On Mon, Jan 20, 2020 at 07:30:40AM +0000, Al Viro wrote:
> <checks ext4>
> Yup, that bug is there as well, all right. Look:
> #ifdef CONFIG_UNICODE
> static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
> const char *str, const struct qstr *name)
> {
> struct qstr qstr = {.name = str, .len = len };
> struct inode *inode = dentry->d_parent->d_inode;
>
> if (!IS_CASEFOLDED(inode) || !EXT4_SB(inode->i_sb)->s_encoding) {
>
> Guess what happens if your (lockless) call of ->d_compare() runs
> into the following sequence:
> CPU1: ext4_d_compare() fetches ->d_parent
> CPU1: takes a hardware interrupt
> CPU2: dentry gets evicted by memory pressure; so is its parent, since
> it was the only thing that used to keep it pinned. Eviction of the parent
> calls dentry_unlink_inode() on the parent, which zeroes its ->d_inode.
> CPU1: comes back
> CPU1: fetches parent's ->d_inode and gets NULL
> CPU1: oopses on null pointer dereference.
>
> It's not impossible to hit. Note that e.g. vfat_cmpi() is not vulnerable
> to that problem - ->d_sb is stable and both the superblock and ->nls_io
> freeing is RCU-delayed.
>
> I hadn't checked ->d_compare() instances for a while; somebody needs to
> do that again, by the look of it. The above definitely is broken;
> no idea how many other instaces had grown such bugs...
f2fs one also has the same bug. Anyway, I'm going down right now, will
check the rest tomorrow morning...
