filelock: don't do security checks on nfsd setlease calls

Zdenek reported seeing some AVC denials due to nfsd trying to set
delegations:

    type=AVC msg=audit(09.11.2023 09:03:46.411:496) : avc:  denied  { lease } for  pid=5127 comm=rpc.nfsd capability=lease  scontext=system_u:system_r:nfsd_t:s0 tcontext=system_u:system_r:nfsd_t:s0 tclass=capability permissive=0

When setting delegations on behalf of nfsd, we don't want to do all of
the normal capabilty and LSM checks. nfsd is a kernel thread and runs
with CAP_LEASE set, so the uid checks end up being a no-op in most cases
anyway.

Some nfsd functions can end up running in normal process context when
tearing down the server. At that point, the CAP_LEASE check can fail and
cause the client to not tear down delegations when expected.

Also, the way the per-fs ->setlease handlers work today is a little
convoluted. The non-trivial ones are wrappers around generic_setlease,
so when they fail due to permission problems they usually they end up
doing a little extra work only to determine that they can't set the
lease anyway. It would be more efficient to do those checks earlier.

Transplant the permission checking from generic_setlease to
vfs_setlease, which will make the permission checking happen earlier on
filesystems that have a ->setlease operation. Add a new kernel_setlease
function that bypasses these checks, and switch nfsd to use that instead
of vfs_setlease.

There is one behavioral change here: prior this patch the
setlease_notifier would fire even if the lease attempt was going to fail
the security checks later. With this change, it doesn't fire until the
caller has passed them. I think this is a desirable change overall. nfsd
is the only user of the setlease_notifier and it doesn't benefit from
being notified about failed attempts.

Cc: Ondrej Mosnáček <omosnacek@gmail.com>
Reported-by: Zdenek Pytela <zpytela@redhat.com>
Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2248830
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20240205-bz2248830-v1-1-d0ec0daecba1@kernel.org
Acked-by: Tom Talpey <tom@talpey.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jtlayton

fs/locks.c

@@ -1925,18 +1925,6 @@ static int generic_delete_lease(struct file *filp, void *owner)
 int generic_setlease(struct file *filp, int arg, struct file_lease **flp,
 			void **priv)
 {
-	struct inode *inode = file_inode(filp);
-	vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(filp), inode);
-	int error;
-
-	if ((!vfsuid_eq_kuid(vfsuid, current_fsuid())) && !capable(CAP_LEASE))
-		return -EACCES;
-	if (!S_ISREG(inode->i_mode))
-		return -EINVAL;
-	error = security_file_lock(filp, arg);
-	if (error)
-		return error;
-
 	switch (arg) {
 	case F_UNLCK:
 		return generic_delete_lease(filp, *priv);
@@ -1987,6 +1975,19 @@ void lease_unregister_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL_GPL(lease_unregister_notifier);
 
+
+int
+kernel_setlease(struct file *filp, int arg, struct file_lease **lease, void **priv)
+{
+	if (lease)
+		setlease_notifier(arg, *lease);
+	if (filp->f_op->setlease)
+		return filp->f_op->setlease(filp, arg, lease, priv);
+	else
+		return generic_setlease(filp, arg, lease, priv);
+}
+EXPORT_SYMBOL_GPL(kernel_setlease);
+
 /**
  * vfs_setlease        -       sets a lease on an open file
  * @filp:	file pointer
@@ -2007,12 +2008,18 @@ EXPORT_SYMBOL_GPL(lease_unregister_notifier);
 int
 vfs_setlease(struct file *filp, int arg, struct file_lease **lease, void **priv)
 {
-	if (lease)
-		setlease_notifier(arg, *lease);
-	if (filp->f_op->setlease)
-		return filp->f_op->setlease(filp, arg, lease, priv);
-	else
-		return generic_setlease(filp, arg, lease, priv);
+	struct inode *inode = file_inode(filp);
+	vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(filp), inode);
+	int error;
+
+	if ((!vfsuid_eq_kuid(vfsuid, current_fsuid())) && !capable(CAP_LEASE))
+		return -EACCES;
+	if (!S_ISREG(inode->i_mode))
+		return -EINVAL;
+	error = security_file_lock(filp, arg);
+	if (error)
+		return error;
+	return kernel_setlease(filp, arg, lease, priv);
 }
 EXPORT_SYMBOL_GPL(vfs_setlease);
 

