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

[Oleg Nesterov <oleg@redhat.com>]

On 09/20, Catalin Marinas wrote:
>
> 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.

But shouldn't the page fault paths on arm64 flush I-cache ?

If alloc_page() returns a page with some random data that resembles a valid
instruction, user-space can't execute this instruction until
special_mapping_fault() installs the page allocated in __create_xol_area().

Again, I know nothing about arm64/icache/etc, I am just curious and trying
to understand...

Oleg.