Re: [Xen-devel] [PATCH 3/3] memory: restrict XENMEM_remove_from_physmap to translated guests

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 3/5/19 1:28 PM, Jan Beulich wrote:
> The commit re-introducing it (14eb3b41d0 ["xen: reinstate previously
> unused XENMEM_remove_from_physmap hypercall"]) as well as the one having
> originally introduced it (d818f3cb7c ["hvm: Use main memory for video
> memory"]) and the one then purging it again (78c3097e4f ["Remove unused
> XENMEM_remove_from_physmap"]) make clear that this operation is intended
> for use on HVM (i.e. translated) guests only. Restrict it at least as
> much, because for PV guests documentation (in the public header) does
> not even match the implementation: It talks about GPFN as input, but
> get_page_from_gfn() assumes a GMFN in the non-translated case (and hands
> back the value passed in).
> 
> Also lift the check in XENMEM_add_to_physmap{,_batch} handling up
> directly into top level hypercall handling, and clarify things in the
> public header accordingly.
> 
> Take the liberty and also replace a pointless use of "current" with a
> more efficient use of an existing local variable (or function parameter
> to be precise).
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Looks good, sorry for the delay:

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

A couple of comments:

> ---
> TBD: Is using P2M_ALLOC here really appropriate? It means e.g.
>      pointlessly populating a PoD slot just to unpopulate it again right
>      away, with the page then free floating, i.e. no longer available
>      for use to replace another PoD slot, and (afaict) no longer
>      accessible by the guest in any way.

Looks like the P2M_ALLOC was introduced in c/s 06e7bfed206. I can't
immediately see any reason to do the allocation -- I think it just must
have been C&P without much thought given as to what was going to happen
next.

> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -4470,9 +4470,6 @@ int xenmem_add_to_physmap_one(
>      mfn_t mfn = INVALID_MFN;
>      p2m_type_t p2mt;
>  
> -    if ( !paging_mode_translate(d) )
> -        return -EACCES;
> -
>      switch ( space )
>      {
>          case XENMAPSPACE_shared_info:
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -815,6 +815,8 @@ int xenmem_add_to_physmap(struct domain
>      long rc = 0;
>      union xen_add_to_physmap_batch_extra extra;
>  
> +    ASSERT(paging_mode_translate(d));

So, just trying to think through the principles behind these two.  We
know that this is never going to be called w/o first calling
xatp_permission_check(); if we assume that such a change will be tested
(i.e., that something with paging_mode_translate() will call this
hypercall before a release), then a single ASSERT() should be enough to
make sure that both functions are updated properly?

I guess that's good enough.  (It's hard not to start to get paranoid
when you ask yourself these sorts of questions.)

 -George


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