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SubjectRe: [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

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