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

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Message-Id: <05872587-E1A0-4714-AF43-7070D72D930C@linuxhacker.ru>
Date:	Fri, 8 Jul 2016 11:14:46 -0400
From:	Oleg Drokin <green@...uxhacker.ru>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM


On Jul 8, 2016, at 7:02 AM, Jeff Layton wrote:

> On Thu, 2016-07-07 at 21:47 -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(-)
>> 
> 
> 
> nit: subject says EPERM, but I think you mean EACCES. The mnemonic I've
> always used is that EPERM is "permanent". IOW, changing permissions
> won't ever allow the user to do something. For instance, unprivileged
> users can never chown a file, so they should get back EPERM there. When
> a directory isn't writeable on a create they should get EACCES since
> they could do the create if the directory were writeable.

Hm, I see, thanks.
Confusing that you get "Permission denied" from perror ;)

>> 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);
>>  	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;
> 
> 
> Ouch. This means two nfsd_permission calls per create operation. If
> it's necessary for correctness then so be it, but is it actually
> documented anywhere (POSIX perhaps?) that we must prefer EEXIST over
> EACCES in this situation?

Opengroup manpage: http://pubs.opengroup.org/onlinepubs/009695399/functions/mkdir.html
newer version is here:
http://pubs.opengroup.org/onlinepubs/9699919799/

They tell us that we absolutely must fail with EEXIST if the name is a symlink
(so we need to lookup it anyway), and also that EEXIST is the failure code
if the path exists.

Are double permission checks really as bad for nfs? it looked like it would
call mostly into VFS so even if first call would be expensive, second call should
be really cheap?

In theory we can do the fh_verify(.., NFSD_MAY_NOP) to skip the first
nfsd_permission() call, but I suspect then we'd violate the spec in the other way:
i.e. mkdir in a directory where you cannot read and execute will give you
EEXIST where as that's when EACCES is desired I imagine.


> 
> -- 
> 
> Jeff Layton <jlayton@...chiereds.net>