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    Subject[PATCH 5.4 122/309] f2fs: fix race conditions in ->d_compare() and ->d_hash()
    Date
    From: Eric Biggers <ebiggers@google.com>

    commit 80f2388afa6ef985f9c5c228e36705c4d4db4756 upstream.

    Since ->d_compare() and ->d_hash() can be called in RCU-walk mode,
    ->d_parent and ->d_inode can be concurrently modified, and in
    particular, ->d_inode may be changed to NULL. For f2fs_d_hash() this
    resulted in a reproducible NULL dereference if a lookup is done in a
    directory being deleted, e.g. with:

    int main()
    {
    if (fork()) {
    for (;;) {
    mkdir("subdir", 0700);
    rmdir("subdir");
    }
    } else {
    for (;;)
    access("subdir/file", 0);
    }
    }

    ... or by running the 't_encrypted_d_revalidate' program from xfstests.
    Both repros work in any directory on a filesystem with the encoding
    feature, even if the directory doesn't actually have the casefold flag.

    I couldn't reproduce a crash in f2fs_d_compare(), but it appears that a
    similar crash is possible there.

    Fix these bugs by reading ->d_parent and ->d_inode using READ_ONCE() and
    falling back to the case sensitive behavior if the inode is NULL.

    Reported-by: Al Viro <viro@zeniv.linux.org.uk>
    Fixes: 2c2eb7a300cd ("f2fs: Support case-insensitive file name lookups")
    Cc: <stable@vger.kernel.org> # v5.4+
    Signed-off-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/f2fs/dir.c | 9 ++++++---
    1 file changed, 6 insertions(+), 3 deletions(-)

    --- a/fs/f2fs/dir.c
    +++ b/fs/f2fs/dir.c
    @@ -1068,24 +1068,27 @@ static int f2fs_d_compare(const struct d
    const char *str, const struct qstr *name)
    {
    struct qstr qstr = {.name = str, .len = len };
    + const struct dentry *parent = READ_ONCE(dentry->d_parent);
    + const struct inode *inode = READ_ONCE(parent->d_inode);

    - if (!IS_CASEFOLDED(dentry->d_parent->d_inode)) {
    + if (!inode || !IS_CASEFOLDED(inode)) {
    if (len != name->len)
    return -1;
    return memcmp(str, name->name, len);
    }

    - return f2fs_ci_compare(dentry->d_parent->d_inode, name, &qstr, false);
    + return f2fs_ci_compare(inode, name, &qstr, false);
    }

    static int f2fs_d_hash(const struct dentry *dentry, struct qstr *str)
    {
    struct f2fs_sb_info *sbi = F2FS_SB(dentry->d_sb);
    const struct unicode_map *um = sbi->s_encoding;
    + const struct inode *inode = READ_ONCE(dentry->d_inode);
    unsigned char *norm;
    int len, ret = 0;

    - if (!IS_CASEFOLDED(dentry->d_inode))
    + if (!inode || !IS_CASEFOLDED(inode))
    return 0;

    norm = f2fs_kmalloc(sbi, PATH_MAX, GFP_ATOMIC);

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