Re: [PATCH] nfsd: make a copy of struct iattr before calling notify_change

[Chuck Lever III <chuck.lever@oracle.com>]

> On May 17, 2023, at 3:05 PM, Jeff Layton <jlayton@kernel.org> wrote:
> 
> On Wed, 2023-05-17 at 17:47 +0000, Chuck Lever III wrote:
>> 
>>> On May 17, 2023, at 12:26 PM, Jeff Layton <jlayton@kernel.org> wrote:
>>> 
>>> notify_change can modify the iattr structure. In particular it can can
>>> end up setting ATTR_MODE when ATTR_KILL_SUID is already set, causing a
>>> BUG() if the same iattr is passed to notify_change more than once.
>>> 
>>> Make a copy of the struct iattr before calling notify_change.
>>> 
>>> Fixes: 34b91dda7124 NFSD: Make nfsd4_setattr() wait before returning NFS4ERR_DELAY
>>> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2207969
>>> Reported-by: Zhi Li <yieli@redhat.com>
>>> Signed-off-by: Jeff Layton <jlayton@kernel.org>
>>> ---
>>> fs/nfsd/vfs.c | 4 +++-
>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>>> index c4ef24c5ffd0..ad0c5cd900b1 100644
>>> --- a/fs/nfsd/vfs.c
>>> +++ b/fs/nfsd/vfs.c
>>> @@ -538,7 +538,9 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp,
>>> 
>>> inode_lock(inode);
>>> for (retries = 1;;) {
>>> - host_err = __nfsd_setattr(dentry, iap);
>>> + struct iattr attrs = *iap;
>> 
>> This construct always makes me queazy. I'm never sure if an
>> initializer inside a loop is "only once" or "every time". I
>> fixed a bug like this once.
>> 
>> But if you've tested it and it addresses the BUG, then let's
>> go with this. I can apply it to nfsd-fixes.
>> 
> 
> 
> I've done some light testing with this kernel, but this was found by Zhi
> while testing with the lustre racer test, so it involves some raciness.
> I've never hit this myself.

Has Zhi tested this fix?


> I'm pretty sure though that this has to be initialized every time. The
> assignment is inside the loop, after all. I'm ok with moving the
> assignment to a different line if you like though:
> 
> struct iattr attrs;
> 
> attrs = *iap;
> ...

Yeah I could do that. I find that easier to read when a loop is
involved; it's unambiguous then what is going on.


>>> +
>>> + host_err = __nfsd_setattr(dentry, &attrs);
>>> if (host_err != -EAGAIN || !retries--)
>>> break;
>>> if (!nfsd_wait_for_delegreturn(rqstp, inode))
>>> -- 
>>> 2.40.1
>>> 
>> 
>> --
>> Chuck Lever
>> 
>> 
> 
> -- 
> Jeff Layton <jlayton@kernel.org>


--
Chuck Lever