[PATCH for 4.20? v4 3/3] xen/riscv: update mfn calculation in pt_mapping_level()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

When pt_update() is called with arguments (..., INVALID_MFN, ..., 0 or 1),
it indicates that a mapping is being destroyed/modifyed.

In the case when modifying or destroying a mapping, it is necessary to
search until a leaf node is found, instead of searching for a page table
entry based on the precalculated `level` and `order`(look at pt_update()).
This is because when `mfn` == INVALID_MFN, the `mask` (in pt_mapping_level())
will take into account only `vfn`, which could accidentally return an
incorrect level, leading to the discovery of an incorrect page table entry.

For example, if `vfn` is page table level 1 aligned, but it was mapped as
page table level 0, then pt_mapping_level() will return `level` = 1, since
only `vfn` (which is page table level 1 aligned) is taken into account when
`mfn` == INVALID_MFN (look at pt_mapping_level()).

Fixes: c2f1ded524 ("xen/riscv: page table handling")
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in v4:
- Move defintion of local variable table inside `else` case as it is
  used only there.
- Change unmap_table(table) to unmap_table(entry) for unifying both
  cases when _pt_walk() is used and when pte is seached on the specified
  level.
- Initialize local variable `entry` to avoid compilation error caused by
  uninitialized variable.
---
Changes in v3:
- Drop ASSERT() for order as it isn't needed anymore.
- Drop PTE_LEAF_SEARCH and use instead level=CONFIG_PAGING_LEVELS;
  refactor connected code correspondingly.
- Calculate order once.
- Drop initializer for local variable order.
- Drop BUG_ON(!pte_is_mapping(*entry)) for the case when leaf searching
  happens as there is a similar check in pt_check_entry(). Look at
  pt.c:41 and pt.c:75.
---
Changes in v2:
 - Introduce PTE_LEAF_SEARCH to tell page table update operation to
   walk down to wherever the leaf entry is.
 - Use introduced PTE_LEAF_SEARCH to not searching pte_t entry twice.
 - Update the commit message.
---
 xen/arch/riscv/pt.c | 97 +++++++++++++++++++++++++++++----------------
 1 file changed, 63 insertions(+), 34 deletions(-)

diff --git a/xen/arch/riscv/pt.c b/xen/arch/riscv/pt.c
index 260a3a9699..ed0587d58b 100644
--- a/xen/arch/riscv/pt.c
+++ b/xen/arch/riscv/pt.c
@@ -249,12 +249,10 @@ pte_t pt_walk(vaddr_t va, unsigned int *pte_level)
 
 /* Update an entry at the level @target. */
 static int pt_update_entry(mfn_t root, vaddr_t virt,
-                           mfn_t mfn, unsigned int target,
+                           mfn_t mfn, unsigned int *target,
                            unsigned int flags)
 {
     int rc;
-    unsigned int level = HYP_PT_ROOT_LEVEL;
-    pte_t *table;
     /*
      * The intermediate page table shouldn't be allocated when MFN isn't
      * valid and we are not populating page table.
@@ -265,41 +263,48 @@ static int pt_update_entry(mfn_t root, vaddr_t virt,
      * combinations of (mfn, flags).
     */
     bool alloc_tbl = !mfn_eq(mfn, INVALID_MFN) || (flags & PTE_POPULATE);
-    pte_t pte, *entry;
-
-    /* convenience aliases */
-    DECLARE_OFFSETS(offsets, virt);
+    pte_t pte, *entry = NULL;
 
