Commit 81759b5b authored Jan 15, 2010 by David Howells Committed by Linus Torvalds Jan 16, 2010

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>

parent efc1a3b1

fs/ramfs/file-nommu.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

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