[PATCH 5/6] NOMMU: Fix race between ramfs truncation and shared mmap

From: David Howells
Date: Fri Jan 08 2010 - 17:05:55 EST

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@xxxxxxxxxx>

fs/ramfs/file-nommu.c | 7 ++++++-
include/linux/sched.h | 2 +-
2 files changed, 7 insertions(+), 2 deletions(-)

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 @@ 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;

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 08a2523..6f7bba9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -389,7 +389,7 @@ arch_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
extern void arch_unmap_area(struct mm_struct *, unsigned long);
extern void arch_unmap_area_topdown(struct mm_struct *, unsigned long);
-extern void arch_pick_mmap_layout(struct mm_struct *mm) {}
+static inline void arch_pick_mmap_layout(struct mm_struct *mm) {}


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