Re: [PATCH] nfsd: make a copy of struct iattr before calling notify_change

[Jeff Layton <jlayton@kernel.org>]

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@redhat.com>
> > Signed-off-by: Jeff Layton <jlayton@kernel.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?

--------------------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--)