Re: [PATCH for-4.14] xen/arm: mm: Access a PT entry before the table is unmapped

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sun, 7 Jun 2020, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> xen_pt_next_level() will retrieve the MFN from the entry right after the
> page-table has been unmapped.
> 
> After calling xen_unmap_table(), there is no guarantee the mapping will
> still be valid. Depending on the implementation, this may result to a
> data abort in Xen.
> 
> Re-order the code to retrieve the MFN before the table is unmapped.
> 
> Fixes: 53abb9a1dcd9 ("xen/arm: mm: Rework Xen page-tables walk during update")
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ---
> 
> I spotted this issue while reworking how page-tables are mapped on Arm64
> during early boot. However the problem should be already there on Arm32.
> 
> I am actually quite amazed we haven't seen any crash on Arm32 because
> there is no direct map. So the mapping will not exist after calling
> xen_unmap_table().
> 
> I suspect this works because unmap_domain_page() doesn't flush the
> TLBs. So the TLB still likely have the entry cached (joy!).

  \0/

Damn!

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

> This patch is a candidate for Xen 4.14 and should also be backported.
> ---
>  xen/arch/arm/mm.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 1b14f4934570..9e2ff7c8005d 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -1036,6 +1036,7 @@ static int xen_pt_next_level(bool read_only, unsigned 
> int level,
>  {
>      lpae_t *entry;
>      int ret;
> +    mfn_t mfn;
>  
>      entry = *table + offset;
>  
> @@ -1053,8 +1054,10 @@ static int xen_pt_next_level(bool read_only, unsigned 
> int level,
>      if ( lpae_is_mapping(*entry, level) )
>          return XEN_TABLE_SUPER_PAGE;
>  
> +    mfn = lpae_get_mfn(*entry);
> +
>      xen_unmap_table(*table);
> -    *table = xen_map_table(lpae_get_mfn(*entry));
> +    *table = xen_map_table(mfn);
>  
>      return XEN_TABLE_NORMAL_PAGE;
>  }
> -- 
> 2.17.1
>