Re: [PATCH v3] ksmbd: fix use-after-free and NULL deref in smb_grant_oplock()

[Namjae Jeon]

On Tue, Mar 17, 2026 at 3:53 PM Werner Kasselman <werner@xxxxxxxxxx> wrote:
>
> smb_grant_oplock() has two issues in the oplock publication sequence:
>
> 1) opinfo is linked into ci->m_op_list (via opinfo_add) before
>    add_lease_global_list() is called.  If add_lease_global_list()
>    fails (kmalloc returns NULL), the error path frees the opinfo
>    via __free_opinfo() while it is still linked in ci->m_op_list.
>    Concurrent m_op_list readers (opinfo_get_list, or direct iteration
>    in smb_break_all_levII_oplock) dereference the freed node.
>
> 2) opinfo->o_fp is assigned after add_lease_global_list() publishes
>    the opinfo on the global lease list.  A concurrent
>    find_same_lease_key() can walk the lease list and dereference
>    opinfo->o_fp->f_ci while o_fp is still NULL.
>
> Fix by restructuring the publication sequence to eliminate post-publish
> failure:
>
> - Set opinfo->o_fp before any list publication (fixes NULL deref).
> - Preallocate lease_table via alloc_lease_table() before opinfo_add()
>   so add_lease_global_list() becomes infallible after publication.
> - Keep the original m_op_list publication order (opinfo_add before
>   lease list) so concurrent opens via same_client_has_lease() and
>   opinfo_get_list() still see the in-flight grant.
> - Use opinfo_put() instead of __free_opinfo() on err_out so that
>   the RCU-deferred free path is used.
>
> This also requires splitting add_lease_global_list() to take a
> preallocated lease_table and changing its return type from int to void,
> since it can no longer fail.
>
> Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
> Fixes: 1dfd062caa16 ("ksmbd: fix use-after-free by using call_rcu() for oplock_info")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Werner Kasselman <werner@xxxxxxxxxxx>
Applied it to #ksmbd-for-next-next.
Thanks!