-    table = map_table(root);
-    for ( ; level > target; level-- )
+    if ( *target == CONFIG_PAGING_LEVELS )
+        entry = _pt_walk(virt, target);
+    else
     {
-        rc = pt_next_level(alloc_tbl, &table, offsets[level]);
-        if ( rc == XEN_TABLE_MAP_NOMEM )
+        pte_t *table;
+        unsigned int level = HYP_PT_ROOT_LEVEL;
+        /* convenience aliases */
+        DECLARE_OFFSETS(offsets, virt);
+
+        table = map_table(root);
+        for ( ; level > *target; level-- )
         {
-            rc = -ENOMEM;
-            goto out;
+            rc = pt_next_level(alloc_tbl, &table, offsets[level]);
+            if ( rc == XEN_TABLE_MAP_NOMEM )
+            {
+                rc = -ENOMEM;
+                goto out;
+            }
+
+            if ( rc == XEN_TABLE_MAP_NONE )
+            {
+                rc = 0;
+                goto out;
+            }
+
+            if ( rc != XEN_TABLE_NORMAL )
+                break;
         }
 
-        if ( rc == XEN_TABLE_MAP_NONE )
+        if ( level != *target )
         {
-            rc = 0;
+            dprintk(XENLOG_ERR,
+                    "%s: Shattering superpage is not supported\n", __func__);
+            rc = -EOPNOTSUPP;
             goto out;
         }
 
-        if ( rc != XEN_TABLE_NORMAL )
-            break;
-    }
-
-    if ( level != target )
-    {
-        dprintk(XENLOG_ERR,
-                "%s: Shattering superpage is not supported\n", __func__);
-        rc = -EOPNOTSUPP;
-        goto out;
+        entry = table + offsets[level];
     }
 
-    entry = table + offsets[level];
-
     rc = -EINVAL;
     if ( !pt_check_entry(*entry, mfn, flags) )
         goto out;
@@ -331,7 +336,8 @@ static int pt_update_entry(mfn_t root, vaddr_t virt,
     rc = 0;
 
  out:
-    unmap_table(table);
+    if ( entry )
+        unmap_table(entry);
 
     return rc;
 }
@@ -424,17 +430,40 @@ static int pt_update(vaddr_t virt, mfn_t mfn,
 
     while ( left )
     {
-        unsigned int order, level;
-
-        level = pt_mapping_level(vfn, mfn, left, flags);
-        order = XEN_PT_LEVEL_ORDER(level);
+        unsigned int order, level = CONFIG_PAGING_LEVELS;
 
-        ASSERT(left >= BIT(order, UL));
+        /*
+         * In the case when modifying or destroying a mapping, it is necessary
+         * to search until a leaf node is found, instead of searching for
+         * a page table entry based on the precalculated `level` and `order`
+         * (look at pt_update()).
+         * This is because when `mfn` == INVALID_MFN, the `mask`(in
+         * pt_mapping_level()) will take into account only `vfn`, which could
+         * accidentally return an incorrect level, leading to the discovery of
+         * an incorrect page table entry.
+         *
+         * For example, if `vfn` is page table level 1 aligned, but it was
+         * mapped as page table level 0, then pt_mapping_level() will return
+         * `level` = 1, since only `vfn` (which is page table level 1 aligned)
+         * is taken into account when `mfn` == INVALID_MFN
+         * (look at pt_mapping_level()).
+         *
+         * To force searching until a leaf node is found is necessary to have
+         * `level` == CONFIG_PAGING_LEVELS which is a default value for
+         * `level`.
+         *
+         * For other cases (when a mapping is not being modified or destroyed),
+         * pt_mapping_level() should be used.
+         */
+        if ( !mfn_eq(mfn, INVALID_MFN) || (flags & PTE_POPULATE) )
+            level = pt_mapping_level(vfn, mfn, left, flags);
 
-        rc = pt_update_entry(root, vfn << PAGE_SHIFT, mfn, level, flags);
+        rc = pt_update_entry(root, vfn << PAGE_SHIFT, mfn, &level, flags);
         if ( rc )
             break;
 
+        order = XEN_PT_LEVEL_ORDER(level);
+
         vfn += 1UL << order;
         if ( !mfn_eq(mfn, INVALID_MFN) )
             mfn = mfn_add(mfn, 1UL << order);
-- 
2.48.1