linux-kernel - Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

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Date:   Tue, 28 Sep 2021 17:08:00 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

On Tue, Sep 28, 2021 at 11:50:37PM +0000, Fenghua Yu wrote:
> If xfeatures's feature bit is 0, xsaves will not write its init value to the
> memory due to init optimization. So the xsaves will do nothing and the
> state is not initialized and may have random data.

> Setting TIF_NEED_FPU_LOAD cannot guaranteed to execute XRSTORS on exiting
> to user. In fpregs_restore_userregs():
> 	if (!fpregs_state_valid(fpu, cpu)) {
> 		...
>                 __restore_fpregs_from_fpstate(&fpu->state, mask);
> 		...
> 	}
> 
> fpregs state should be invalid to get the XRSTROS executed.
> 
> So setting TIF_NEED_FPU_LOAD may get the FPU register unchanged on exiting
> to user.

Does this help?
Changed lines marked with //<<<<<

-Tony

void *begin_update_one_xsave_feature(struct task_struct *tsk,
				     enum xfeature xfeature, bool full)
{
	struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
	struct xregs_state *xinit = &init_fpstate.xsave;
	u64 fmask = 1ull << xfeature;
	void *addr;

	BUG_ON(!(xsave->header.xcomp_bv & fmask));

	fpregs_lock();

	addr = __raw_xsave_addr(xsave, xfeature);

	if (full || tsk != current) {
		memcpy(addr, __raw_xsave_addr(xinit, xfeature), xstate_sizes[xfeature]);
		goto out;
	}

	if (!(xsave->header.xfeatures & fmask)) {
		xsave->header.xfeatures |= fmask;	//<<<<<
		xsaves(xsave, fmask);
	}

out:
	xsave->header.xfeatures |= fmask;
	return addr;
}

void finish_update_one_xsave_feature(struct task_struct *tsk)
{
	set_ti_thread_flag(task_thread_info(tsk), TIF_NEED_FPU_LOAD);
	if (tsk == current)				//<<<<<
		__cpu_invalidate_fpregs_state();	//<<<<<
	fpregs_unlock();
}