On Thu, Jan 03, 2019 at 01:41:04PM +0530, Sahitya Tummala wrote: > In the vfs_statx() context, during path lookup, the dentry gets > added to sd->s_dentry via configfs_attach_attr(). In the end, > vfs_statx() kills the dentry by calling path_put(), which invokes > configfs_d_iput(). Ideally, this dentry must be removed from > sd->s_dentry but it doesn't if the sd->s_count >= 3. As a result, > sd->s_dentry is holding reference to a stale dentry pointer whose > memory is already freed up. This results in use-after-free issue, > when this stale sd->s_dentry is accessed later in > configfs_readdir() path. > > This issue can be easily reproduced, by running the LTP test case - > sh fs_racer_file_list.sh /config > (https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/fs/racer/fs_racer_file_list.sh) > > Signed-off-by: Sahitya Tummala <[email protected]> > --- > fs/configfs/dir.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c > index 39843fa..a5906c5 100644 > --- a/fs/configfs/dir.c > +++ b/fs/configfs/dir.c > @@ -66,7 +66,7 @@ static void configfs_d_iput(struct dentry * dentry, > * configfs_attach_attr and set sd->s_dentry to null > * even it's still in use. > */ > - if (atomic_read(&sd->s_count) <= 2) > + if (sd->s_dentry == dentry) > sd->s_dentry = NULL; > > spin_unlock(&configfs_dirent_lock);
Please, update the comment above as well - it refers the increment of sd->s_count, which becomes stale. I agree that the problem is real and the fix is sane (and that's what should've been done in the first place, back in "configfs: fix race between dentry put and lookup", which has introduced that logics).
