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 <[email protected]>
---
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;
On Sat, Sep 05, 2009 at 12:25:37AM -0400, Nicolas Pitre wrote:
> 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 <[email protected]>
Acked-by: "Theodore Ts'o" <[email protected]>
> 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.
This is an ext2-only issue; for journalling reasons ext3 and ext4 read
and write the directory via buffer heads. So this issue won't apply
for ext3 or ext4.
- Ted