Subject: Re: [PATCH bpf-next 2/7] set_memory: introduce set_memory_[ro|x]_noalias

From: Peter Zijlstra <[email-masked]>
Date: Wed, 17 Nov 2021 23:01:32 +0100

On Wed, Nov 17, 2021 at 09:36:27PM +0000, Song Liu wrote:
> 
> 
> > On Nov 16, 2021, at 12:00 AM, Peter Zijlstra <[email-masked]> wrote:
> > 
> > On Mon, Nov 15, 2021 at 11:13:42PM -0800, Song Liu wrote:
> >> These allow setting ro/x for module_alloc() mapping, while leave the
> >> linear mapping rw/nx.
> > 
> > This needs a very strong rationale for *why*. How does this not
> > trivially circumvent W^X ?
> 
> In this case, we want to have multiple BPF programs sharing the 2MB page. 
> When the JIT engine is working on one program, we would rather existing
> BPF programs on the same page stay on RO+X mapping (the module_alloc() 
> address). The solution in this version is to let the JIT engine write to 
> the page via linear address. 
> 
> An alternative is to only use the module_alloc() address, and flip the 
> read-only bit (of the whole 2MB page) back and forth. However, this 
> requires some serialization among different JIT jobs. 

Neither options seem acceptible to me as they both violate W^X.

Please have a close look at arch/x86/kernel/alternative.c:__text_poke()
for how we modify active text. I think that or something very similar is
the only option. By having an alias in a special (user) address space
that is not accessible by any other CPU, only the poking CPU can expoit
this (temporary) hole, which is a much larger ask than any of the
proposed options.