fs/nfsd/nfs4layouts.c

@@ -170,7 +170,7 @@ nfsd4_free_layout_stateid(struct nfs4_stid *stid)
 	spin_unlock(&fp->fi_lock);
 
 	if (!nfsd4_layout_ops[ls->ls_layout_type]->disable_recalls)
-		vfs_setlease(ls->ls_file->nf_file, F_UNLCK, NULL, (void **)&ls);
+		kernel_setlease(ls->ls_file->nf_file, F_UNLCK, NULL, (void **)&ls);
 	nfsd_file_put(ls->ls_file);
 
 	if (ls->ls_recalled)
@@ -199,8 +199,7 @@ nfsd4_layout_setlease(struct nfs4_layout_stateid *ls)
 	fl->c.flc_pid = current->tgid;
 	fl->c.flc_file = ls->ls_file->nf_file;
 
-	status = vfs_setlease(fl->c.flc_file, fl->c.flc_type, &fl,
-			      NULL);
+	status = kernel_setlease(fl->c.flc_file, fl->c.flc_type, &fl, NULL);
 	if (status) {
 		locks_free_lease(fl);
 		return status;

fs/nfsd/nfs4state.c

@@ -1249,7 +1249,7 @@ static void nfs4_unlock_deleg_lease(struct nfs4_delegation *dp)
 
 	WARN_ON_ONCE(!fp->fi_delegees);
 
-	vfs_setlease(nf->nf_file, F_UNLCK, NULL, (void **)&dp);
+	kernel_setlease(nf->nf_file, F_UNLCK, NULL, (void **)&dp);
 	put_deleg_file(fp);
 }
 
@@ -5532,8 +5532,8 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
 	if (!fl)
 		goto out_clnt_odstate;
 
-	status = vfs_setlease(fp->fi_deleg_file->nf_file,
-			      fl->c.flc_type, &fl, NULL);
+	status = kernel_setlease(fp->fi_deleg_file->nf_file,
+				      fl->c.flc_type, &fl, NULL);
 	if (fl)
 		locks_free_lease(fl);
 	if (status)
@@ -5571,7 +5571,7 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
 
 	return dp;
 out_unlock:
-	vfs_setlease(fp->fi_deleg_file->nf_file, F_UNLCK, NULL, (void **)&dp);
+	kernel_setlease(fp->fi_deleg_file->nf_file, F_UNLCK, NULL, (void **)&dp);
 out_clnt_odstate:
 	put_clnt_odstate(dp->dl_clnt_odstate);
 	nfs4_put_stid(&dp->dl_stid);

include/linux/filelock.h

@@ -208,6 +208,7 @@ struct file_lease *locks_alloc_lease(void);
 int __break_lease(struct inode *inode, unsigned int flags, unsigned int type);
 void lease_get_mtime(struct inode *, struct timespec64 *time);
 int generic_setlease(struct file *, int, struct file_lease **, void **priv);
+int kernel_setlease(struct file *, int, struct file_lease **, void **);
 int vfs_setlease(struct file *, int, struct file_lease **, void **);
 int lease_modify(struct file_lease *, int, struct list_head *);
 
@@ -378,6 +379,12 @@ static inline int generic_setlease(struct file *filp, int arg,
 	return -EINVAL;
 }
 
+static inline int kernel_setlease(struct file *filp, int arg,
+			       struct file_lease **lease, void **priv)
+{
+	return -EINVAL;
+}
+
 static inline int vfs_setlease(struct file *filp, int arg,
 			       struct file_lease **lease, void **priv)
 {