netdev - Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble memory as ROX

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Date: Thu, 1 Jun 2023 14:00:15 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble
 memory as ROX

On Thu, Jun 01, 2023 at 04:54:27PM +0000, Edgecombe, Rick P wrote:
> It is just a local flush, but I wonder how much text_poke()ing is too
> much. A lot of the are even inside loops. Can't it do the batch version
> at least?
> 
> The other thing, and maybe this is in paranoia category, but it's
> probably at least worth noting. Before the modules were not made
> executable until all of the code was finalized. Now they are made
> executable in an intermediate state and then patched later. It might
> weaken the CFI stuff, but also it just kind of seems a bit unbounded
> for dealing with executable code.

I believe bpf starts out by initializing new executable memory with
illegal opcodes, maybe we should steal that and make it standard.

> Preparing the modules in a separate RW mapping, and then text_poke()ing
> the whole thing in when you are done would resolve both of these.

text_poke() _does_ create a separate RW mapping.

The thing that sucks about text_poke() is that it always does a full TLB
flush, and AFAICT that's not remotely needed. What it really wants to be
doing is conceptually just

kmap_local()
mempcy()
kunmap_loca()
flush_icache();

...except that kmap_local() won't actually create a new mapping on
non-highmem architectures, so text_poke() open codes it.