On Wed, Nov 17, 2021 at 09:36:27PM +0000, Song Liu wrote: > > > > On Nov 16, 2021, at 12:00 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
