Re: [PATCH] nfsd: free nfsd_file by gc after adding it to lru list

Chuck Lever
2025-01-13T14:07:44Z

Hello!

On 1/12/25 9:59 PM, Li Lingfeng wrote:
> In nfsd_file_put, after inserting the nfsd_file into the nfsd_file_lru
> list, gc may be triggered in another thread and immediately release this
> nfsd_file, which will lead to a UAF when accessing this nfsd_file again.

Do you happen to have a reproducer that can trigger this issue?

> All the places where unhash is done will also perform lru_remove, so there
> is no need to do lru_remove separately here. After inserting the nfsd_file
> into the nfsd_file_lru list, it can be released by relying on gc.
>
> Fixes: 4a0e73e635e3 ("NFSD: Leave open files out of the filecache LRU")

The code that is being replaced below was introduced in ac3a2585f018
("nfsd: rework refcounting in filecache"). This fix won't apply to
kernels that have only 4a0e73e635e3 but not ac3a2585f018, for instance.

At the very least we need to add "Cc: stable@vger.kernel.org # v6.2" but
I'm not convinced "Fixes: 4a0e73e635e3" is correct.

> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
> ---
> fs/nfsd/filecache.c | 12 ++----------
> 1 file changed, 2 insertions(+), 10 deletions(-)
>
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index a1cdba42c4fa..37b65cb1579a 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -372,18 +372,10 @@ nfsd_file_put(struct nfsd_file *nf)
> /* Try to add it to the LRU. If that fails, decrement. */
> if (nfsd_file_lru_add(nf)) {
> /* If it's still hashed, we're done */
> - if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
> + if (list_lru_count(&nfsd_file_lru))
> nfsd_file_schedule_laundrette();
> - return;
> - }

The above change does not seem to be related to the fix described
in the patch description. Can you help me understand why this is
needed?

> - /*
> - * We're racing with unhashing, so try to remove it from
> - * the LRU. If removal fails, then someone else already
> - * has our reference.
> - */
> - if (!nfsd_file_lru_remove(nf))
> - return;
> + return;
> }
> }
> if (refcount_dec_and_test(&nf->nf_ref))

--
Chuck Lever