mm: migration: avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks

Page migration requires rmap to be able to find all ptes mapping a page
at all times, otherwise the migration entry can be instantiated, but it
is possible to leave one behind if the second rmap_walk fails to find
the page.  If this page is later faulted, migration_entry_to_page() will
call BUG because the page is locked indicating the page was migrated by
the migration PTE not cleaned up. For example

  kernel BUG at include/linux/swapops.h:105!
  invalid opcode: 0000 [#1] PREEMPT SMP
  ...
  Call Trace:
   [<ffffffff810e951a>] handle_mm_fault+0x3f8/0x76a
   [<ffffffff8130c7a2>] do_page_fault+0x44a/0x46e
   [<ffffffff813099b5>] page_fault+0x25/0x30
   [<ffffffff8114de33>] load_elf_binary+0x152a/0x192b
   [<ffffffff8111329b>] search_binary_handler+0x173/0x313
   [<ffffffff81114896>] do_execve+0x219/0x30a
   [<ffffffff8100a5c6>] sys_execve+0x43/0x5e
   [<ffffffff8100320a>] stub_execve+0x6a/0xc0
  RIP  [<ffffffff811094ff>] migration_entry_wait+0xc1/0x129

There is a race between shift_arg_pages and migration that triggers this
bug.  A temporary stack is setup during exec and later moved.  If
migration moves a page in the temporary stack and the VMA is then removed
before migration completes, the migration PTE may not be found leading to
a BUG when the stack is faulted.

This patch causes pages within the temporary stack during exec to be
skipped by migration.  It does this by marking the VMA covering the
temporary stack with an otherwise impossible combination of VMA flags.
These flags are cleared when the temporary stack is moved to its final
location.

[kamezawa.hiroyu@jp.fujitsu.com: idea for having migration skip temporary stacks]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/exec.c b/fs/exec.c
index e6e94c6..9badbc0 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -242,9 +242,10 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	 * use STACK_TOP because that can depend on attributes which aren't
 	 * configured yet.
 	 */
+	BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
 	vma->vm_end = STACK_TOP_MAX;
 	vma->vm_start = vma->vm_end - PAGE_SIZE;
-	vma->vm_flags = VM_STACK_FLAGS;
+	vma->vm_flags = VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
 	err = insert_vm_struct(mm, vma);
@@ -616,6 +617,7 @@ int setup_arg_pages(struct linux_binprm *bprm,
 	else if (executable_stack == EXSTACK_DISABLE_X)
 		vm_flags &= ~VM_EXEC;
 	vm_flags |= mm->def_flags;
+	vm_flags |= VM_STACK_INCOMPLETE_SETUP;
 
 	ret = mprotect_fixup(vma, &prev, vma->vm_start, vma->vm_end,
 			vm_flags);
@@ -630,6 +632,9 @@ int setup_arg_pages(struct linux_binprm *bprm,
 			goto out_unlock;
 	}
 
+	/* mprotect_fixup is overkill to remove the temporary stack flags */
+	vma->vm_flags &= ~VM_STACK_INCOMPLETE_SETUP;
+
 	stack_expand = 131072UL; /* randomly 32*4k (or 2*64k) pages */
 	stack_size = vma->vm_end - vma->vm_start;
 	/*