Re: [patch V4 09/65] x86/fpu: Sanitize xstateregs_set()

[Andrei Vagin]

On Wed, Jun 23, 2021 at 5:24 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
....
>
> -       /*
> -        * mxcsr reserved bits must be masked to zero for security reasons.
> -        */
> -       xsave->i387.mxcsr &= mxcsr_feature_mask;
> -
> -       /*
> -        * In case of failure, mark all states as init:
> -        */
> -       if (ret)
> -               fpstate_init(&fpu->state);
> +       fpu__prepare_write(fpu);
> +       ret = copy_kernel_to_xstate(&fpu->state.xsave, kbuf ?: tmpbuf);

This change breaks gVisor. Now, when we set a new fpu state without
fp,sse and ymm
via ptrace, mxcsr isn't reset to the default value. The issue is
reproduced only on hosts
without xsaves. On hosts with xsaves, it works as expected.

Thanks,
Andrei