[xen stable-4.10] AMD/IOMMU: update live PTEs atomically

[patchbot@xxxxxxx]

commit 56f8da75e1d42047549da304afb20bd72d572c84
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Oct 20 15:21:19 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 20 15:21:19 2020 +0200

    AMD/IOMMU: update live PTEs atomically
    
    Updating a live PTE bitfield by bitfield risks the compiler re-ordering
    the individual updates as well as splitting individual updates into
    multiple memory writes. Construct the new entry fully in a local
    variable, do the check to determine the flushing needs on the thus
    established new entry, and then write the new entry by a single insn.
    
    Similarly using memset() to clear a PTE is unsafe, as the order of
    writes the function does is, at least in principle, undefined.
    
    This is part of XSA-347.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Paul Durrant <paul@xxxxxxx>
    master commit: 3b055121c5410e2c3105d6d06aa24ca0d58868cd
    master date: 2020-10-20 14:22:52 +0200
---
 xen/drivers/passthrough/amd/iommu_map.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/xen/drivers/passthrough/amd/iommu_map.c 
b/xen/drivers/passthrough/amd/iommu_map.c
index 85ce9405cf..3c29788d9a 100644
--- a/xen/drivers/passthrough/amd/iommu_map.c
+++ b/xen/drivers/passthrough/amd/iommu_map.c
@@ -41,7 +41,7 @@ static void clear_iommu_pte_present(unsigned long l1_mfn, 
unsigned long gfn)
 
     table = map_domain_page(_mfn(l1_mfn));
     pte = table + pfn_to_pde_idx(gfn, IOMMU_PAGING_MODE_LEVEL_1);
-    *pte = 0;
+    write_atomic(pte, 0);
     unmap_domain_page(table);
 }
 
@@ -49,7 +49,7 @@ static bool_t set_iommu_pde_present(u32 *pde, unsigned long 
next_mfn,
                                     unsigned int next_level,
                                     bool_t iw, bool_t ir)
 {
-    uint64_t addr_lo, addr_hi, maddr_next;
+    uint64_t addr_lo, addr_hi, maddr_next, full;
     u32 entry;
     bool need_flush = false, old_present;
 
@@ -106,7 +106,7 @@ static bool_t set_iommu_pde_present(u32 *pde, unsigned long 
next_mfn,
     if ( next_level == IOMMU_PAGING_MODE_LEVEL_0 )
         set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, entry,
                              IOMMU_PTE_FC_MASK, IOMMU_PTE_FC_SHIFT, &entry);
-    pde[1] = entry;
+    full = (uint64_t)entry << 32;
 
     /* mark next level as 'present' */
     set_field_in_reg_u32((u32)addr_lo >> PAGE_SHIFT, 0,
@@ -118,7 +118,9 @@ static bool_t set_iommu_pde_present(u32 *pde, unsigned long 
next_mfn,
     set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, entry,
                          IOMMU_PDE_PRESENT_MASK,
                          IOMMU_PDE_PRESENT_SHIFT, &entry);
-    pde[0] = entry;
+    full |= entry;
+
+    write_atomic((uint64_t *)pde, full);
 
     return need_flush;
 }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.10