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

[Jeff Layton]

On Wed, 2025-01-15 at 10:03 -0500, Chuck Lever wrote:
> On 1/14/25 2:39 PM, Jeff Layton wrote:
> > On Tue, 2025-01-14 at 14:27 -0500, Jeff Layton wrote:
> > > On Mon, 2025-01-13 at 10:59 +0800, 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.
> > > > 
> > > > 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")
> > > > 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;
> > > > -			}
> > > >   
> > > > -			/*
> > > > -			 * 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))
> > > 
> > > I think this looks OK. Filecache bugs are particularly nasty though, so
> > > let's run this through a nice long testing cycle.
> > > 
> > > Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > 
> > Actually, I take it back. This is problematic in another way.
> > 
> > In this case, we're racing with another task that is unhashing the
> > object, but we've put it on the LRU ourselves. What guarantee do we
> > have that the unhashing and removal from the LRU didn't occur before
> > this task called nfsd_file_lru_add()? That's why we attempt to remove
> > it here -- we can't rely on the task that unhashed it to do so at that
> > point.
> > 
> > What might be best is to take and hold the rcu_read_lock() before doing
> > the nfsd_file_lru_add, and just release it after we do these racy
> > checks. That should make it safe to access the object.
> > 
> > Thoughts?
> 
> Holding the RCU read lock will keep the dereferences safe since
> nfsd_file objects are freed only after an RCU grace period. But will the
> logic in nfsd_file_put() work properly on totally dead nfsd_file
> objects? I don't see a specific failure mode there, but I'm not very
> imaginative.
> 
> Overall, I think RCU would help.
> 

It should be safe to call nfsd_file_lru_add() with the rcu_read_lock()
held. After that we're just looking at the nf_flags() and the nf_lru
list head. On a dead file, HASHED will be clear and the
nfsd_file_lru_remove() call will be a no-op (the list_head will be
empty).

Li Lingfeng, do you want to propose a patch for this? Unfortunately,
your reproducer won't work after that, since you can't sleep with the
rcu_read_lock held.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>