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

[Dave Hansen]

On 7/13/22 21:04, Andrei Vagin 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.
Is gVisor some out-of-tree kernel code or is it just an proprietary KVM
user?  In other words, is this an issue with the upstream kernel itself
or does it require kernel modification?