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

From: Nicolas Pitre
Date: Sat Sep 05 2009 - 00:26:14 EST

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);
+ }
return 0;
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/