Re: [PATCH 1/2] nfs: Fix spurios EPERM when mkdir of existing dentry

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> On Jul 7, 2016, at 13:07, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote:
> 
> 
> On Jul 7, 2016, at 12:59 PM, Trond Myklebust wrote:
> 
>> 
>>> On Jul 7, 2016, at 12:52, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote:
>>> 
>>> 
>>> On Jul 7, 2016, at 12:16 PM, Trond Myklebust wrote:
>>> 
>>>> 
>>>>> On Jul 7, 2016, at 01:53, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote:
>>>>> 
>>>>> It's great when we can shave an extra RPC, but not at the expense
>>>>> of correctness.
>>>>> We should not return EPERM (from vfs_create/mknod/mkdir) if the
>>>>> name already exists, even if we have no write access in parent.
>>>>> 
>>>>> Since the check in nfs_permission is clearly not enough to stave
>>>>> off this, just throw in the extra READ access to actually
>>>>> go through.
>>>>> 
>>>>> Signed-off-by: Oleg Drokin <green@xxxxxxxxxxxxxx>
>>>>> ---
>>>>> fs/nfs/dir.c | 4 +++-
>>>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
>>>>> index d8015a0..8c7835b 100644
>>>>> --- a/fs/nfs/dir.c
>>>>> +++ b/fs/nfs/dir.c
>>>>> @@ -1383,8 +1383,10 @@ struct dentry *nfs_lookup(struct inode *dir, struct dentry * dentry, unsigned in
>>>>> 	/*
>>>>> 	 * If we're doing an exclusive create, optimize away the lookup
>>>>> 	 * but don't hash the dentry.
>>>>> +	 * This optimization only works if we can write in the parent.
>>>>> 	 */
>>>>> -	if (nfs_is_exclusive_create(dir, flags))
>>>>> +	if (nfs_is_exclusive_create(dir, flags) &&
>>>>> +	    (inode_permission(dir, MAY_WRITE | MAY_READ | MAY_EXEC) == 0))
>>>>> 		return NULL;
>>>>> 
>>>> 
>>>> NACK. The only write permission we should care about on the client side is whether or not the filesystem is mounted read-only. All other permissions are checked by the server.
>>> 
>>> Right. This was mostly a discussion piece.
>>> The problem here is nfs_permission() returns 0 if you check for
>>> inode_permission(dir, MAY_WRITE | MAY_EXEC) (as in may_create), but then
>>> some other checks in the kernel still catch on to the fact that the directory
>>> is not writeable, so we have a premature failure with EPERM and server never sees
>>> this request which breaks things.
>> 
>> Are these VFS level checks? Which ones?
> 
> Yes, VFS level I believe.
> For Lustre it's may_create() from vfs_mkdir() that stops us short
> and the Lustre patch is effective.
> but for NFS this must be something else and I did not trace
> it completely. One of the security checks, I guess?
> if NFS patch is changed to check inode_permission(dir, MAY_OPEN | MAY_EXEC)
> as in Lustre, that returns 0 no matter what.
> 
> This is trivial to reproduce too.

So, should we be ignoring the MAY_EXEC flag when the VFS asks for inode_permission(dir, MAY_WRITE|MAY_EXEC)? I suspect that is the problem here.

> 
>>> (the read-only mount is not handled as well at the moment of course and my patch
>>> does not address this issue either, but it's easier to address in the VFS, like
>>> in filename_create() or something).
>>> 
>>> I see that two major consumers of this nfs_permission MAY_WRITE|!MAY_READ check
>>> are creates and deletes and with deletes we had a lookup already, so it already
>>> looked up the child and revalidated the parent.
>>> For creates, a revalidation still might be needed, I guess and that was the main driver
>>> behind this check? And that only when you do current dir creates, because otherwise
>>> the parent would have been revalidated in lookup?
>>> Is this the major case why that check is actually there?
>>> 
>>> Just trying to see how to approach this better without breaking the applications.

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