Message-ID: <5923519a4c8f6bb6d5ccd2697447b313fb61bba3.camel@kernel.org>
Date: Mon, 27 Jan 2025 08:32:18 -0500
From: Jeff Layton <jlayton@...nel.org>
To: Chuck Lever <chuck.lever@...cle.com>, NeilBrown <neilb@...e.de>
Cc: Olga Kornievskaia <okorniev@...hat.com>, Dai Ngo <Dai.Ngo@...cle.com>, 
 Tom Talpey <tom@...pey.com>, Salvatore Bonaccorso <carnil@...ian.org>,
 linux-nfs@...r.kernel.org, 	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: validate the nfsd_serv pointer before calling
 svc_wake_up

On Mon, 2025-01-27 at 08:22 -0500, Chuck Lever wrote:
> On 1/27/25 8:07 AM, Jeff Layton wrote:
> > On Mon, 2025-01-27 at 11:15 +1100, NeilBrown wrote:
> > > On Mon, 27 Jan 2025, Jeff Layton wrote:
> > > > On Mon, 2025-01-27 at 08:53 +1100, NeilBrown wrote:
> > > > > On Sun, 26 Jan 2025, Jeff Layton wrote:
> > > > > > On Sun, 2025-01-26 at 13:39 +1100, NeilBrown wrote:
> > > > > > > On Sun, 26 Jan 2025, Jeff Layton wrote:
> > > > > > > > nfsd_file_dispose_list_delayed can be called from the filecache
> > > > > > > > laundrette, which is shut down after the nfsd threads are shut down and
> > > > > > > > the nfsd_serv pointer is cleared. If nn->nfsd_serv is NULL then there
> > > > > > > > are no threads to wake.
> > > > > > > > 
> > > > > > > > Ensure that the nn->nfsd_serv pointer is non-NULL before calling
> > > > > > > > svc_wake_up in nfsd_file_dispose_list_delayed. This is safe since the
> > > > > > > > svc_serv is not freed until after the filecache laundrette is cancelled.
> > > > > > > > 
> > > > > > > > Fixes: ffb402596147 ("nfsd: Don't leave work of closing files to a work queue")
> > > > > > > > Reported-by: Salvatore Bonaccorso <carnil@...ian.org>
> > > > > > > > Closes: https://lore.kernel.org/linux-nfs/7d9f2a8aede4f7ca9935a47e1d405643220d7946.camel@kernel.org/
> > > > > > > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > > > > > > > ---
> > > > > > > > This is only lightly tested, but I think it will fix the bug that
> > > > > > > > Salvatore reported.
> > > > > > > > ---
> > > > > > > >   fs/nfsd/filecache.c | 11 ++++++++++-
> > > > > > > >   1 file changed, 10 insertions(+), 1 deletion(-)
> > > > > > > > 
> > > > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > > > > > index e91c164b5ea21507659904690533a19ca43b1b64..fb2a4469b7a3c077de2dd750f43239b4af6d37b0 100644
> > > > > > > > --- a/fs/nfsd/filecache.c
> > > > > > > > +++ b/fs/nfsd/filecache.c
> > > > > > > > @@ -445,11 +445,20 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
> > > > > > > >   						struct nfsd_file, nf_gc);
> > > > > > > >   		struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
> > > > > > > >   		struct nfsd_fcache_disposal *l = nn->fcache_disposal;
> > > > > > > > +		struct svc_serv *serv;
> > > > > > > >   
> > > > > > > >   		spin_lock(&l->lock);
> > > > > > > >   		list_move_tail(&nf->nf_gc, &l->freeme);
> > > > > > > >   		spin_unlock(&l->lock);
> > > > > > > > -		svc_wake_up(nn->nfsd_serv);
> > > > > > > > +
> > > > > > > > +		/*
> > > > > > > > +		 * The filecache laundrette is shut down after the
> > > > > > > > +		 * nn->nfsd_serv pointer is cleared, but before the
> > > > > > > > +		 * svc_serv is freed.
> > > > > > > > +		 */
> > > > > > > > +		serv = nn->nfsd_serv;
> > > > > > > 
> > > > > > > I wonder if this should be READ_ONCE() to tell the compiler that we
> > > > > > > could race with clearing nn->nfsd_serv.  Would the comment still be
> > > > > > > needed?
> > > > > > > 
> > > > > > 
> > > > > > I think we need a comment at least. The linkage between the laundrette
> > > > > > and the nfsd_serv being set to NULL is very subtle. A READ_ONCE()
> > > > > > doesn't convey that well, and is unnecessary here.
> > > > > 
> > > > > Why do you say "is unnecessary here" ?
> > > > > If the code were
> > > > >     if (nn->nfsd_serv)
