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

Oleg Drokin
2016-07-08T21:53:32Z

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?

> 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