[Xen-devel] [PATCH v3 for-4.10 1/2] x86/mm: fix potential race conditions in map_pages_to_xen().

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

From: Min He <min.he@xxxxxxxxx>

In map_pages_to_xen(), a L2 page table entry may be reset to point to
a superpage, and its corresponding L1 page table need be freed in such
scenario, when these L1 page table entries are mapping to consecutive
page frames and having the same mapping flags.

However, variable `pl1e` is not protected by the lock before L1 page table
is enumerated. A race condition may happen if this code path is invoked
simultaneously on different CPUs.

For example, `pl1e` value on CPU0 may hold an obsolete value, pointing
to a page which has just been freed on CPU1. Besides, before this page
is reused, it will still be holding the old PTEs, referencing consecutive
page frames. Consequently the `free_xen_pagetable(l2e_to_l1e(ol2e))` will
be triggered on CPU0, resulting the unexpected free of a normal page.

This patch fixes the above problem by protecting the `pl1e` with the lock.

Also, there're other potential race conditions. For instance, the L2/L3
entry may be modified concurrently on different CPUs, by routines such as
map_pages_to_xen(), modify_xen_mappings() etc. To fix this, this patch will
check the _PAGE_PRESENT and _PAGE_PSE flags, after the spinlock is obtained,
for the corresponding L2/L3 entry.

Signed-off-by: Min He <min.he@xxxxxxxxx>
Signed-off-by: Yi Zhang <yi.z.zhang@xxxxxxxxx>
Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
---
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Changes in v3:
According to comments from Jan Beulich:
  - use local variable instead of dereference pointer to pte to check flag.
  - also check the _PAGE_PRESENT for L2E/L3E.
Others:
  - Commit message changes.

Changes in v2:
According to comments from Jan Beulich:
  - check PSE of pl2e and pl3e, and skip the re-consolidation if set.
  - commit message changes, e.g. add "From :" tag etc.
  - code style changes.
  - introduce a seperate patch to resolve the similar issue in
    modify_xen_mappings().
---
 xen/arch/x86/mm.c | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index a20fdca..1697be9 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4844,9 +4844,29 @@ int map_pages_to_xen(
             {
                 unsigned long base_mfn;
 
-                pl1e = l2e_to_l1e(*pl2e);
                 if ( locking )
                     spin_lock(&map_pgdir_lock);
+
+                ol2e = *pl2e;
+                /*
+                 * L2E may be already cleared, or set to a superpage, by
+                 * concurrent paging structure modifications on other CPUs.
+                 */
+                if ( !(l2e_get_flags(ol2e) & _PAGE_PRESENT) )
+                {
+                    if ( locking )
+                        spin_unlock(&map_pgdir_lock);
+                    continue;
+                }
+
+                if ( l2e_get_flags(ol2e) & _PAGE_PSE )
+                {
+                    if ( locking )
+                        spin_unlock(&map_pgdir_lock);
+                    goto check_l3;
+                }
+
+                pl1e = l2e_to_l1e(ol2e);
                 base_mfn = l1e_get_pfn(*pl1e) & ~(L1_PAGETABLE_ENTRIES - 1);
                 for ( i = 0; i < L1_PAGETABLE_ENTRIES; i++, pl1e++ )
                     if ( (l1e_get_pfn(*pl1e) != (base_mfn + i)) ||
@@ -4854,7 +4874,6 @@ int map_pages_to_xen(
                         break;
                 if ( i == L1_PAGETABLE_ENTRIES )
                 {
-                    ol2e = *pl2e;
                     l2e_write_atomic(pl2e, l2e_from_pfn(base_mfn,
                                                         l1f_to_lNf(flags)));
                     if ( locking )
@@ -4880,7 +4899,20 @@ int map_pages_to_xen(
 
             if ( locking )
                 spin_lock(&map_pgdir_lock);
+
             ol3e = *pl3e;
+            /*
+             * L3E may be already cleared, or set to a superpage, by
+             * concurrent paging structure modifications on other CPUs.
+             */
+            if ( !(l3e_get_flags(ol3e) & _PAGE_PRESENT) ||
+                (l3e_get_flags(ol3e) & _PAGE_PSE) )
+            {
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
+                continue;
+            }
+
             pl2e = l3e_to_l2e(ol3e);
             base_mfn = l2e_get_pfn(*pl2e) & ~(L2_PAGETABLE_ENTRIES *
                                               L1_PAGETABLE_ENTRIES - 1);
-- 
2.5.0


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