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

[Oleg Nesterov <oleg@redhat.com>]

On 09/22, Will Deacon wrote:
>
> On Fri, Sep 20, 2024 at 07:32:23PM +0200, Oleg Nesterov wrote:
> > 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...
>
> We defer the icache maintenance until set_pte_at() time, where we call
> __sync_icache_dcache() if we're installing a present, executable user
> eintry.

And to me this looks as if __sync_icache_dcache() must be called when
user space tries to fault-in the page allocated in __create_xol_area()
and returned by special_mapping_fault().

So I still don't understand the problem.

Oleg.