Re: [RFC PATCH] x86: entry: flush the cache if syscall error

[Kristen C Accardi <kristen@linux.intel.com>]

On Tue, 2018-09-11 at 20:02 +0200, Jann Horn wrote:
> On Mon, Sep 10, 2018 at 9:14 PM Kristen Carlson Accardi
> <kristen@linux.intel.com> wrote:
> > This patch aims to make it harder to perform cache timing attacks
> > on data
> > left behind by system calls. If we have an error returned from a
> > syscall,
> > flush the L1 cache.
> 
> How much protection does this provide, given that it e.g. doesn't
> flush L2/L3 and doesn't prevent data leakage through hyperthreading
> and cache coherency? Is an L2/L3-based attack expected to be harder
> than an L1D-based one?

My reasoning here is that L2/L3 caches can be partitioned using
something like CAT (maybe), but L1 cannot. So IMO L1 is the case that
needs coverage. Also, while this doesn't address a specific exploit,
the idea is that attacks on data in L1D are more common, and the
performance penalty for L2/L3 flushes would be too high without a
specific exploit in mind.