Re: [PATCH] kprobes: Enable tracing for mololithic kernel images

Luis Chamberlain
2022-06-09T13:24:53Z

On Thu, Jun 09, 2022 at 05:48:52AM +0200, Christoph Hellwig wrote:
> On Wed, Jun 08, 2022 at 01:26:19PM -0700, Luis Chamberlain wrote:
> > No, that was removed because it has only one user.
>
> That is only part of the story. The other part is that the overall
> kernel simply does not have any business allocating exutable memory.
> Executable memory is a very special concept for modules or module-like
> code like kprobes, and should not be exposed as a general concept.

It is not just modules and kprobes, it is also ftrace and bpf too now.
So while it should not be used everywhere calling it module_alloc()
is just confusing at this point. Likewise, module_alloc_huge() is
being proposed too and I'd rather we deal with this properly in aligment
of taking care of the rename as well.

If the concern is to restrict access we can use the module namespace stuff
so to ensure only intended users get access to it.

> Especially as executable memory really should not also be writable
> for security reasons. In other words, we should actually never
> allocate executable memory, every. We might seal memory and then
> mark it executable after having written to it, which is how modules
> and kprobes are implemented on all modern Linux ports anyway.

The respective free *should* do the executable bits, and there
is no generic way to do this for all archs and so it is open coded
today. In fact some architectures need further work / help and so
split up the module data and exect already on v5.19+ with the new
ARCH_WANTS_MODULES_DATA_IN_VMALLOC. See this thread for details:

https://lkml.kernel.org/r/Yo1XTN441qbNTLGR@bombadil.infradead.org

Doing this work is not easy, but if we're going to do it, it must
be done right.

Luis