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>
diff --git a/fs/ramfs/file-nommu.c b/fs/ramfs/file-nommu.c
index 2efc571..2665313 100644
--- a/fs/ramfs/file-nommu.c
+++ b/fs/ramfs/file-nommu.c
@@ -131,6 +131,8 @@
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 @@
) {
/* 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;
}
