for -tip

Signed-off-by: Siarhei Liakh <sliakh.lkml@xxxxxxxxx>
Signed-off-by: Xuxian Jiang <jiang@xxxxxxxxxxx>
---

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 4814d35..6dbcb6f 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -123,6 +123,17 @@ config DEBUG_NX_TEST
         and the software setup of this feature.
         If in doubt, say "N"

+config DEBUG_SET_1ST_MB_RWNX
+       bool "Set 1st MB of kernel space mapping as RW+NX"
+       default n
+       depends on X86
+       ---help---
+         This option helps to catch unintended modifications/execution of
+         1st megabyte of kernel address space. Such protection may interfere
+         with run-time code patching and legacy BIOS services residing within
+         the 1st MB of physical address space.
+         If in doubt, say "N".
+
 config 4KSTACKS
       bool "Use 4Kb for kernel stacks instead of 8Kb"
       depends on X86_32
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index 5cb3f0f..22ccf08 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -52,6 +52,12 @@
 #include <asm/page_types.h>
 #include <asm/init.h>

+#ifdef CONFIG_DEBUG_SET_1ST_MB_RWNX
+#define TEXT_START _text
+#else
+#define TEXT_START PAGE_OFFSET
+#endif
+
 unsigned long highstart_pfn, highend_pfn;

 static noinline int do_test_wp_bit(void);
@@ -225,7 +231,8 @@ page_table_range_init(unsigned long start,
unsigned long end, pgd_t *pgd_base)

 static inline int is_kernel_text(unsigned long addr)
 {
-       if (addr >= PAGE_OFFSET && addr <= (unsigned long)__init_end)
+       if (addr >= (unsigned long)TEXT_START
+               && addr <= (unsigned long)__init_end)
               return 1;
       return 0;
 }
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index cf07c26..595398e 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -256,13 +256,14 @@ static inline pgprot_t
static_protections(pgprot_t prot, unsigned long address,
 {
       pgprot_t forbidden = __pgprot(0);

+#ifndef CONFIG_DEBUG_SET_1ST_MB_RWNX
       /*
        * The BIOS area between 640k and 1Mb needs to be executable for
        * PCI BIOS based config access (CONFIG_PCI_GOBIOS) support.
        */
       if (within(pfn, BIOS_BEGIN >> PAGE_SHIFT, BIOS_END >> PAGE_SHIFT))
               pgprot_val(forbidden) |= _PAGE_NX;
-
+#endif
       /*
        * The kernel text needs to be executable for obvious reasons
        * Does not cover __inittext since that is gone later on. On
diff --git a/arch/x86/pci/pcbios.c b/arch/x86/pci/pcbios.c
index 1c975cc..ec3b3db 100644
--- a/arch/x86/pci/pcbios.c
+++ b/arch/x86/pci/pcbios.c
@@ -8,6 +8,7 @@
 #include <linux/uaccess.h>
 #include <asm/pci_x86.h>
 #include <asm/pci-functions.h>
+#include <asm/cacheflush.h>

 /* BIOS32 signature: "_32_" */
 #define BIOS32_SIGNATURE       (('_' << 0) + ('3' << 8) + ('2' << 16)
+ ('_' << 24))
@@ -58,6 +59,16 @@ static struct {
       unsigned short segment;
 } bios32_indirect = { 0, __KERNEL_CS };

+/* Set two consecutive pages as read-only and executable */
+void set_2_pages_rox(unsigned long addr)
+{
+#ifdef CONFIG_DEBUG_SET_1ST_MB_RWNX
+       unsigned long pg_address = PFN_DOWN(addr + PAGE_OFFSET) << PAGE_SHIFT;
+       set_memory_ro(pg_address, 2);
+       set_memory_x(pg_address, 2);
+#endif
+}
+
 /*
 * Returns the entry point for the given service, NULL on error
 */
@@ -82,15 +93,19 @@ static unsigned long bios32_service(unsigned long service)
       local_irq_restore(flags);

       switch (return_code) {
-               case 0:
-                       return address + entry;
-               case 0x80:      /* Not present */
-                       printk(KERN_WARNING "bios32_service(0x%lx):
not present\n", service);
-                       return 0;
-               default: /* Shouldn't happen */
-                       printk(KERN_WARNING "bios32_service(0x%lx):
returned 0x%x -- BIOS bug!\n",
-                               service, return_code);
-                       return 0;
+       case 0: /* Service present, set proper page access */
+               address += entry;
+               set_2_pages_rox(address);
+               return address;
+       case 0x80: /* Not present */
+               printk(KERN_WARNING "bios32_service(0x%lx): not present\n",
+                       service);
+               return 0;
+       default: /* Shouldn't happen */
+               printk(KERN_WARNING "bios32_service(0x%lx): "
+                       "returned 0x%x -- BIOS bug!\n",
+                       service, return_code);
+               return 0;
       }
 }

@@ -331,6 +346,7 @@ static struct pci_raw_ops * __devinit pci_find_bios(void)
                       DBG("PCI: BIOS32 Service Directory entry at 0x%lx\n",
                                       bios32_entry);
                       bios32_indirect.address = bios32_entry + PAGE_OFFSET;
+                       set_2_pages_rox(bios32_entry);
                       if (check_pcibios())
                               return &pci_bios_access;
               }
--
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