In the Linux kernel, the following vulnerability has been resolved:
ceph: prevent use-after-free in encode_cap_msg()
In fs/ceph/caps.c, in encode_cap_msg(), use after free error was caught by KASAN at this line - ceph_buffer_get(arg->xattr_buf);. This implies before the refcount could be increment here, it was freed.
In same file, in handle_cap_grant() refcount is decremented by this line - ceph_buffer_put(ci->i_xattrs.blob);. It appears that a race occurred and resource was freed by the latter line before the former line could increment it.
encode_cap_msg() is called by __send_cap() and __send_cap() is called by ceph_check_caps() after calling __prep_cap(). __prep_cap() is where arg->xattr_buf is assigned to ci->i_xattrs.blob. This is the spot where the refcount must be increased to prevent use after free error.