[RFC] reworked NX protection for kernel data

From: matthieu castet
Date: Thu Aug 12 2010 - 18:24:56 EST

Hi,

because nobody seems to work on "NX protection for kernel data" patches [1], I decided to rework them a bit :
- remove unneeded alignment in vmlinux.lds
- call mark_nxdata_nx in mark_rodata_ro instead of free_initmem
- fix a bug in my machine where memory was x and rw after kernel data
- static_protections for bios rom is only enabled when CONFIG_PCI_BIOS (no x64)
- bios rom is set to x only when probing pcibios (this won't be done is mmconfig is supported).

A second patch is more radical : it make the ROM bios memory (0xe0000-0x100000) ro and x, but only for system
supporting NX (to avoid breaking very old machine). I don't know if it is safe.

I am waiting for your comment before reposting all the patches (module and improper large page preservation)
I only test it on a 32bit NX cpu.


Matthieu

[1]
Note: this patch depends on "Correct improper large page preservation" patch

This patch expands functionality of CONFIG_DEBUG_RODATA to set main
(static) kernel data area as NX.
The following steps are taken to achieve this:
1. Linker script is adjusted so .text always starts and ends on a page bound
2. Linker script is adjusted so .rodata and .data always start and
end on a page boundary
3. void mark_nxdata_nx(void) added to arch/x86/mm/init.c with actual
functionality: NX is set for all pages from _etext through _end.
4. mark_nxdata_nx() called from free_initmem() (after init has been released
5. free_init_pages() sets released memory NX in arch/x86/mm/init.c

The results of patch application may be observed in the diff of kernel page
table dumps:
--- data_nx_pt_before.txt 2009-10-13 07:48:59.000000000 -0400
+++ data_nx_pt_after.txt 2009-10-13 07:26:46.000000000 -0400
@@ -2,8 +2,9 @@
0x00000000-0xc0000000 3G pmd
---[ Kernel Mapping ]---
0xc0000000-0xc0100000 1M RW GLB x pte
-0xc0100000-0xc048d000 3636K ro GLB x pte
-0xc048d000-0xc0600000 1484K RW GLB x pte
+0xc0100000-0xc0381000 2564K ro GLB x pte
+0xc0381000-0xc048d000 1072K ro GLB NX pte
+0xc048d000-0xc0600000 1484K RW GLB NX pte
0xc0600000-0xf7800000 882M RW PSE GLB NX pmd
0xf7800000-0xf79fe000 2040K RW GLB NX pte
0xf79fe000-0xf7a00000 8K pte

The patch have been developed for Linux 2.6.34-rc2 x86 by Siarhei Liakh
<sliakh.lkml@xxxxxxxxx> and Xuxian Jiang <jiang@xxxxxxxxxxx>.

V1: initial patch for 2.6.30
V2: patch for 2.6.31-rc7
V3: moved all code into arch/x86, adjusted credits
V4: fixed ifdef, removed credits from CREDITS
V5: fixed an address calculation bug in mark_nxdata_nx()
V6: added acked-by and PT dump diff to commit log
V7: minor adjustments for -tip

Signed-off-by: Siarhei Liakh <sliakh.lkml@xxxxxxxxx>
Signed-off-by: Xuxian Jiang <jiang@xxxxxxxxxxx>
Acked-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Reviewed-by: James Morris <jmorris@xxxxxxxxx>
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index d0bb522..958ac45 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -69,7 +69,7 @@ jiffies_64 = jiffies;

PHDRS {
text PT_LOAD FLAGS(5); /* R_E */
- data PT_LOAD FLAGS(7); /* RWE */
+ data PT_LOAD FLAGS(6); /* RW_ */
#ifdef CONFIG_X86_64
user PT_LOAD FLAGS(5); /* R_E */
#ifdef CONFIG_SMP
@@ -116,6 +116,10 @@ SECTIONS

