nommu: fix race between ramfs truncation and shared mmap

Fix the race between the truncation of a ramfs file and an attempt to make
a shared mmap of region of that file.

The problem is that do_mmap_pgoff() calls f_op->get_unmapped_area() to
verify that the file region is made of contiguous pages and to find its
base address - but there isn't any locking to guarantee this region until
vma_prio_tree_insert() is called by add_vma_to_mm().

Note that moving the functionality into f_op->mmap() doesn't help as that
is also called before vma_prio_tree_insert().

Instead make ramfs_nommu_check_mappings() grab nommu_region_sem whilst it
does its checks.  This means that this function will wait whilst mmaps
take place.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Greg Ungerer <gerg@snapgear.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/ramfs/file-nommu.c b/fs/ramfs/file-nommu.c
index 2efc57173fd7..266531343aae 100644
--- a/fs/ramfs/file-nommu.c
+++ b/fs/ramfs/file-nommu.c
@@ -131,6 +131,8 @@ static int ramfs_nommu_check_mappings(struct inode *inode,
         struct vm_area_struct *vma;
         struct prio_tree_iter iter;
 
+        down_write(&nommu_region_sem);
+
         /* search for VMAs that fall within the dead zone */
         vma_prio_tree_foreach(vma, &iter, &inode->i_mapping->i_mmap,
                               newsize >> PAGE_SHIFT,
@@ -138,10 +140,13 @@ static int ramfs_nommu_check_mappings(struct inode *inode,
                               ) {
                 /* found one - only interested if it's shared out of the page
                  * cache */
-                if (vma->vm_flags & VM_SHARED)
+                if (vma->vm_flags & VM_SHARED) {
+                        up_write(&nommu_region_sem);
                         return -ETXTBSY; /* not quite true, but near enough */
+                }
         }
 
+        up_write(&nommu_region_sem);
         return 0;
 }