[PATCH V2] mm/powerpc: Fix _PAGE_PTE breaking swapoff

[Hugh Dickins]

On Mon, 11 Jan 2016, Aneesh Kumar K.V wrote:

> Core kernel expect swp_entry_t to be consisting of
> only swap type and swap offset. We should not leak pte bits to
> swp_entry_t. This breaks swapoff which use the swap type and offset
> to build a swp_entry_t and later compare that to the swp_entry_t
> obtained from linux page table pte. Leaking pte bits to swp_entry_t
> breaks that comparison and results in us looping in try_to_unuse.
> 
> The stack trace can be anywhere below try_to_unuse() in mm/swapfile.c,
> since swapoff is circling around and around that function, reading from
> each used swap block into a page, then trying to find where that page
> belongs, looking at every non-file pte of every mm that ever swapped.
> 
> Reported-by: Hugh Dickins <hughd at google.com>
> Suggested-by: Hugh Dickins <hughd at google.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>

I think we've seen enough of my name above, but if it helps further
Acked-by: Hugh Dickins <hughd at google.com>

Though I don't find the code comment below on swp_entry_t enlightening -
your commit description above is much more helpful.  If I were writing it,
I might say... hmm, it's too hard: given all the convolutions, I gave up.

> ---
> Changes from V1:
> * improve change log and code comment
> 
>  arch/powerpc/include/asm/book3s/64/pgtable.h | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index 03c1a5a21c0c..cecb971674a8 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -158,9 +158,14 @@ static inline void pgd_set(pgd_t *pgdp, unsigned long val)
>  #define __swp_entry(type, offset)	((swp_entry_t) { \
>  					((type) << _PAGE_BIT_SWAP_TYPE) \
>  					| ((offset) << PTE_RPN_SHIFT) })
> -
> -#define __pte_to_swp_entry(pte)		((swp_entry_t) { pte_val((pte)) })
> -#define __swp_entry_to_pte(x)		__pte((x).val)
> +/*
> + * swp_entry_t should be independent of pte bits. We build a swp_entry_t from
> + * swap type and offset we get from swap and convert that to pte to
> + * find a matching pte in linux page table.
> + * Clear bits not found in swap entries here
> + */
> +#define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val((pte)) & ~_PAGE_PTE })
> +#define __swp_entry_to_pte(x)	__pte((x).val | _PAGE_PTE)
>  
>  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>  #define _PAGE_SWP_SOFT_DIRTY   (1UL << (SWP_TYPE_BITS + _PAGE_BIT_SWAP_TYPE))
> -- 
> 2.5.0