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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

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

-- 
Catalin