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Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state
To:     Dave Hansen <dave.hansen@intel.com>,
        "Chang S. Bae" <chang.seok.bae@intel.com>, bp@suse.de,
        tglx@linutronix.de, mingo@kernel.org, x86@kernel.org
Cc:     len.brown@intel.com, jing2.liu@intel.com, ravi.v.shankar@intel.com,
        linux-kernel@vger.kernel.org
References: <20210523193259.26200-1-chang.seok.bae@intel.com>
 <20210523193259.26200-15-chang.seok.bae@intel.com>
 <af093744-6f68-ff51-f40b-4db234b363d8@intel.com>
From:   Andy Lutomirski <luto@kernel.org>
Message-ID: <872cb0a2-3659-2e6c-52a8-33f1a2f0a2cd@kernel.org>
Date:   Wed, 16 Jun 2021 11:12:27 -0700
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On 6/16/21 9:27 AM, Dave Hansen wrote:
> 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?
> 

Wait, really?  I somehow thought that XSAVE* would write out the actual
state even if XFD=1.

This seems like it's asking for some kind of bug.