mm: fix use-after-free in sys_remap_file_pages

remap_file_pages calls mmap_region, which may merge the VMA with other
existing VMAs, and free "vma".  This can lead to a use-after-free bug.
Avoid the bug by remembering vm_flags before calling mmap_region, and
not trying to dereference vma later.

Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/mm/fremap.c b/mm/fremap.c
index 5bff08147768..bbc4d660221a 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -208,9 +208,10 @@ get_write_lock:
                 if (mapping_cap_account_dirty(mapping)) {
                         unsigned long addr;
                         struct file *file = get_file(vma->vm_file);
+                        /* mmap_region may free vma; grab the info now */
+                        vm_flags = vma->vm_flags;
 
-                        addr = mmap_region(file, start, size,
-                                        vma->vm_flags, pgoff);
+                        addr = mmap_region(file, start, size, vm_flags, pgoff);
                         fput(file);
                         if (IS_ERR_VALUE(addr)) {
                                 err = addr;
@@ -218,7 +219,7 @@ get_write_lock:
                                 BUG_ON(addr != start);
                                 err = 0;
                         }
-                        goto out;
+                        goto out_freed;
                 }
                 mutex_lock(&mapping->i_mmap_mutex);
                 flush_dcache_mmap_lock(mapping);
@@ -253,6 +254,7 @@ get_write_lock:
 out:
         if (vma)
                 vm_flags = vma->vm_flags;
+out_freed:
         if (likely(!has_write_lock))
                 up_read(&mm->mmap_sem);
         else