[PATCH] ext2: fix unbalanced kmap()/kunmap()

[Nicolas Pitre]

In ext2_rename(), dir_page is acquired through ext2_dotdot().  It is 
then released through ext2_set_link() but only if old_dir != new_dir. 
Failing that, the pkmap reference count is never decremented and the 
page remains pinned forever.  Repeat that a couple times with highmem 
pages and all pkmap slots get exhausted, and every further kmap() calls 
end up stalling on the pkmap_map_wait queue at which point the whole 
system comes to a halt.

Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxxx>
---

I ran into this issue while testing highmem on ARM by running the git 
test suite in a loop with 3 parallel instances.  Using the right mv 
sequence in a script would constitute a pretty simple recipe for a 
local DoS on systems running ext2 and highmem.  No idea if ext3 or ext4 
have the same issue.

diff --git a/fs/ext2/namei.c b/fs/ext2/namei.c
index e1dedb0..78d9b92 100644
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -362,6 +362,10 @@ static int ext2_rename (struct inode * old_dir, struct dentry * old_dentry,
 	if (dir_de) {
 		if (old_dir != new_dir)
 			ext2_set_link(old_inode, dir_de, dir_page, new_dir, 0);
+		else {
+			kunmap(dir_page);
+			page_cache_release(dir_page);
+		}
 		inode_dec_link_count(old_dir);
 	}
 	return 0;
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