Re: [PATCH 2/3] x86: fix do_update_va_mapping_otherdomain() wrt translated domains

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 12/10/17 11:00, Jan Beulich wrote:
> While I can't seem to find any users of this hypercall (being a likely
> explanation of why the problem wasn't noticed so far), just like for

Judging by c/s a51ed685b which shifted
__HYPERVISOR_update_va_mapping_otherdomain's hypercall number to make
space for __HYPERVISOR_grant_table_op, I'd have said the chance of it
being used was slim.  However,

andrewcoop@andrewcoop:/local/xen.git/xen$ git checkout a51ed685
andrewcoop@andrewcoop:/local/xen.git/xen$ git grep update_va_mapping_otherdomain -- :/
../linux-2.6.7-xen-sparse/drivers/xen/blkback/blkback.c:320:    if ( HYPERVISOR_update_va_mapping_otherdomain(
../linux-2.6.7-xen-sparse/drivers/xen/blkback/blkback.c:404:        mcl[i].op = __HYPERVISOR_update_va_mapping_otherdomain;
../linux-2.6.7-xen-sparse/drivers/xen/netback/netback.c:516:        mcl[0].op = __HYPERVISOR_update_va_mapping_otherdomain;
../linux-2.6.7-xen-sparse/include/asm-xen/hypervisor.h:458:static inline int HYPERVISOR_update_va_mapping_otherdomain(
../linux-2.6.7-xen-sparse/include/asm-xen/hypervisor.h:464:        : "=a" (ret) : "0" (__HYPERVISOR_update_va_mapping_otherdomain), 
arch/x86/memory.c:1264:int do_update_va_mapping_otherdomain(unsigned long page_nr, 
arch/x86/x86_32/entry.S:723:        .long SYMBOL_NAME(do_update_va_mapping_otherdomain)
include/hypervisor-ifs/hypervisor-if.h:50:#define __HYPERVISOR_update_va_mapping_otherdomain 22


It certainly was used at that point in history.  If none of that code
has survived into more recent version {blk,net}back, it is probably that
the hypercall isn't used any more.

> do_mmu_update() paged-out and shared page handling is needed here. Move
> all this logic into mod_l1_entry(), which then also results in no
> longer
> - doing any of this handling for non-present PTEs,
> - acquiring two temporary page references when one is already more than
>   enough.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> Now that L1 entry handling in do_mmu_update() is sufficiently similar
> again to that of L2/L3/L4 entries, I wonder whether it wouldn't it be
> better for the function to refuse pg_owner != pt_owner for L2/L3/L4
> updates. Right now the passed in foreign domain ID is simply ignored
> in that case (except for the XSM check).

I can't see anything good coming from having pg_owner != pt_owner in non
L1 pagetables.  Explicit rejection is certainly better than doing the
wrong thing silently under the hood.

Do you want to do a separate patch for that, or fold it into this one?

> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -1632,7 +1632,6 @@ static int mod_l1_entry(l1_pgentry_t *pl
>  
>      if ( l1e_get_flags(nl1e) & _PAGE_PRESENT )
>      {
> -        /* Translate foreign guest addresses. */
>          struct page_info *page = NULL;
>  
>          if ( unlikely(l1e_get_flags(nl1e) & l1_disallow_mask(pt_dom)) )
> @@ -1642,9 +1641,35 @@ static int mod_l1_entry(l1_pgentry_t *pl
>              return -EINVAL;
>          }
>  
> +        /* Translate foreign guest address. */
>          if ( paging_mode_translate(pg_dom) )
>          {
> -            page = get_page_from_gfn(pg_dom, l1e_get_pfn(nl1e), NULL, P2M_ALLOC);
> +            p2m_type_t p2mt;
> +            p2m_query_t q = l1e_get_flags(nl1e) & _PAGE_RW ?
> +                            P2M_ALLOC | P2M_UNSHARE : P2M_ALLOC;
> +
> +            page = get_page_from_gfn(pg_dom, l1e_get_pfn(nl1e), &p2mt, q);
> +
> +            if ( p2m_is_paged(p2mt) )
> +            {
> +                if ( page )
> +                    put_page(page);
> +                p2m_mem_paging_populate(pg_dom, l1e_get_pfn(nl1e));
> +                return -ENOENT;
> +            }
> +
> +            if ( p2mt == p2m_ram_paging_in && !page )
> +                return -ENOENT;
> +
> +            /* Did our attempt to unshare fail? */
> +            if ( (q & P2M_UNSHARE) && p2m_is_shared(p2mt) )
> +            {
> +                /* We could not have obtained a page ref. */
> +                ASSERT(!page);
> +                /* And mem_sharing_notify has already been called. */
> +                return -ENOMEM;
> +            }
> +
>              if ( !page )
>                  return -EINVAL;
>              nl1e = l1e_from_page(page, l1e_get_flags(nl1e));
> @@ -3315,47 +3340,10 @@ long do_mmu_update(
>                  switch ( page->u.inuse.type_info & PGT_type_mask )
>                  {
>                  case PGT_l1_page_table:
> -                {
> -                    l1_pgentry_t l1e = l1e_from_intpte(req.val);
> -                    p2m_type_t l1e_p2mt = p2m_ram_rw;
> -                    struct page_info *target = NULL;
> -                    p2m_query_t q = (l1e_get_flags(l1e) & _PAGE_RW) ?
> -                                        P2M_UNSHARE : P2M_ALLOC;
> -
> -                    if ( paging_mode_translate(pg_owner) )
> -                        target = get_page_from_gfn(pg_owner, l1e_get_pfn(l1e),
> -                                                   &l1e_p2mt, q);
> -
> -                    if ( p2m_is_paged(l1e_p2mt) )
> -                    {
> -                        if ( target )
> -                            put_page(target);
> -                        p2m_mem_paging_populate(pg_owner, l1e_get_pfn(l1e));
> -                        rc = -ENOENT;
> -                        break;
> -                    }
> -                    else if ( p2m_ram_paging_in == l1e_p2mt && !target )
> -                    {
> -                        rc = -ENOENT;
> -                        break;
> -                    }
> -                    /* If we tried to unshare and failed */
> -                    else if ( (q & P2M_UNSHARE) && p2m_is_shared(l1e_p2mt) )
> -                    {
> -                        /* We could not have obtained a page ref. */
> -                        ASSERT(target == NULL);
> -                        /* And mem_sharing_notify has already been called. */
> -                        rc = -ENOMEM;
> -                        break;
> -                    }
> -
> -                    rc = mod_l1_entry(va, l1e, mfn,
> +                    rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
>                                        cmd == MMU_PT_UPDATE_PRESERVE_AD, v,
>                                        pg_owner);
> -                    if ( target )
> -                        put_page(target);
> -                }
> -                break;
> +                    break;
>                  case PGT_l2_page_table:
>                      rc = mod_l2_entry(va, l2e_from_intpte(req.val), mfn,
>                                        cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
> @@ -3367,7 +3355,7 @@ long do_mmu_update(
>                  case PGT_l4_page_table:
>                      rc = mod_l4_entry(va, l4e_from_intpte(req.val), mfn,
>                                        cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
> -                break;
> +                    break;

If we are tidying up the style, could we also get some newlines between
break and case?

Either way, Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

>                  case PGT_writable_page:
>                      perfc_incr(writable_mmu_updates);
>                      if ( paging_write_guest_entry(v, va, req.val, _mfn(mfn)) )
>
>


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