Re: [PATCH v2 0/6] x86/alternatives: text_poke() fixes

[Nadav Amit]

at 10:17 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Thu, Sep 06, 2018 at 05:01:25PM +0000, Nadav Amit wrote:
>> In addition, there might be a couple of issues with your fix:
> 
> It boots on my box ;-)
> 
>> 1. __set_pte_vaddr() is not used exclusive by set_fixmap(). This means
>> the warning might be wrong, but also means that these code patches (Xenâs
>> set_pte_mfn(), CPU-entry-area setup) needs to be checked. And as you said
>> before, someone might use this function for other purposes as well.
> 
> CEA is fine, that actually needs it too.
> 
> The one thing I missed out on earlier is the below chunk, that is no
> longer needed now that cea_set_pte() actually does the right thing.
> 
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index b7b01d762d32..14ad97fa0749 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -293,12 +293,6 @@ static void ds_update_cea(void *cea, void *addr, size_t size, pgprot_t prot)
> preempt_disable();
> for (; msz < size; msz += PAGE_SIZE, pa += PAGE_SIZE, cea += PAGE_SIZE)
> cea_set_pte(cea, pa, prot);
> -
> - /*
> -  * This is a cross-CPU update of the cpu_entry_area, we must shoot down
> -  * all TLB entries for it.
> -  */
> - flush_tlb_kernel_range(start, start + size);
> preempt_enable();
> }
> 
> @@ -310,8 +304,6 @@ static void ds_clear_cea(void *cea, size_t size)
> preempt_disable();
> for (; msz < size; msz += PAGE_SIZE, cea += PAGE_SIZE)
> cea_set_pte(cea, 0, PAGE_NONE);
> -
> - flush_tlb_kernel_range(start, start + size);
> preempt_enable();
> }
> 
> 
>> 2. Printing the virtual address can break KASLR.
> 
> Local KASLR is a myth.. but sure, we can fix the print.
> 
>> 3. The WARN() can introduce some overhead since checking if IRQs are
>> disabled takes considerably long time. Perhaps VM_WARN() or something is
>> better. I realize this code-path is not on the hot-path though...
> 
> Yeah, if it triggers you have bigger problems. We can make it
> WARN_ONCE() I suppose.
> 
>> 4. I guess flush_tlb_kernel_range() should also have something like
>> VM_WARN_ON(irqs_disabled()), just as an additional general sanity check.
> 
> It has, it's hidden in kernel/smp.c:smp_call_function_many().

Right. Thanks.

> 
>> Let me know if you want me to make these changes and include your patch in
>> the set.
> 
> The set is no longer needed. text_poke() is fine and correct with this
> one patch.

It depends what security you want. Some may consider even the short
time-window in which the kernel code is writable from other cores as
insufficient for security.

In addition, the set removes the need for remote TLB shootdowns that
text_poke() - with this fix - requires.