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

[Andy Lutomirski]

On Thu, Oct 11, 2018 at 11:55 AM Kristen Carlson Accardi
<kristen@xxxxxxxxxxxxxxx> 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.
>
> It's important to note that this patch is not addressing any specific
> exploit, nor is it intended to be a complete defense against anything.
> It is intended to be a low cost way of eliminating some of side effects
> of a failed system call.
>
> A performance test using sysbench on one hyperthread and a script which
> attempts to repeatedly access files it does not have permission to access
> on the other hyperthread found no significant performance impact.
>

> +__visible inline void l1_cache_flush(struct pt_regs *regs)
> +{
> +       if (IS_ENABLED(CONFIG_SYSCALL_FLUSH) &&
> +           static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
> +               if (regs->ax == 0 || regs->ax == -EAGAIN ||
> +                   regs->ax == -EEXIST || regs->ax == -ENOENT ||
> +                   regs->ax == -EXDEV || regs->ax == -ETIMEDOUT ||
> +                   regs->ax == -ENOTCONN || regs->ax == -EINPROGRESS)
> +                       return;
> +
> +               wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
> +       }
> +}

Ugh.

What exactly is this trying to protect against?  And how many cycles
should we expect L1D_FLUSH to take?

ISTM that, if we have a situation where the L1D can be read by user
code, we lose, via hyperthreading, successful syscalls, /dev/random,
and may other vectors.  This seems like a small mitigation at a rather
large cost.