Re: [PATCH 1/2] x86/mm/pti: Check unaligned address for pmd clone in pti_clone_pagetable()

[Dave Hansen]

On 11/30/20 7:25 AM, Lai Jiangshan wrote:
> The commit 825d0b73cd752("x86/mm/pti: Handle unaligned address gracefully
> in pti_clone_pagetable()") handles unaligned address well for unmapped
> PUD/PMD etc. But unaligned address for pmd_large() or PTI_CLONE_PMD is also
> needed to be aware.

That 825d0b73cd752 changelog says:

>     pti_clone_pmds() assumes that the supplied address is either:
>     
>      - properly PUD/PMD aligned
>     or
>      - the address is actually mapped which means that independently
>        of the mapping level (PUD/PMD/PTE) the next higher mapping
>        exists.

... and that was the root of the bug.  If there was a large, unmapped
area, it would skip a PUD_SIZE or PMD_SIZE *area* instead of skipping to
the *next* pud/pmd.

The case being patched here is from a *present* PTE/PMD, so it's a
mapped area, not a hole.

That said, I think the previous changelog was wrong.  An unaligned
address to a mapped, large (2M) region followed by a smaller (4k) region
would skip too far into the 4k region.

That said, I'm not sure I like this fix.  If someone is explicitly
asking to clone a PMD (which pti_clone_pgtable() forces you to do), they
better align the address.