linux-kernel - Re: [PATCH v1 2/4] x86/tdx: Add validation of userspace MMIO instructions

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Message-ID: <ZrDKkD4M4AKOFqb8@example.org>
Date: Mon, 5 Aug 2024 14:50:24 +0200
From: Alexey Gladkov <legion@...nel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yuan Yao <yuan.yao@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Yuntao Wang <ytcoode@...il.com>, Kai Huang <kai.huang@...el.com>,
	Baoquan He <bhe@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	cho@...rosoft.com, decui@...rosoft.com, John.Starks@...rosoft.com
Subject: Re: [PATCH v1 2/4] x86/tdx: Add validation of userspace MMIO
 instructions

On Fri, Aug 02, 2024 at 10:41:17AM +0300, Kirill A. Shutemov wrote:
> On Tue, Jul 30, 2024 at 07:35:57PM +0200, Alexey Gladkov (Intel) wrote:
> > +static int valid_vaddr(struct ve_info *ve, enum insn_mmio_type mmio, int size,
> > +			unsigned long vaddr)
> > +{
> > +	phys_addr_t phys_addr;
> > +	bool writable = false;
> > +
> > +	/* It's not fatal. This can happen due to swap out or page migration. */
> > +	if (get_phys_addr(vaddr, &phys_addr, &writable) || (ve->gpa != cc_mkdec(phys_addr)))
> > +		return -EAGAIN;
> 
> I think we need big fat comment here why these checks are needed.
> 
> We have ve->gpa and it was valid at the time we got ve_info. But after we
> get ve_info, we enable interrupts allowing tlb shootdown and therefore
> munmap() in parallel thread of the process.
> 
> So by the time we've got here ve->gpa might be unmapped from the process,
> the device it belongs to removed from system and something else could be
> plugged in its place.
> 
> That's why we need to re-check if the GPA is still mapped and writable if
> we are going to write to it.

Make sense. I will add bigger comment here.

> > +
> > +	/* Check whether #VE info matches the instruction that was decoded. */
> > +	switch (mmio) {
> > +	case INSN_MMIO_WRITE:
> > +	case INSN_MMIO_WRITE_IMM:
> > +		if (!writable || !(ve->exit_qual & EPT_VIOLATION_ACC_WRITE))
> > +			return -EFAULT;
> > +		break;
> > +	case INSN_MMIO_READ:
> > +	case INSN_MMIO_READ_ZERO_EXTEND:
> > +	case INSN_MMIO_READ_SIGN_EXTEND:
> > +		if (!(ve->exit_qual & EPT_VIOLATION_ACC_READ))
> > +			return -EFAULT;
> > +		break;
> > +	default:
> > +		WARN_ONCE(1, "Unsupported mmio instruction: %d", mmio);
> > +		return -EINVAL;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov
> 

-- 
Rgrds, legion