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FAILED: patch "[PATCH] ext4: fix an use-after-free issue about data=journal" failed to apply to 5.4-stable tree

PATCH [email protected] 1 participant 1 message Jan 24, 2022

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The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5c48a7df91499e371ef725895b2e2d21a126e227 Mon Sep 17 00:00:00 2001
From: Zhang Yi <[email protected]>
Date: Sat, 25 Dec 2021 17:09:37 +0800
Subject: [PATCH] ext4: fix an use-after-free issue about data=journal
writeback mode

Our syzkaller report an use-after-free issue that accessing the freed buffer_head on the writeback page in __ext4_journalled_writepage(). The problem is that if there was a truncate racing with the data=journalled writeback procedure, the writeback length could become zero and bget_one() refuse to get buffer_head's refcount, then the truncate procedure release buffer once we drop page lock, finally, the last ext4_walk_page_buffers() trigger the use-after-free problem.

sync truncate
ext4_sync_file()
file_write_and_wait_range()

                                   ext4_setattr(0)
                                    inode->i_size = 0

ext4_writepage()
len = 0
__ext4_journalled_writepage()
page_bufs = page_buffers(page) ext4_walk_page_buffers(bget_one) <- does not get refcount
do_invalidatepage()
free_buffer_head()
ext4_walk_page_buffers(page_bufs) <- trigger use-after-free

After commit bdf96838aea6 ("ext4: fix race between truncate and __ext4_journalled_writepage()"), we have already handled the racing case, so the bget_one() and bput_one() are not needed. So this patch simply remove these hunk, and recheck the i_size to make it safe.

Fixes bdf96838aea6 ("ext4: fix race between truncate and __ext4_journalled_writepage()")
Signed-off-by Zhang Yi <[email protected]>
Cc [email protected]
Link https://lore.kernel.org/r/[email protected] open here
Signed-off-by Theodore Ts'o <[email protected]>
fs/ext4/inode.c +10 -27
index bca9951634d9..68070f34f0cf 100644
@@ -1845,30 +1845,16 @@ int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 	return 0;
 }
 
-static int bget_one(handle_t *handle, struct inode *inode,
-		    struct buffer_head *bh)
-{
-	get_bh(bh);
-	return 0;
-}
-
-static int bput_one(handle_t *handle, struct inode *inode,
-		    struct buffer_head *bh)
-{
-	put_bh(bh);
-	return 0;
-}
-
 static int __ext4_journalled_writepage(struct page *page,
 				       unsigned int len)
 {
 	struct address_space *mapping = page->mapping;
 	struct inode *inode = mapping->host;
-	struct buffer_head *page_bufs = NULL;
 	handle_t *handle = NULL;
 	int ret = 0, err = 0;
 	int inline_data = ext4_has_inline_data(inode);
 	struct buffer_head *inode_bh = NULL;
+	loff_t size;
 
 	ClearPageChecked(page);
 

@@ -1878,14 +1864,6 @@ static int __ext4_journalled_writepage(struct page *page,
 		inode_bh = ext4_journalled_write_inline_data(inode, len, page);
 		if (inode_bh == NULL)
 			goto out;
-	} else {
-		page_bufs = page_buffers(page);
-		if (!page_bufs) {
-			BUG();
-			goto out;
-		}
-		ext4_walk_page_buffers(handle, inode, page_bufs, 0, len,
-				       NULL, bget_one);
 	}
 	/*
 	 * We need to release the page lock before we start the

@@ -1906,7 +1884,8 @@ static int __ext4_journalled_writepage(struct page *page,
 
 	lock_page(page);
 	put_page(page);
-	if (page->mapping != mapping) {
+	size = i_size_read(inode);
+	if (page->mapping != mapping || page_offset(page) > size) {
 		/* The page got truncated from under us */
 		ext4_journal_stop(handle);
 		ret = 0;

@@ -1916,6 +1895,13 @@ static int __ext4_journalled_writepage(struct page *page,
 	if (inline_data) {
 		ret = ext4_mark_inode_dirty(handle, inode);
 	} else {
+		struct buffer_head *page_bufs = page_buffers(page);
+
+		if (page->index == size >> PAGE_SHIFT)
+			len = size & ~PAGE_MASK;
+		else
+			len = PAGE_SIZE;
+
 		ret = ext4_walk_page_buffers(handle, inode, page_bufs, 0, len,
 					     NULL, do_journal_get_write_access);
 

@@ -1936,9 +1922,6 @@ static int __ext4_journalled_writepage(struct page *page,
 out:
 	unlock_page(page);
 out_no_pagelock:
-	if (!inline_data && page_bufs)
-		ext4_walk_page_buffers(NULL, inode, page_bufs, 0, len,
-				       NULL, bput_one);
 	brelse(inode_bh);
 	return ret;
 }