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

Oleg Drokin
2016-07-22T15:13:31Z

On Jul 22, 2016, at 6:55 AM, J. Bruce Fields wrote:

> 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@linuxhacker.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.

Hm, in fact indeed. I was just too worked up about the client side, but on the
server side there was a real lookup already, so it does look workable.

>
> --b.