Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

[Andy Lutomirski]

On Wed, Sep 23, 2020 at 3:20 PM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On 9/23/2020 3:08 PM, Dave Hansen wrote:
> > On 9/23/20 3:06 PM, Yu, Yu-cheng wrote:
> >> I think I'll add a check here for (r + 8) >= TASK_SIZE_MAX.  It is
> >> better than getting a fault.
> >
> > There's also wrmsr_safe().
> >
> Yes, thanks.
>
> Since I am going to change this to:
>
> fpu__prepare_write(), then write to the XSAVES area.
>
> The kernel does not expect XRSTORS to fail ("Bad FPU state detected..."
> message).  So maybe still check the address first.

Surely there are plenty of ways to use ptrace() to poke garbage into
the FPU state.  We should be able to handle this type of failure
somewhat gracefully.