Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

[Alexei Starovoitov]

On Wed, Oct 30, 2019 at 3:06 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Oct 29, 2019 at 05:27:43PM +0000, Edgecombe, Rick P wrote:
> > On Mon, 2019-10-28 at 22:00 +0100, Peter Zijlstra wrote:
>
> > > That should be limited to the module range. Random data maps could
> > > shatter the world.
> >
> > BPF has one vmalloc space allocation for the byte code and one for the module
> > space allocation for the JIT. Both get RO also set on the direct map alias of
> > the pages, and reset RW when freed.
>
> Argh, I didn't know they mapped the bytecode RO; why does it do that? It
> can throw out the bytecode once it's JIT'ed.

because of endless security "concerns" that some folks had.
Like what if something can exploit another bug in the kernel
and modify bytecode that was already verified
then interpreter will execute that modified bytecode.
Sort of similar reasoning why .text is read-only.
I think it's not a realistic attack, but I didn't bother to argue back then.
The mere presence of interpreter itself is a real security concern.
People that care about speculation attacks should
have CONFIG_BPF_JIT_ALWAYS_ON=y,
so modifying bytecode via another exploit will be pointless.
Getting rid of RO for bytecode will save a ton of memory too,
since we won't need to allocate full page for each small programs.