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

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Date:	Fri, 22 Jul 2016 06:55:27 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Oleg Drokin <green@...uxhacker.ru>
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 Fri, Jul 22, 2016 at 02:35:26AM -0400, Oleg Drokin wrote:
> 
> 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.

static inline int may_create(struct inode *dir, struct dentry *child)
{
        audit_inode_child(dir, child, AUDIT_TYPE_CHILD_CREATE);
        if (child->d_inode)
                return -EEXIST;
	...

So it looks OK to me.

--b.