Subject: [PATCH 3/4 V7] [tip:x86/mm] NX protection for kernel data

From: Siarhei Liakh <[email-masked]>
Date: Sun, 06 Jun 2010 21:01:18 -0400

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 boundary
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
<[email-masked]> and Xuxian Jiang <[email-masked]>.

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 <[email-masked]>
Signed-off-by: Xuxian Jiang <[email-masked]>
Acked-by: Arjan van de Ven <[email-masked]>
Reviewed-by: James Morris <[email-masked]>
---

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 2cc2497..7f82438 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
@@ -108,6 +108,8 @@ SECTIONS
 		IRQENTRY_TEXT
 		*(.fixup)
 		*(.gnu.warning)
+		/* .text should occupy whole number of pages */
+		. = ALIGN(PAGE_SIZE);
 		/* End of text section */
 		_etext = .;
 	} :text = 0x9090
@@ -121,6 +123,7 @@ SECTIONS
 	X64_ALIGN_DEBUG_RODATA_END
 
 	/* Data */
+	. = ALIGN(PAGE_SIZE);
 	.data : AT(ADDR(.data) - LOAD_OFFSET) {
 		/* Start of data section */
 		_sdata = .;
@@ -143,6 +146,8 @@ SECTIONS
 		/* rarely changed data like cpu maps */
 		READ_MOSTLY_DATA(INTERNODE_CACHE_BYTES)
 
+		/* .data should occupy whole number of pages */
+		. = ALIGN(PAGE_SIZE);
 		/* End of data section */
 		_edata = .;
 	} :data
@@ -307,7 +312,7 @@ 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 452ee5b..240dcb0 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -361,8 +361,9 @@ 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);
@@ -377,11 +378,29 @@ void free_init_pages(char *what, unsigned long begin, unsigned long end)
 #endif
 }
 
+void mark_nxdata_nx(void)
+{
+#ifdef CONFIG_DEBUG_RODATA
+	/*
+	 * When this called, init has already been executed and released,
+	 * so everything past _etext sould be NX.
+	 */
+	unsigned long start = PAGE_ALIGN((unsigned long)(&_etext));
+	unsigned long size = PAGE_ALIGN((unsigned long)(&_end)) - start;
+
+	printk(KERN_INFO "NX-protecting the kernel data: %lx, %lu pages\n",
+		start, size >> PAGE_SHIFT);
+	set_memory_nx(start, size >> PAGE_SHIFT);
+#endif
+}
+
 void free_initmem(void)
 {
 	free_init_pages("unused kernel memory",
 			(unsigned long)(&__init_begin),
 			(unsigned long)(&__init_end));
+	/* Set kernel's data as NX */
+	mark_nxdata_nx();
 }
 
 #ifdef CONFIG_BLK_DEV_INITRD