[v1] x86/ept: pass correct level to p2m_entry_modify

[Xen-devel] [PATCH] x86/ept: pass correct level to p2m_entry_modify

Posted by Roger Pau Monne 6 years, 7 months ago

EPT differs from NPT and shadow when translating page orders to levels
in the physmap page tables. EPT page tables level for order 0 pages is
0, while NPT and shadow instead use 1, ie: EPT page tables levels
starts at 0 while NPT and shadow starts at 1.

Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
one to the level, in order to match NPT and shadow usage.

While there also fix p2m_entry_modify BUG condition to trigger when
foreign or ioreq entries with level different than 0 are attempted.
That should allow to catch future errors related to the level
parameter.

Fixes: c7a4c0 ('x86/mm: split p2m ioreq server pages special handling into helper')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Jun Nakajima <jun.nakajima@intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Wei Liu <wl@xen.org>
---
 xen/arch/x86/mm/p2m-ept.c | 2 +-
 xen/include/asm-x86/p2m.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
index e3044bee2e..6b8468c793 100644
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -51,7 +51,7 @@ static int atomic_write_ept_entry(struct p2m_domain *p2m,
                                   int level)
 {
     int rc = p2m_entry_modify(p2m, new.sa_p2mt, entryptr->sa_p2mt,
-                              _mfn(new.mfn), _mfn(entryptr->mfn), level);
+                              _mfn(new.mfn), _mfn(entryptr->mfn), level + 1);
 
     if ( rc )
         return rc;
diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
index 09ef7e02fd..756929d5c0 100644
--- a/xen/include/asm-x86/p2m.h
+++ b/xen/include/asm-x86/p2m.h
@@ -946,7 +946,7 @@ static inline int p2m_entry_modify(struct p2m_domain *p2m, p2m_type_t nt,
                                    p2m_type_t ot, mfn_t nfn, mfn_t ofn,
                                    unsigned int level)
 {
-    BUG_ON(level > 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));
+    BUG_ON(level != 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));
 
     if ( level != 1 || (nt == ot && mfn_eq(nfn, ofn)) )
         return 0;
-- 
2.20.1 (Apple Git-117)


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Re: [Xen-devel] [PATCH] x86/ept: pass correct level to p2m_entry_modify

Posted by Jan Beulich 6 years, 7 months ago

On 03.07.2019 11:43, Roger Pau Monne wrote:
> EPT differs from NPT and shadow when translating page orders to levels
> in the physmap page tables. EPT page tables level for order 0 pages is
> 0, while NPT and shadow instead use 1, ie: EPT page tables levels
> starts at 0 while NPT and shadow starts at 1.
> 
> Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
> one to the level, in order to match NPT and shadow usage.
> 
> While there also fix p2m_entry_modify BUG condition to trigger when
> foreign or ioreq entries with level different than 0 are attempted.
> That should allow to catch future errors related to the level
> parameter.
> 
> Fixes: c7a4c0 ('x86/mm: split p2m ioreq server pages special handling into helper')

A 6-digit hash is definitely too short in the long run. I understand
that this then wants backporting to the 4.12 tree.

> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -946,7 +946,7 @@ static inline int p2m_entry_modify(struct p2m_domain *p2m, p2m_type_t nt,
>                                      p2m_type_t ot, mfn_t nfn, mfn_t ofn,
>                                      unsigned int level)
>   {
> -    BUG_ON(level > 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));
> +    BUG_ON(level != 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));

Wouldn't you better leave this alone and add BUG_ON(!level)?

Jan
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Re: [Xen-devel] [PATCH] x86/ept: pass correct level to p2m_entry_modify

Posted by Roger Pau Monné 6 years, 7 months ago

On Wed, Jul 03, 2019 at 10:22:03AM +0000, Jan Beulich wrote:
> On 03.07.2019 11:43, Roger Pau Monne wrote:
> > EPT differs from NPT and shadow when translating page orders to levels
> > in the physmap page tables. EPT page tables level for order 0 pages is
> > 0, while NPT and shadow instead use 1, ie: EPT page tables levels
> > starts at 0 while NPT and shadow starts at 1.
> > 
> > Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
> > one to the level, in order to match NPT and shadow usage.
> > 
> > While there also fix p2m_entry_modify BUG condition to trigger when
> > foreign or ioreq entries with level different than 0 are attempted.
> > That should allow to catch future errors related to the level
> > parameter.
> > 
> > Fixes: c7a4c0 ('x86/mm: split p2m ioreq server pages special handling into helper')
> 
> A 6-digit hash is definitely too short in the long run. I understand
> that this then wants backporting to the 4.12 tree.

Yes.

Is there consensus on how many digits to use 8, 12, 16?

> > --- a/xen/include/asm-x86/p2m.h
> > +++ b/xen/include/asm-x86/p2m.h
> > @@ -946,7 +946,7 @@ static inline int p2m_entry_modify(struct p2m_domain *p2m, p2m_type_t nt,
> >                                      p2m_type_t ot, mfn_t nfn, mfn_t ofn,
> >                                      unsigned int level)
> >   {
> > -    BUG_ON(level > 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));
> > +    BUG_ON(level != 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));
> 
> Wouldn't you better leave this alone and add BUG_ON(!level)?

That's an option also. I guess your check is better because it will
trigger for any call with level == 0, while mine would only do if such
call is also to add an entry of type ioreq or foreign.

Thanks, Roger.

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Re: [Xen-devel] [PATCH] x86/ept: pass correct level to p2m_entry_modify

Posted by Jan Beulich 6 years, 7 months ago

On 03.07.2019 13:01, Roger Pau Monné  wrote:
> On Wed, Jul 03, 2019 at 10:22:03AM +0000, Jan Beulich wrote:
>> On 03.07.2019 11:43, Roger Pau Monne wrote:
>>> EPT differs from NPT and shadow when translating page orders to levels
>>> in the physmap page tables. EPT page tables level for order 0 pages is
>>> 0, while NPT and shadow instead use 1, ie: EPT page tables levels
>>> starts at 0 while NPT and shadow starts at 1.
>>>
>>> Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
>>> one to the level, in order to match NPT and shadow usage.
>>>
>>> While there also fix p2m_entry_modify BUG condition to trigger when
>>> foreign or ioreq entries with level different than 0 are attempted.
>>> That should allow to catch future errors related to the level
>>> parameter.
>>>
>>> Fixes: c7a4c0 ('x86/mm: split p2m ioreq server pages special handling into helper')
>>
>> A 6-digit hash is definitely too short in the long run. I understand
>> that this then wants backporting to the 4.12 tree.
> 
> Yes.
> 
> Is there consensus on how many digits to use 8, 12, 16?

Consensus - no, I don't think so. But anything below 8 seems
open for collisions in the foreseeable future (albeit I don't
know if git has separate hash name spaces for commits and
objects in general). I've been using 10 for the last so many
years ...

Jan
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