[PATCH v2] arm: p2m.c bug-fix: hypervisor hang on __p2m_get_mem_access

[Corneliu ZUZU <czuzu@bitdefender.com>]

When __p2m_get_mem_access gets called, the p2m lock is already taken
by either get_page_from_gva or p2m_get_mem_access.
Possible code paths:
1)	-> get_page_from_gva
		-> p2m_mem_access_check_and_get_page
			-> __p2m_get_mem_access
2)	-> p2m_get_mem_access
		-> __p2m_get_mem_access

In both cases if __p2m_get_mem_access subsequently gets to
call p2m_lookup (happens if !radix_tree_lookup(...)), a hypervisor
hang will occur, since p2m_lookup also spin-locks on the p2m lock.

This bug-fix simply replaces the p2m_lookup call from __p2m_get_mem_access
with a call to __p2m_lookup and also adds an ASSERT to ensure that the p2m lock
is already taken upon __p2m_get_mem_access entry.

Signed-off-by: Corneliu ZUZU <czuzu@bitdefender.com>

---
Changed since v1:
  * added p2m-lock ASSERT
---
 xen/arch/arm/p2m.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 2190908..e8e6db4 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -468,6 +468,8 @@ static int __p2m_get_mem_access(struct domain *d, gfn_t gfn,
 #undef ACCESS
     };
 
+    ASSERT(spin_is_locked(&p2m->lock));
+
     /* If no setting was ever set, just return rwx. */
     if ( !p2m->mem_access_enabled )
     {
@@ -490,7 +492,7 @@ static int __p2m_get_mem_access(struct domain *d, gfn_t gfn,
          * No setting was found in the Radix tree. Check if the
          * entry exists in the page-tables.
          */
-        paddr_t maddr = p2m_lookup(d, gfn_x(gfn) << PAGE_SHIFT, NULL);
+        paddr_t maddr = __p2m_lookup(d, gfn_x(gfn) << PAGE_SHIFT, NULL);
         if ( INVALID_PADDR == maddr )
             return -ESRCH;
 
-- 
2.5.0