Re: [PATCH v4] mm/vmalloc: randomize vmalloc() allocations

[Topi Miettinen]

On 15.3.2021 19.47, Uladzislau Rezki wrote:
> On Mon, Mar 15, 2021 at 09:16:26AM -0700, Kees Cook wrote:
> > On Mon, Mar 15, 2021 at 01:24:10PM +0100, Uladzislau Rezki wrote:
> > > On Mon, Mar 15, 2021 at 11:04:42AM +0200, Topi Miettinen wrote:
> > > > What's the problem with that? It seems to me that nothing relies on specific
> > > > addresses of the chunks, so it should be possible to randomize these too.
> > > > Also the alignment is honored.
> > > My concern are:
> > >
> > > - it is not a vmalloc allocator;
> > > - per-cpu allocator allocates chunks, thus it might be it happens only once. It does not allocate it often;
> >
> > That's actually the reason to randomize it: if it always ends up in the
> > same place at every boot, it becomes a stable target for attackers.
> Probably we can randomize a base address only once when pcpu-allocator
> allocates a fist chunk during the boot.
>
> > > - changing it will likely introduce issues you are not aware of;
> > > - it is not supposed to be interacting with vmalloc allocator. Read the
> > >    comment under pcpu_get_vm_areas();
> > >
> > > Therefore i propose just not touch it.
> >
> > How about splitting it from this patch instead? Then it can get separate
> > testing, etc.
> It should be split as well as tested.

Would you prefer another kernel option `randomize_percpu_allocator=1`, 
or would it be OK to make it a flag in `randomize_vmalloc`, like 
`randomize_vmalloc=3`? Maybe the latter would not be compatible with 
static branches.

-Topi

> --
> Vlad Rezki