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>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/filelock.h b/include/linux/filelock.h
index 553d65a88048..aabd4bdf7eba 100644
--- a/include/linux/filelock.h
+++ b/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)
 {