> > > > >              svc_wake_up(nn->nfsd_serv);
> > > > > that would be wrong as nn->nfds_serv could be set to NULL between the
> > > > > two.
> > > > > And the C compile is allowed to load the value twice because the C memory
> > > > > model declares that would have the same effect.
> > > > > While I doubt it would actually change how the code is compiled, I think
> > > > > we should have READ_ONCE() here (and I've been wrong before about what
> > > > > the compiler will actually do).
> > > > > 
> > > > > 
> > > > 
> > > > It's unnecessary because the outcome of either case is acceptable.
> > > > 
> > > > When racing with shutdown, either it's NULL and the laundrette won't
> > > > call svc_wake_up(), or it's non-NULL and it will. In the non-NULL case,
> > > > the call to svc_wake_up() will be a no-op because the threads are shut
> > > > down.
> > > > 
> > > > The vastly common case in this code is that this pointer will be non-
> > > > NULL, because the server is running (i.e. not racing with shutdown). I
> > > > don't see the need in making all of those accesses volatile.
> > > 
> > > One of us is confused.  I hope it isn't me.
> > > 
> > 
> > It's probably me. I think you have a much better understanding of
> > compiler design than I do. Still...
> > 
> > > The hypothetical problem I see is that the C compiler could generate
> > > code to load the value "nn->nfsd_serv" twice.  The first time it is not
> > > NULL, the second time it is NULL.
> > > The first is used for the test, the second is passed to svc_wake_up().
> > > 
> > > Unlikely though this is, it is possible and READ_ONCE() is designed
> > > precisely to prevent this.
> > > To quote from include/asm-generic/rwonce.h it will
> > >   "Prevent the compiler from merging or refetching reads"
> > > 
> > > A "volatile" access does not add any cost (in this case).  What it does
> > > is break any aliasing that the compile might have deduced.
> > > Even if the compiler thinks it has "nn->nfsd_serv" in a register, it
> > > won't think it has the result of READ_ONCE(nn->nfsd_serv) in that register.
> > > And if it needs the result of a previous READ_ONCE(nn->nfsd_serv) it
> > > won't decide that it can just read nn->nfsd_serv again.  It MUST keep
> > > the result of READ_ONCE(nn->nfsd_serv) somewhere until it is not needed
> > > any more.
> > 
> > I'm mainly just considering the resulting pointer. There are two
> > possible outcomes to the fetch of nn->nfsd_serv. Either it's a valid
> > pointer that points to the svc_serv, or it's NULL. The resulting code
> > can handle either case, so it doesn't seem like adding READ_ONCE() will
> > create any material difference here.
> > 
> > Maybe I should ask it this way: What bad outcome could result if we
> > don't add READ_ONCE() here?
> 
> Neil just described it. The compiler would generate two load operations,
> one for the test and one for the function call argument. The first load
> can retrieve a non-NULL address, and the second a NULL address.
> 
> I agree a READ_ONCE() is necessary.
> 
> 

Now I'm confused:

                struct svc_serv *serv;

		[...]

                /*
                 * The filecache laundrette is shut down after the
                 * nn->nfsd_serv pointer is cleared, but before the
                 * svc_serv is freed.
                 */
                serv = nn->nfsd_serv;
                if (serv)
                        svc_wake_up(serv);

This code is explicitly asking to fetch nn->nfsd_serv into the serv
variable, and then is testing that copy of the pointer and passing it
into svc_wake_up().

How is the compiler allowed to suddenly refetch a NULL pointer into
serv after testing that serv is non-NULL?
-- 
Jeff Layton <jlayton@...nel.org>

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