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>

diff --git a/mm/fremap.c b/mm/fremap.c
index 5bff0814776870e2974e6e436ad99f99f264e05a..bbc4d660221ac4e514e24d49ce140b25ad5364d2 100644 (file)
--- 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