[PATCH v2 1/4] Fix XIP fault vs truncate race

[Ross Zwisler <ross.zwisler@linux.intel.com>]

From: Matthew Wilcox <matthew.r.wilcox@intel.com>

Pagecache faults recheck i_size after taking the page lock to ensure that
the fault didn't race against a truncate.  We don't have a page to lock
in the XIP case, so use the i_mmap_mutex instead.  It is locked in the
truncate path in unmap_mapping_range() after updating i_size.  So while
we hold it in the fault path, we are guaranteed that either i_size has
already been updated in the truncate path, or that the truncate will
subsequently call zap_page_range_single() and so remove the mapping we
have just inserted.

There is a window of time in which i_size has been reduced and the
thread has a mapping to a page which will be removed from the file,
but this is harmless as the page will not be allocated to a different
purpose before the thread's access to it is revoked.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
---
 mm/filemap_xip.c |   24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
index d8d9fe3..c8d23e9 100644
--- a/mm/filemap_xip.c
+++ b/mm/filemap_xip.c
@@ -260,8 +260,17 @@ again:
 		__xip_unmap(mapping, vmf->pgoff);
 
 found:
+		/* We must recheck i_size under i_mmap_mutex */
+		mutex_lock(&mapping->i_mmap_mutex);
+		size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
+							PAGE_CACHE_SHIFT;
+		if (unlikely(vmf->pgoff >= size)) {
+			mutex_unlock(&mapping->i_mmap_mutex);
+			return VM_FAULT_SIGBUS;
+		}
 		err = vm_insert_mixed(vma, (unsigned long)vmf->virtual_address,
 							xip_pfn);
+		mutex_unlock(&mapping->i_mmap_mutex);
 		if (err == -ENOMEM)
 			return VM_FAULT_OOM;
 		/*
@@ -285,16 +294,27 @@ found:
 		}
 		if (error != -ENODATA)
 			goto out;
+
+		/* We must recheck i_size under i_mmap_mutex */
+		mutex_lock(&mapping->i_mmap_mutex);
+		size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
+							PAGE_CACHE_SHIFT;
+		if (unlikely(vmf->pgoff >= size)) {
+			ret = VM_FAULT_SIGBUS;
+			goto unlock;
+		}
 		/* not shared and writable, use xip_sparse_page() */
 		page = xip_sparse_page();
 		if (!page)
-			goto out;
+			goto unlock;
 		err = vm_insert_page(vma, (unsigned long)vmf->virtual_address,
 							page);
 		if (err == -ENOMEM)
-			goto out;
+			goto unlock;
 
 		ret = VM_FAULT_NOPAGE;
+unlock:
+		mutex_unlock(&mapping->i_mmap_mutex);
 out:
 		write_seqcount_end(&xip_sparse_seq);
 		mutex_unlock(&xip_sparse_mutex);
-- 
1.7.10.4