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

[Chuck Lever]

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@xxxxxxxxxxxxxxx # v6.2" but
I'm not convinced "Fixes: 4a0e73e635e3" is correct.


> Signed-off-by: Li Lingfeng <lilingfeng3@xxxxxxxxxx>
> ---
>   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