Re: [PATCH 2/3] arm64: use PAGE_KERNEL_ROX directly in alloc_insn_page

[Will Deacon]

On Tue, Jun 23, 2020 at 11:37:14AM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 10:07:58AM +0100, Will Deacon wrote:
> > On Tue, Jun 23, 2020 at 11:05:05AM +0200, Christoph Hellwig wrote:
> > > On Sat, Jun 20, 2020 at 07:16:16PM -0700, Andrew Morton wrote:
> > > > On Thu, 18 Jun 2020 08:43:06 +0200 Christoph Hellwig <hch@xxxxxx> wrote:
> > > > > --- a/arch/arm64/kernel/probes/kprobes.c
> > > > > +++ b/arch/arm64/kernel/probes/kprobes.c
> > > > > @@ -120,15 +120,9 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
> > > > >  
> > > > >  void *alloc_insn_page(void)
> > > > >  {
> > > > > -	void *page;
> > > > > -
> > > > > -	page = vmalloc_exec(PAGE_SIZE);
> > > > > -	if (page) {
> > > > > -		set_memory_ro((unsigned long)page, 1);
> > > > > -		set_vm_flush_reset_perms(page);
> > > > > -	}
> > > > > -
> > > > > -	return page;
> > > > > +	return __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
> > > > > +			GFP_KERNEL, PAGE_KERNEL_ROX, VM_FLUSH_RESET_PERMS,
> > > > > +			NUMA_NO_NODE, __func__);
> > > > >  }
> > > > >  
> > > > >  /* arm kprobe: install breakpoint in text */
> > > > 
> > > > But why.  I think this is just a cleanup, doesn't address any runtime issue?
> > > 
> > > It doesn't "fix" an issue - it just simplifies and speeds up the code.
> > 
> > Ok, but I don't understand the PLT comment from Peter in
> > 20200618092754.GF576905@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx:
> > 
> >   | I think this has the exact same range issue as the x86 user. But it
> >   | might be less fatal if their PLT magic can cover the full range.
> > 
> > Peter, please could you elaborate on your concern? I feel like I'm missing
> > some context.
> 
> On x86 we can only directly call code in a (signed) 32bit immediate
> range (2G) and our kernel text and module range are constrained by that.
> 
> IIRC ARM64 has an even smaller immediate range and needs to play fixup
> games with trampolines or somesuch (there was an ARM specific name for
> it that I've misplaced again). Does that machinery cover the entire
> vmalloc space or are you only able to fix up for a smaller range?
> 
> Your arch/arm64/kernel/module.c:module_alloc() implementation seems to
> have an explicit module range different from the full vmalloc range, I'm
> thinking this is for a reason.

Ah, gotcha. In this case, we're talking about the kprobe out-of-line
buffer. We don't directly branch to that; instead we take a BRK exception
and either exception return + singlestep the OOL buffer, or we simulate
the instruction if it's doing anything PC-relative, so I don't see the
need for a PLT.

Will