EXCEPTION_TABLE(16) :text = 0x9090

+#if defined(CONFIG_DEBUG_RODATA)
+ /* .text should occupy whole number of pages */
+ . = ALIGN(PAGE_SIZE);
+#endif
X64_ALIGN_DEBUG_RODATA_BEGIN
RO_DATA(PAGE_SIZE)
X64_ALIGN_DEBUG_RODATA_END
@@ -307,7 +311,7 @@ SECTIONS
__bss_start = .;
*(.bss..page_aligned)
*(.bss)
- . = ALIGN(4);
+ . = ALIGN(PAGE_SIZE);
__bss_stop = .;
}

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index b278535..bfacdbe 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -362,8 +362,9 @@ void free_init_pages(char *what, unsigned long begin, unsigned long end)
/*
* We just marked the kernel text read only above, now that
* we are going to free part of that, we need to make that
- * writeable first.
+ * writeable and non-executable first.
*/
+ set_memory_nx(begin, (end - begin) >> PAGE_SHIFT);
set_memory_rw(begin, (end - begin) >> PAGE_SHIFT);

printk(KERN_INFO "Freeing %s: %luk freed\n", what, (end - begin) >> 10);
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index bca7909..85a1a75 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -225,7 +225,7 @@ 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 && addr <= (unsigned long)__init_end)
return 1;
return 0;
}
@@ -1033,6 +1033,22 @@ void set_kernel_text_ro(void)
set_pages_ro(virt_to_page(start), size >> PAGE_SHIFT);
}

+static void mark_nxdata_nx(void)
+{
+ /*
+ * When this called, init has already been executed and released,
+ * so everything past _etext sould be NX.
+ */
+ unsigned long start = PFN_ALIGN(_etext);
+ /* this comes from is_kernel_text upper limit. Also HPAGE where used */
+ unsigned long size = (((unsigned long)__init_end + HPAGE_SIZE) & HPAGE_MASK)- start;
+
+ if (__supported_pte_mask & _PAGE_NX)
+ printk(KERN_INFO "NX-protecting the kernel data: %luk\n",
+ size >> 10);
+ set_pages_nx(virt_to_page(start), size >> PAGE_SHIFT);
+}
+
void mark_rodata_ro(void)
{
unsigned long start = PFN_ALIGN(_text);
@@ -1067,6 +1083,7 @@ void mark_rodata_ro(void)
printk(KERN_INFO "Testing CPA: write protecting again\n");
set_pages_ro(virt_to_page(start), size >> PAGE_SHIFT);
#endif
+ mark_nxdata_nx();
}
#endif

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index ee41bba..0dec7f2 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -761,7 +761,7 @@ void mark_rodata_ro(void)
unsigned long start = PFN_ALIGN(_text);
unsigned long rodata_start =
((unsigned long)__start_rodata + PAGE_SIZE - 1) & PAGE_MASK;
- unsigned long end = (unsigned long) &__end_rodata_hpage_align;
+ unsigned long end = (((unsigned long)__init_end + HPAGE_SIZE) & HPAGE_MASK);
unsigned long text_end = PAGE_ALIGN((unsigned long) &__stop___ex_table);
unsigned long rodata_end = PAGE_ALIGN((unsigned long) &__end_rodata);
unsigned long data_start = (unsigned long) &_sdata;
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 91ce756..d130dc5 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -261,8 +261,10 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
* The BIOS area between 640k and 1Mb needs to be executable for
* PCI BIOS based config access (CONFIG_PCI_GOBIOS) support.
*/
+#ifdef CONFIG_PCI_BIOS
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
diff --git a/arch/x86/pci/pcbios.c b/arch/x86/pci/pcbios.c
index 2492d16..34ccb73 100644
--- a/arch/x86/pci/pcbios.c
+++ b/arch/x86/pci/pcbios.c
@@ -9,6 +9,7 @@
#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))
@@ -420,11 +421,24 @@ int pcibios_set_irq_routing(struct pci_dev *dev, int pin, int irq)
}
EXPORT_SYMBOL(pcibios_set_irq_routing);

