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

[Tom Talpey <tom@talpey.com>]

On 2/5/2024 3:16 PM, Jeff Layton wrote:
> On Tue, 2024-02-06 at 06:59 +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.
>>
>> NeilBrown
>>
>>
> 
> Ahh yeah. Good point. I'm fine with including that if Christian wants to
> fold it in.
> 
> Thanks for the review!

[re-replying to fix cc linux-cifs@ /vger/. kernel.org ...]

cifsfs.c is a new file being added to the change, cc-ing linux-cifs
for awareness. The SMB3 protocol does support leases on directories, so
this vfs change might be important to note in the future.

In the meantime, for cifsfs.c:

Acked-by: Tom Talpey <tom@talpey.com>

Tom.

>>> This patch is based on top of a merge of Christian's vfs.file branch
>>> (which has the file_lock/lease split). There is a small merge confict
>>> with Chuck's nfsd-next patch, but it should be fairly simple to resolve.
>>> ---
>>>   fs/locks.c               | 43 +++++++++++++++++++++++++------------------
>>>   fs/nfsd/nfs4layouts.c    |  5 ++---
>>>   fs/nfsd/nfs4state.c      |  8 ++++----
>>>   include/linux/filelock.h |  7 +++++++
>>>   4 files changed, 38 insertions(+), 25 deletions(-)
>>>
>>> diff --git a/fs/locks.c b/fs/locks.c
>>> index 33c7f4a8c729..26d52ef5314a 100644
>>> --- a/fs/locks.c
>>> +++ b/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);
>>>   
>>> diff --git a/fs/nfsd/nfs4layouts.c b/fs/nfsd/nfs4layouts.c
>>> index 4fa21b74a981..4c0d00bdfbb1 100644
>>> --- a/fs/nfsd/nfs4layouts.c
>>> +++ b/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;
>>> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
>>> index b2c8efb5f793..6d52ecba8e9c 100644
>>> --- a/fs/nfsd/nfs4state.c
>>> +++ b/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);
>>> diff --git a/include/linux/filelock.h b/include/linux/filelock.h
>>> index 4a5ad26962c1..cd6c1c291de9 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 *);
>>>   
>>> @@ -357,6 +358,12 @@ 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)
>>>   {
>>>
>>> ---
>>> base-commit: 1499e59af376949b062cdc039257f811f6c1697f
>>> change-id: 20240202-bz2248830-03e6c7506705
>>>
>>> Best regards,
>>> -- 
>>> Jeff Layton <jlayton@kernel.org>
>>>
>>>
>>>
>>
>