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

[Will Deacon <will@kernel.org>]

On Mon, Sep 23, 2024 at 09:57:14AM +0800, Liao, Chang wrote:
> 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?

No, I think Oleg's right. The initial cache maintenance will happen when the
executable pte is installed. However, we should use __GFP_ZERO anyway
because I don't think it's a good idea to map an uninitialised page into
userspace.

Will