+/* according some bios specification
+ * http://members.datafast.net.au/dft0802/specs/bios21.pdf, we could
+ * restrict the x zone to some pages and make it ro. But we
+ * really don't care, processor with NX support should be able to use
+ * mmconfig for pci that disable pcibios...
+ */
+static inline void set_bios_x(void)
+{
+ set_memory_x(PAGE_OFFSET + BIOS_BEGIN, (BIOS_END - BIOS_BEGIN) >> PAGE_SHIFT);
+}
+
void __init pci_pcbios_init(void)
{
- if ((pci_probe & PCI_PROBE_BIOS)
- && ((raw_pci_ops = pci_find_bios()))) {
- pci_bios_present = 1;
+ if (pci_probe & PCI_PROBE_BIOS) {
+ set_bios_x();
+ if ((raw_pci_ops = pci_find_bios())) {
+ pci_bios_present = 1;
+ }
}
}

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index d130dc5..5fd5f4d 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -258,12 +258,13 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
pgprot_t required = __pgprot(0);

/*
- * The BIOS area between 640k and 1Mb needs to be executable for
+ * The BIOS area between 896k and 1Mb (ROM) needs to be executable for
* PCI BIOS based config access (CONFIG_PCI_GOBIOS) support.
*/
#ifdef CONFIG_PCI_BIOS
- if (within(pfn, BIOS_BEGIN >> PAGE_SHIFT, BIOS_END >> PAGE_SHIFT))
- pgprot_val(forbidden) |= _PAGE_NX;
+ if ((__supported_pte_mask & _PAGE_NX) &&
+ within(pfn, 0xe0000 >> PAGE_SHIFT, BIOS_END >> PAGE_SHIFT))
+ pgprot_val(forbidden) |= _PAGE_NX|_PAGE_RW;
#endif

/*
diff --git a/arch/x86/pci/pcbios.c b/arch/x86/pci/pcbios.c
index 34ccb73..ac35d6f 100644
--- a/arch/x86/pci/pcbios.c
+++ b/arch/x86/pci/pcbios.c
@@ -11,6 +11,7 @@
#include <asm/pci-functions.h>
#include <asm/cacheflush.h>

+#define ROM_BEGIN 0xe0000
/* BIOS32 signature: "_32_" */
#define BIOS32_SIGNATURE (('_' << 0) + ('3' << 8) + ('2' << 16) + ('_' << 24))

@@ -301,7 +302,7 @@ static struct pci_raw_ops * __devinit pci_find_bios(void)
* 0xe0000 through 0xfffff for a valid BIOS32 structure.
*/

- for (check = (union bios32 *) __va(0xe0000);
+ for (check = (union bios32 *) __va(ROM_BEGIN);
check <= (union bios32 *) __va(0xffff0);
++check) {
long sig;
@@ -424,18 +425,20 @@ EXPORT_SYMBOL(pcibios_set_irq_routing);
/* according some bios specification
* http://members.datafast.net.au/dft0802/specs/bios21.pdf, we could
* restrict the x zone to some pages and make it ro. But we
- * really don't care, processor with NX support should be able to use
- * mmconfig for pci that disable pcibios...
+ * really don't care, system rom should always be ro and x.
*/
-static inline void set_bios_x(void)
+static inline void set_bios_rox(void)
{
- set_memory_x(PAGE_OFFSET + BIOS_BEGIN, (BIOS_END - BIOS_BEGIN) >> PAGE_SHIFT);
+ if (__supported_pte_mask & _PAGE_NX) {
+ set_memory_ro(PAGE_OFFSET + ROM_BEGIN, (BIOS_END - ROM_BEGIN) >> PAGE_SHIFT);
+ set_memory_x(PAGE_OFFSET + ROM_BEGIN, (BIOS_END - ROM_BEGIN) >> PAGE_SHIFT);
+ }
}

void __init pci_pcbios_init(void)
{
if (pci_probe & PCI_PROBE_BIOS) {
- set_bios_x();
+ set_bios_rox();
if ((raw_pci_ops = pci_find_bios())) {
pci_bios_present = 1;
}