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

From: Ingo Molnar
Date: Fri Apr 25 2008 - 12:35:24 EST



* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxx>
Date: Fri Apr 25 17:07:03 CEST 2008

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
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);
--
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