linux-kernel - Re: [PATCH] nfsd: make a copy of struct iattr before calling notify

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Date:   Wed, 24 May 2023 07:57:08 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "Chuck Lever" <chuck.lever@...cle.com>,
        "Zhi Li" <yieli@...hat.com>, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: make a copy of struct iattr before calling notify_change

On Tue, 23 May 2023, Jeff Layton wrote:
> On Mon, 2023-05-22 at 12:45 +1000, NeilBrown wrote:
> > On Thu, 18 May 2023, Jeff Layton wrote:
> > > notify_change can modify the iattr structure. In particular it can can
> > > end up setting ATTR_MODE when ATTR_KILL_SUID is already set, causing a
> > > BUG() if the same iattr is passed to notify_change more than once.
> > > 
> > > Make a copy of the struct iattr before calling notify_change.
> > > 
> > > Fixes: 34b91dda7124 NFSD: Make nfsd4_setattr() wait before returning NFS4ERR_DELAY
> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2207969
> > > Reported-by: Zhi Li <yieli@...hat.com>
> > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > > ---
> > >  fs/nfsd/vfs.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > index c4ef24c5ffd0..ad0c5cd900b1 100644
> > > --- a/fs/nfsd/vfs.c
> > > +++ b/fs/nfsd/vfs.c
> > > @@ -538,7 +538,9 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > >  
> > >  	inode_lock(inode);
> > >  	for (retries = 1;;) {
> > > -		host_err = __nfsd_setattr(dentry, iap);
> > > +		struct iattr attrs = *iap;
> > > +
> > > +		host_err = __nfsd_setattr(dentry, &attrs);
> > 
> > I think this needs something to ensure a well meaning by-passer doesn't
> > try to "optimise" it back to the way it was.
> > Maybe make "iap" const?  Or add a comment?  Or both?
> > 
> 
> We can't make iap const, as we have to call nfsd_sanitize_attrs on it,
> and that will change things in it. I think we'll probably have to settle
> for a comment. Chuck, can we fold the patch below in to this one?

Thanks :-)

NeilBrown


> 
> --------------------8<------------------
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 2a3687cdf926..817effd63730 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -538,6 +538,11 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	for (retries = 1;;) {
>  		struct iattr attrs;
>  
> +		/*
> +		 * notify_change can alter the iattr in ways that make it
> +		 * unsuitable for submission multiple times. Make a copy
> +		 * for every loop.
> +		 */
>  		attrs = *iap;
>  		host_err = __nfsd_setattr(dentry, &attrs);
>  		if (host_err != -EAGAIN || !retries--)
> 
>