linux-kernel - Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned

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Date:   Thu, 16 Jun 2022 04:19:26 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org, ak@...ux.intel.com,
        dan.j.williams@...el.com, david@...hat.com, hpa@...or.com,
        linux-kernel@...r.kernel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCHv4 3/3] x86/tdx: Handle load_unaligned_zeropad()
 page-cross to a shared page

On Wed, Jun 15, 2022 at 04:34:48PM -0700, Dave Hansen wrote:
> On 6/15/22 15:52, Kirill A. Shutemov wrote:
> >> +	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
> >> +	if (vaddr / PAGE_SIZE != (vaddr + size) / PAGE_SIZE)
> > Oops. I just realized it has off-by-one. It supposed to be:
> > 
> > 	if (vaddr / PAGE_SIZE != (vaddr + size - 1) / PAGE_SIZE)
> 
> That was bugging me.  Glad you caught this.
> 
> Wouldn't this be more obviously correct?
> 
> 	if (ALIGN_DOWN(vaddr,        PAGE_SIZE) !=
> 	    ALIGN_DOWN(vaddr + size, PAGE_SIZE))
> 		...

I don't think it fixes anything.

Consider the case when vaddr is 4092 and size is 4. This is legitimate
access -- aligned and not page crosser.

The left side of the check will be evaluated to 0 and the right will be 1
which is wrong and will get us -EFAULT.

I cannot think of a helper that would fit the need.

-- 
 Kirill A. Shutemov