mm: introduce VM_POPULATE flag to better deal with racy userspace programs

The vm_populate() code populates user mappings without constantly
holding the mmap_sem.  This makes it susceptible to racy userspace
programs: the user mappings may change while vm_populate() is running,
and in this case vm_populate() may end up populating the new mapping
instead of the old one.

In order to reduce the possibility of userspace getting surprised by
this behavior, this change introduces the VM_POPULATE vma flag which
gets set on vmas we want vm_populate() to work on.  This way
vm_populate() may still end up populating the new mapping after such a
race, but only if the new mapping is also one that the user has
requested (using MAP_SHARED, MAP_LOCKED or mlock) to be populated.

Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Tested-by: Andy Lutomirski <luto@amacapital.net>
Cc: Greg Ungerer <gregungerer@westnet.com.au>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 3 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 8826c77513a9..39a3944e1658 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1306,9 +1306,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
         }
 
         addr = mmap_region(file, addr, len, vm_flags, pgoff);
-        if (!IS_ERR_VALUE(addr) &&
-            ((vm_flags & VM_LOCKED) ||
-             (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
+        if (!IS_ERR_VALUE(addr) && (vm_flags & VM_POPULATE))
                 *populate = true;
         return addr;
 }