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

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Date:	Thu, 21 Jul 2016 16:34:15 -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 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.

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