linux-kernel - Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM

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Date:	Fri, 22 Jul 2016 02:35:26 -0400
From:	Oleg Drokin <green@...uxhacker.ru>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Jeff Layton <jlayton@...chiereds.net>, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM


On Jul 21, 2016, at 9:57 PM, J. Bruce Fields wrote:

> On Thu, Jul 21, 2016 at 04:37:40PM -0400, Oleg Drokin wrote:
>> 
>> On Jul 21, 2016, at 4:34 PM, J. Bruce Fields wrote:
>> 
>>> On Fri, Jul 08, 2016 at 05:53:19PM -0400, Oleg Drokin wrote:
>>>> 
>>>> On Jul 8, 2016, at 4:54 PM, J. Bruce Fields wrote:
>>>> 
>>>>> On Thu, Jul 07, 2016 at 09:47:46PM -0400, Oleg Drokin wrote:
>>>>>> It looks like we are bit overzealous about failing mkdir/create/mknod
>>>>>> with permission denied if the parent dir is not writeable.
>>>>>> Need to make sure the name does not exist first, because we need to
>>>>>> return EEXIST in that case.
>>>>>> 
>>>>>> Signed-off-by: Oleg Drokin <green@...uxhacker.ru>
>>>>>> ---
>>>>>> A very similar problem exists with symlinks, but the patch is more
>>>>>> involved, so assuming this one is ok, I'll send a symlink one separately.
>>>>>> fs/nfsd/nfs4proc.c |  6 +++++-
>>>>>> fs/nfsd/vfs.c      | 11 ++++++++++-
>>>>>> 2 files changed, 15 insertions(+), 2 deletions(-)
>>>>>> 
>>>>>> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
>>>>>> index de1ff1d..0067520 100644
>>>>>> --- a/fs/nfsd/nfs4proc.c
>>>>>> +++ b/fs/nfsd/nfs4proc.c
>>>>>> @@ -605,8 +605,12 @@ nfsd4_create(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>>>>>> 
>>>>>> 	fh_init(&resfh, NFS4_FHSIZE);
>>>>>> 
>>>>>> +	/*
>>>>>> +	 * We just check thta parent is accessible here, nfsd_* do their
>>>>>> +	 * own access permission checks
>>>>>> +	 */
>>>>>> 	status = fh_verify(rqstp, &cstate->current_fh, S_IFDIR,
>>>>>> -			   NFSD_MAY_CREATE);
>>>>>> +			   NFSD_MAY_EXEC);
>>>>>> 	if (status)
>>>>>> 		return status;
>>>>>> 
>>>>>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>>>>>> index 6fbd81e..6a45ec6 100644
>>>>>> --- a/fs/nfsd/vfs.c
>>>>>> +++ b/fs/nfsd/vfs.c
>>>>>> @@ -1161,7 +1161,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
>>>>>> 	if (isdotent(fname, flen))
>>>>>> 		goto out;
>>>>>> 
>>>>>> -	err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_CREATE);
>>>>>> +	/*
>>>>>> +	 * Even though it is a create, first we see if we are even allowed
>>>>>> +	 * to peek inside the parent
>>>>>> +	 */
>>>>>> +	err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_EXEC);
>>>>> 
>>>>> Looks like in the v3 case we haven't actually locked the directory yet
>>>>> at this point so this check is a little race-prone.
>>>> 
>>>> In reality this check is not really needed, I suspect.
>>>> When we call vfs_create/mknod/mkdir later on, it has it's own permission check
>>>> anyway so if there was a race and somebody changed dir access in the middle,
>>>> there's going to be another check anyway and it would be caught.
>>>> Unless there's some weird server-side permission wiggling as well that makes it
>>>> ineffective, but I imagine that one cannot really change in a racy way?
>>> 
>>> Yeah, I think I'll just change those NFSD_MAY_EXEC's to NFSD_MAY_NOP's.
>>> We still need the fh_verify there since it's also what does the
>>> filehandle->dentry translation, but we don't need permission checking
>>> here yet.
>> 
>> This will likely need an extra test to ensure that when you
>> do mkdir where you do not have exec permissions, you would get EACCES instead
>> of EEXIST, otherwise that would be information leakage, no?
>> Or do you think the second time we do nfsd_permission, that would be covered?
> 
> No, you're right, for some reason I thought that the check for a
> positive inode didn't happen till later.  But actually the logic is
> basically:
> 
> 	lock inode
> 	lookup_one_len
> 	return nfserr_exist if looked up dentry is positive.
> 	check for create permission
> 	vfs_create
> 
> So, yes, the initial MAY_EXEC test's needed to prevent that information
> leak.
> 
> That said... I wonder why it's done that way?  Seems to me we could just
> tremove that nfserr_exist check and the vfs would handle it for us....
> I'll try that.

It won't work because the very first thing vfs_create does is may_create(),
and so you get EACCES right there instead of the EEXIST.

> 
> --b.
> 
>> 
>>> Applying with that one change.  (And I'll followup with some additional
>>> minor cleanup of the create code.)
>>> 
>>> --b.
>>> 
>>>> 
>>>>> I wonder why the code's structured that way--it's confusing.
>>>> 
>>>> Probably years of accumulated "damage" ;)
>>>> 
>>>>> --b.
>>>>> 
>>>>>> 	if (err)
>>>>>> 		goto out;
>>>>>> 
>>>>>> @@ -1211,6 +1215,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
>>>>>> 		goto out; 
>>>>>> 	}
>>>>>> 
>>>>>> +	/* Now let's see if we actually have permissions to create */
>>>>>> +	err = nfsd_permission(rqstp, fhp->fh_export, dentry, NFSD_MAY_CREATE);
>>>>>> +	if (err)
>>>>>> +		goto out;
>>>>>> +
>>>>>> 	if (!(iap->ia_valid & ATTR_MODE))
>>>>>> 		iap->ia_mode = 0;
>>>>>> 	iap->ia_mode = (iap->ia_mode & S_IALLUGO) | type;
>>>>>> -- 
>>>>>> 2.7.4