[PATCH] arm64: uprobes: Optimize cache flushes for xol slot

[Liao, Chang]

在 2024/9/20 23:32, Catalin Marinas 写道:
> On Fri, Sep 20, 2024 at 04:58:31PM +0800, Liao, Chang wrote:
>>
>>
>> 在 2024/9/19 22:18, Oleg Nesterov 写道:
>>> On 09/19, Liao Chang wrote:
>>>>
>>>> --- a/arch/arm64/kernel/probes/uprobes.c
>>>> +++ b/arch/arm64/kernel/probes/uprobes.c
>>>> @@ -17,12 +17,16 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
>>>>  	void *xol_page_kaddr = kmap_atomic(page);
>>>>  	void *dst = xol_page_kaddr + (vaddr & ~PAGE_MASK);
>>>>
>>>> +	if (!memcmp(dst, src, len))
>>>> +		goto done;
>>>
>>> can't really comment, I know nothing about arm64...
>>>
>>> but don't we need to change __create_xol_area()
>>>
>>> 	-	area->page = alloc_page(GFP_HIGHUSER);
>>> 	+	area->page = alloc_page(GFP_HIGHUSER | __GFP_ZERO);
>>>
>>> to avoid the false positives?
>>
>> Indeed, it would be safer.
>>
>> Could we tolerate these false positives? Even if the page are not reset
>> to zero bits, if the existing bits are the same as the instruction being
>> copied, it still can execute the correct instruction.
> 
> Not if the I-cache has stale data. If alloc_page() returns a page with
> some random data that resembles a valid instruction but there was never
> a cache flush (sync_icache_aliases() on arm64), it's irrelevant whether
> the compare (on the D-cache side) succeeds or not.

Absolutly right, I overlooked the comparsion is still performed in the D-cache.
However, the most important thing is ensuring the I-cache sees the accurate bits,
which is why a cache flush in necessary for each xol slot.

> 
> I think using __GFP_ZERO should do the trick. All 0s is a permanently
> undefined instruction, not something we'd use with xol.

Unfortunately, the comparison assumes the D-cache and I-cache are already
in sync for the slot being copied. But this assumption is flawed if we start
with a page with some random bits and D-cache has not been sychronized with
I-cache. So, besides __GFP_ZERO, should we have a additional cache flush
after page allocation?

> 

-- 
BR
Liao, Chang