linux-kernel - Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

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Date:   Wed, 16 Jun 2021 09:27:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>, bp@...e.de,
        luto@...nel.org, tglx@...utronix.de, mingo@...nel.org,
        x86@...nel.org
Cc:     len.brown@...el.com, jing2.liu@...el.com, ravi.v.shankar@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state

On 5/23/21 12:32 PM, Chang S. Bae wrote:
> @@ -571,6 +612,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
>  
>  	set_thread_flag(TIF_NEED_FPU_LOAD);
>  
> +	xfd_switch(old_fpu, new_fpu);

This seems fragile.

XSAVE* will decline to write out state for feature i when XFD[i]=1 and
will instead write out the init state.  That means that, at this point,
we switch XFD and yet leave state for feature i in place.

That means this *MUST* come before any copy_fpregs_to_fpstate() occurs.

Could we please add some FPU_WARN_ON() checks to ensure that no XSAVE*
is ever performed with XINUSE=1 *and* XFD armed?