x86 Fix test_poke for vmalloced pages

The shadow vmap for DEBUG_RODATA kernel text modification uses virt_to_page to get the pages from the pointer address.

However, I think vmalloc_to_page would be required in case the page is used for modules.

Since only the core kernel text is marked read-only, use kernel_text_address() to make sure we only shadow map the core kernel text, not modules.

It applies on top of the current x86 git tree.

---
 arch/x86/kernel/alternative.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: linux-2.6-x86/arch/x86/kernel/alternative.c
===================================================================
@@ -511,7 +511,7 @@
 	BUG_ON(len > sizeof(long));
 	BUG_ON((((long)addr + len - 1) & ~(sizeof(long) - 1))
 		- ((long)addr & ~(sizeof(long) - 1)));
-	{
+	if (kernel_text_address((unsigned long)addr)) {
 		struct page *pages[2] = { virt_to_page(addr),
 			virt_to_page(addr + PAGE_SIZE) };
 		if (!pages[1])

@@ -522,6 +522,13 @@
 		memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
 		local_irq_restore(flags);
 		vunmap(vaddr);
+	} else {
+		/*
+		 * modules are in vmalloc'ed memory, always writable.
+		 */
+		local_irq_save(flags);
+		memcpy(addr, opcode, len);
+		local_irq_restore(flags);
 	}
 	sync_core();
 	/* Could also do a CLFLUSH here to speed up CPU recovery; but

Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

actually, i think we should mark module text readonly too.

Ingo

I agree, then we would have to use vmalloc_to_page. I guess it has not been done before because there wasn't any mechanism such as text_poke available.

Mathieu

Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

Yes, but in the meantime, the x86 tree would need this patch to make kprobes work correctly on modules.

I suspect that without this fix, with the enhanced hotplug and kprobes patch, kprobes will use text_poke to insert breakpoints in modules (vmalloced pages used), which will map the wrong pages and corrupt random kernel locations instead of updating the correct page.

Work that would write protect the module pages should clearly be done, but it can come in a later time. We have to make sure we interact correctly with the page allocation debugging, as an example.

Here is the patch against x86.git 2.6.25-rc5 :

The shadow vmap for DEBUG_RODATA kernel text modification uses virt_to_page to get the pages from the pointer address.

However, I think vmalloc_to_page would be required in case the page is used for modules.

Since only the core kernel text is marked read-only, use kernel_text_address() to make sure we only shadow map the core kernel text, not modules.

---
 arch/x86/kernel/alternative.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: linux-2.6-x86/arch/x86/kernel/alternative.c
===================================================================
@@ -511,7 +511,7 @@
 	BUG_ON(len > sizeof(long));
 	BUG_ON((((long)addr + len - 1) & ~(sizeof(long) - 1))
 		- ((long)addr & ~(sizeof(long) - 1)));
-	{
+	if (kernel_text_address((unsigned long)addr)) {
 		struct page *pages[2] = { virt_to_page(addr),
 			virt_to_page(addr + PAGE_SIZE) };
 		if (!pages[1])

@@ -522,6 +522,13 @@
 		memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
 		local_irq_restore(flags);
 		vunmap(vaddr);
+	} else {
+		/*
+		 * modules are in vmalloc'ed memory, always writable.
+		 */
+		local_irq_save(flags);
+		memcpy(addr, opcode, len);
+		local_irq_restore(flags);
 	}
 	sync_core();
 	/* Could also do a CLFLUSH here to speed up CPU recovery; but

Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68