Hi Jeff-
> On Oct 1, 2022, at 5:59 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> nfsd_file is RCU-freed, so it's possible that one could be found that's
> in the process of being freed and the memory recycled. Ensure we hold
> the rcu_read_lock while attempting to get a reference on the object.
I'm OK with entertaining clean-up patches in this code, but I
am skeptical that this patch addresses the concern enumerated
in bug #394. As you've pointed out to me before, the "UAF on
DELEGRETURN crashes" appeared well before v5.19, which is the
kernel release where this bit of code was introduced.
> Cc: NeilBrown <neilb@xxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> fs/nfsd/filecache.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index d5c57360b418..f4f75ae2e4ea 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -1077,10 +1077,12 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
>
> retry:
> /* Avoid allocation if the item is already in cache */
> + rcu_read_lock();
> nf = rhashtable_lookup_fast(&nfsd_file_rhash_tbl, &key,
> nfsd_file_rhash_params);
> if (nf)
> nf = nfsd_file_get(nf);
> + rcu_read_unlock();
Again:
657 static inline void *rhashtable_lookup_fast(
658 struct rhashtable *ht, const void *key,
659 const struct rhashtable_params params)
660 {
661 void *obj;
662
663 rcu_read_lock();
664 obj = rhashtable_lookup(ht, key, params);
665 rcu_read_unlock();
666
667 return obj;
668 }
Since rhashtable_lookup() itself is a public API, please
just call that in nfsd_file_do_acquire() after explicitly
taking the RCU read lock.
> if (nf)
> goto wait_for_construction;
>
> @@ -1090,21 +1092,21 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> goto out_status;
> }
>
> + rcu_read_lock();
> nf = rhashtable_lookup_get_insert_key(&nfsd_file_rhash_tbl,
> &key, &new->nf_rhash,
> nfsd_file_rhash_params);
> + if (!IS_ERR_OR_NULL(nf)) {
> + nf = nfsd_file_get(nf);
Note that nfsd_file_get() can still return NULL.
> + nfsd_file_slab_free(&new->nf_rcu);
Why is the slab_free call now inside the RCU critical section?
Granted this should be a rare case, but this adds unnecessary
latency while the read lock is held.
> + }
> + rcu_read_unlock();
Is there a problem replacing rhashtable_lookup_get_insert_key()
with rhashtable_lookup_insert_key() and just retrying the normal
lookup if insertion returns EEXIST? That way, an nfsd_file_get()
is necessary only at the rhashtable_lookup() call site above.
> if (!nf) {
> nf = new;
@new was just released above, so won't this set @nf to point
to freed memory in some cases?
> goto open_file;
> }
> if (IS_ERR(nf))
> goto insert_err;
> - nf = nfsd_file_get(nf);
> - if (nf == NULL) {
> - nf = new;
> - goto open_file;
> - }
> - nfsd_file_slab_free(&new->nf_rcu);
>
> wait_for_construction:
> wait_on_bit(&nf->nf_flags, NFSD_FILE_PENDING, TASK_UNINTERRUPTIBLE);
> --
> 2.37.3
--
Chuck Lever
