MIPS: Avoid mcheck by flushing page range in huge_ptep_set_access_flags()

Problem:

1) Huge page mapping of anonymous memory is initially invalid.  Will be
   faulted in by copy-on-write mechanism.

2) Userspace attempts store at the end of the huge mapping.

3) TLB Refill exception handler fill TLB with a normal (4K sized)
   invalid page at the end of the huge mapping virtual address range.

4) Userspace restarted, and re-attempts the store at the end of the
   huge mapping.

5) Page from #3 is invalid, we get a fault and go to the hugepage
   fault handler.  This tries to map a huge page and calls
   huge_ptep_set_access_flags() to install the mapping.

6) We just call the generic ptep_set_access_flags() to set up the page
   tables, but the flush there assumes a normal (4K sized) page and
   only tries to flush the first part of the huge page virtual address
   out of the TLB, since the existing entry from step #3 doesn't
   conflict, nothing is flushed.

7) We attempt to load the mapping into the TLB, but because it
   conflicts with the entry from step #3, we get a Machine Check
   exception.

The fix: Flush the entire rage covered by the huge page in
huge_ptep_set_access_flags(), and remove the optimization in
local_flush_tlb_range() so that the flush actually does the correct
thing.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Hillf Danton <dhillf@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/4661/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
(cherry picked from commit dd617f258cc39d36be26afee9912624a2d23112c)

1 files changed, 4 insertions, 14 deletions

diff --git a/arch/mips/mm/tlb-r4k.c b/arch/mips/mm/tlb-r4k.c
index 4b9b935a070e..88e79ad6f811 100644
--- a/arch/mips/mm/tlb-r4k.c
+++ b/arch/mips/mm/tlb-r4k.c
@@ -120,18 +120,11 @@ void local_flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 
         if (cpu_context(cpu, mm) != 0) {
                 unsigned long size, flags;
-                int huge = is_vm_hugetlb_page(vma);
 
                 ENTER_CRITICAL(flags);
-                if (huge) {
-                        start = round_down(start, HPAGE_SIZE);
-                        end = round_up(end, HPAGE_SIZE);
-                        size = (end - start) >> HPAGE_SHIFT;
-                } else {
-                        start = round_down(start, PAGE_SIZE << 1);
-                        end = round_up(end, PAGE_SIZE << 1);
-                        size = (end - start) >> (PAGE_SHIFT + 1);
-                }
+                start = round_down(start, PAGE_SIZE << 1);
+                end = round_up(end, PAGE_SIZE << 1);
+                size = (end - start) >> (PAGE_SHIFT + 1);
                 if (size <= current_cpu_data.tlbsize/2) {
                         int oldpid = read_c0_entryhi();
                         int newpid = cpu_asid(cpu, mm);
@@ -140,10 +133,7 @@ void local_flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
                                 int idx;
 
                                 write_c0_entryhi(start | newpid);
-                                if (huge)
-                                        start += HPAGE_SIZE;
-                                else
-                                        start += (PAGE_SIZE << 1);
+                                start += (PAGE_SIZE << 1);
                                 mtc0_tlbw_hazard();
                                 tlb_probe();
                                 tlb_probe_hazard();