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    SubjectRe: vfat: Broken case-insensitive support for UTF-8
    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...

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