Re: [PATCH] filelock: don't do security checks on nfsd setlease calls

["NeilBrown" <neilb@suse.de>]

On Wed, 07 Feb 2024, Christian Brauner wrote:
> On Tue, Feb 06, 2024 at 06:59:49AM +1100, NeilBrown wrote:
> > On Mon, 05 Feb 2024, Jeff Layton wrote:
> > > 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>
> > 
> > Reviewed-by: NeilBrown <neilb@suse.de>
> > 
> > It definitely nice to move all the security and sanity check early.
> > This patch allows a minor clean-up in cifs which could possibly be
> > included:
> > diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
> > index 2a4a4e3a8751..0f142d1ec64f 100644
> > 
> > --- a/fs/smb/client/cifsfs.c
> > +++ b/fs/smb/client/cifsfs.c
> > @@ -1094,9 +1094,6 @@ cifs_setlease(struct file *file, int arg, struct file_lock **lease, void **priv)
> >  	struct inode *inode = file_inode(file);
> >  	struct cifsFileInfo *cfile = file->private_data;
> >  
> > -	if (!(S_ISREG(inode->i_mode)))
> > -		return -EINVAL;
> > -
> >  	/* Check if file is oplocked if this is request for new lease */
> >  	if (arg == F_UNLCK ||
> >  	    ((arg == F_RDLCK) && CIFS_CACHE_READ(CIFS_I(inode))) ||
> > 
> > 
> > as ->setlease() is now never called for non-ISREG files.
> 
> I've added the following on top. I've made you author and added your
> SoB. Please tell me if you have any problems with this:

No problems at all - thanks for doing this!

NeilBrown


> 
> From d30e52329760873bf0d7984a442cace3a4b5f39d Mon Sep 17 00:00:00 2001
> From: NeilBrown <neilb@suse.de>
> Date: Tue, 6 Feb 2024 14:08:57 +0100
> Subject: [PATCH] smb: remove redundant check
> 
> ->setlease() is never called on non-regular files now. So remove the
> check from cifs_setlease().
> 
> Link: https://lore.kernel.org/r/170716318935.13976.13465352731929804157@noble.neil.brown.name
> Signed-off-by: NeilBrown <neilb@suse.de>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/smb/client/cifsfs.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
> index 5eee5b00547f..cbcb98d5f2d7 100644
> --- a/fs/smb/client/cifsfs.c
> +++ b/fs/smb/client/cifsfs.c
> @@ -1094,9 +1094,6 @@ cifs_setlease(struct file *file, int arg, struct file_lease **lease, void **priv
>  	struct inode *inode = file_inode(file);
>  	struct cifsFileInfo *cfile = file->private_data;
>  
> -	if (!(S_ISREG(inode->i_mode)))
> -		return -EINVAL;
> -
>  	/* Check if file is oplocked if this is request for new lease */
>  	if (arg == F_UNLCK ||
>  	    ((arg == F_RDLCK) && CIFS_CACHE_READ(CIFS_I(inode))) ||
> -- 
> 2.43.0
> 
>