[xen master] x86/P2M: synchronize fast and slow paths of p2m_get_page_from_gfn()

[patchbot@xxxxxxx]

commit a8325f981ce4ff8ac8bcc73735f357846b0a0fbb
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Mar 31 09:21:12 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Mar 31 09:21:12 2025 +0200

    x86/P2M: synchronize fast and slow paths of p2m_get_page_from_gfn()
    
    Handling of both grants and foreign pages was different between the two
    paths.
    
    While permitting access to grants would be desirable, doing so would
    require more involved handling; undo that for the time being. In
    particular the page reference obtained would prevent the owning domain
    from changing e.g. the page's type (after the grantee has released the
    last reference of the grant). Instead perhaps another reference on the
    grant would need obtaining. Which in turn would require determining
    which grant that was.
    
    Foreign pages in any event need permitting on both paths.
    
    Introduce a helper function to be used on both paths, such that
    respective checking differs in just the extra "to be unshared" condition
    on the fast path.
    
    While there adjust the sanity check for foreign pages: Don't leak the
    reference on release builds when on a debug build the assertion would
    have triggered. (Thanks to Roger for the suggestion.)
    
    Fixes: 80ea7af17269 ("x86/mm: Introduce get_page_from_gfn()")
    Fixes: 50fe6e737059 ("pvh dom0: add and remove foreign pages")
    Fixes: cbbca7be4aaa ("x86/p2m: make p2m_get_page_from_gfn() handle grant 
case correctly")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/mm/p2m.c | 68 +++++++++++++++++++++++++++++----------------------
 1 file changed, 39 insertions(+), 29 deletions(-)

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 4f8a781842..3a39b5d124 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -328,12 +328,46 @@ void p2m_put_gfn(struct p2m_domain *p2m, gfn_t gfn)
     gfn_unlock(p2m, gfn_x(gfn), 0);
 }
 
+static struct page_info *get_page_from_mfn_and_type(
+    const struct domain *d, mfn_t mfn, p2m_type_t t)
+{
+    struct page_info *page;
+
+    if ( !mfn_valid(mfn) )
+        return NULL;
+
+    page = mfn_to_page(mfn);
+
+    if ( p2m_is_ram(t) )
+    {
+        if ( p2m_is_shared(t) )
+            d = dom_cow;
+
+        if ( get_page(page, d) )
+            return page;
+    }
+    else if ( unlikely(p2m_is_foreign(t)) )
+    {
+        const struct domain *fdom = page_get_owner_and_reference(page);
+
+        if ( fdom )
+        {
+            if ( likely(fdom != d) )
+                return page;
+            ASSERT_UNREACHABLE();
+            put_page(page);
+        }
+    }
+
+    return NULL;
+}
+
 /* Atomically look up a GFN and take a reference count on the backing page. */
 struct page_info *p2m_get_page_from_gfn(
     struct p2m_domain *p2m, gfn_t gfn,
     p2m_type_t *t, p2m_access_t *a, p2m_query_t q)
 {
-    struct page_info *page = NULL;
+    struct page_info *page;
     p2m_access_t _a;
     p2m_type_t _t;
     mfn_t mfn;
@@ -347,26 +381,9 @@ struct page_info *p2m_get_page_from_gfn(
         /* Fast path: look up and get out */
         p2m_read_lock(p2m);
         mfn = p2m_get_gfn_type_access(p2m, gfn, t, a, 0, NULL, 0);
-        if ( p2m_is_any_ram(*t) && mfn_valid(mfn)
-             && !((q & P2M_UNSHARE) && p2m_is_shared(*t)) )
-        {
-            page = mfn_to_page(mfn);
-            if ( unlikely(p2m_is_foreign(*t)) || unlikely(p2m_is_grant(*t)) )
-            {
-                struct domain *fdom = page_get_owner_and_reference(page);
-
-                ASSERT(!p2m_is_foreign(*t) || fdom != p2m->domain);
-                if ( fdom == NULL )
-                    page = NULL;
-            }
-            else
-            {
-                struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
-
-                if ( !get_page(page, d) )
-                    page = NULL;
-            }
-        }
+        page = !(q & P2M_UNSHARE) || !p2m_is_shared(*t)
+               ? get_page_from_mfn_and_type(p2m->domain, mfn, *t)
+               : NULL;
         p2m_read_unlock(p2m);
 
         if ( page )
@@ -380,14 +397,7 @@ struct page_info *p2m_get_page_from_gfn(
 
     /* Slow path: take the write lock and do fixups */
     mfn = get_gfn_type_access(p2m, gfn_x(gfn), t, a, q, NULL);
-    if ( p2m_is_ram(*t) && mfn_valid(mfn) )
-    {
-        struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
-
-        page = mfn_to_page(mfn);
-        if ( !get_page(page, d) )
-            page = NULL;
-    }
+    page = get_page_from_mfn_and_type(p2m->domain, mfn, *t);
     put_gfn(p2m->domain, gfn_x(gfn));
 
     return page;
--
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