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

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Message-ID: <mnncaxyk6jsbtlxk6xo5jvs7mzirp3ituyf7anequxy6xjjijm@ogkxlksd4gi6>
Date: Fri, 2 Aug 2024 10:41:17 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: "Alexey Gladkov (Intel)" <legion@...nel.org>
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 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.

> +
> +	/* 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