fork: copy mm's vm usage counters under mmap_sem

If a forking process has a thread calling (un)mmap (silly but still),
the child process may have some of its mm's vm usage counters (total_vm
and friends) screwed up, because currently they are copied from oldmm
w/o holding any locks (memcpy in dup_mm).

This patch moves the counters initialization to dup_mmap() to be called
under oldmm->mmap_sem, which eliminates any possibility of race.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 5a547a59a38a..aff84f84b0d3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -374,6 +374,11 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
          */
         down_write_nested(&mm->mmap_sem, SINGLE_DEPTH_NESTING);
 
+        mm->total_vm = oldmm->total_vm;
+        mm->shared_vm = oldmm->shared_vm;
+        mm->exec_vm = oldmm->exec_vm;
+        mm->stack_vm = oldmm->stack_vm;
+
         rb_link = &mm->mm_rb.rb_node;
         rb_parent = NULL;
         pprev = &mm->mmap;