Re: [PATCH 1/1] x86: fix text_poke

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > Not sure how the fixmap is better. It's pretty much equivalent, 
> > isn't it? Perhaps a little cheaper, but the code shouldn't be 
> > performance critical.
> 
> I have no really strong opinions. However, we do have a *lot* of lock 
> prefixes in the kernel, and fixmaps are a lot cheaper than vmap(). It 
> may not be performance-critical, but for me the "locks" section for 
> the kernel is 0x8060 bytes long, which would seem to say that this is 
> called four thousand times for each suspend and resume.
> 
> With each invocation being thousands of instructions and a cross-CPU 
> IPI for the tlb flush, that kind of stuff adds up. We're likely 
> talking real fractions of a second, rather than milliseconds.

the other thing is atomicity - your new version of text_poke() is 
evidently atomic - while vmap() does a kmalloc which might sleep. 
Atomicity for something as fragile as code-patching never hurts, so i 
definitely like your version more.

it's also the more familar API - set_fixmap() is used more frequently 
than vmap() - hence less danger of doing something wrong.

in fact i'd do the extra sanity check below as well on top of your patch 
- all core kernel text pages are PageReserved so the one below would 
have caught the memory corruption right at its source.

	Ingo

---------------->
Subject: x86: harden kernel code patching
From: Ingo Molnar <mingo@elte.hu>
Date: Fri Apr 25 17:07:03 CEST 2008

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/kernel/alternative.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux/arch/x86/kernel/alternative.c
===================================================================
--- linux.orig/arch/x86/kernel/alternative.c
+++ linux/arch/x86/kernel/alternative.c
@@ -522,6 +522,8 @@ void *__kprobes text_poke(void *addr, co
 		unsigned long virt = fix_to_virt(FIX_POKE);
 		phys &= PAGE_MASK;
 
+		WARN_ON(!PageReserved(virt_to_page(addr)));
+
 		spin_lock_irqsave(&poke_lock, flags);
 		set_fixmap(FIX_POKE, phys);
 		memcpy((void *)(virt + offset), opcode, len);