Re: NFS mounts failing when keytab present on client

[Eric Biggers]

On Wed, Mar 28, 2018 at 09:00:14AM +0100, M A Young wrote:
> On Tue, 27 Mar 2018, Eric Biggers wrote:
> 
> > Hi Michael,
> > 
> > On Tue, Mar 27, 2018 at 11:06:14PM +0100, Michael Young wrote:
> > > NFS mounts stopped working on one of my computers after a kernel update from
> > > 4.15.3 to 4.15.4. I traced the problem to the commit
> > > [46e8d06e423c4f35eac7a8b677b713b3ec9b0684] crypto: hash - prevent using
> > > keyed hashes without setting key
> > > and a later kernel with this patch reverted works normally.
> > > 
> > > The problem seems to be related to kerberos as the mount fails when the
> > > keytab is present, but works if I rename the keytab file. This is true even
> > > though the mount is with sec=sys . The mount should also work with sec=krb5
> > > but that also fails in the same way. When the mount fails there are errors
> > > in dmesg like
> > > [ 1232.522816] gss_marshal: gss_get_mic FAILED (851968)
> > > [ 1232.522819] RPC: couldn't encode RPC header, exit EIO
> > > [ 1232.522856] gss_marshal: gss_get_mic FAILED (851968)
> > > [ 1232.522857] RPC: couldn't encode RPC header, exit EIO
> > > [ 1232.522863] NFS: nfs4_discover_server_trunking unhandled error -5.
> > > Exiting with error EIO
> > > [ 1232.525039] gss_marshal: gss_get_mic FAILED (851968)
> > > [ 1232.525042] RPC: couldn't encode RPC header, exit EIO
> > > 
> > > 	Michael Young
> > 
> > Thanks for the bug report.  I think the error is coming from
> > net/sunrpc/auth_gss/gss_krb5_crypto.c.  There are two potential problems I see.
> > The first one, which is definitely a bug, is that make_checksum_hmac_md5()
> > allocates an HMAC transform and request, then does these crypto API calls:
> > 
> > 	crypto_ahash_init()
> > 	crypto_ahash_setkey()
> > 	crypto_ahash_digest()
> > 
> > This is wrong because it makes no sense to init() the HMAC request before the
> > key has been set, and doubly so when it's calling digest() which is shorthand
> > for init() + update() + final().  So I think it just needs to be removed.  You
> > can test the following patch:
> > 
> > diff --git a/net/sunrpc/auth_gss/gss_krb5_crypto.c b/net/sunrpc/auth_gss/gss_krb5_crypto.c
> > index 12649c9fedab..8654494b4d0a 100644
> > --- a/net/sunrpc/auth_gss/gss_krb5_crypto.c
> > +++ b/net/sunrpc/auth_gss/gss_krb5_crypto.c
> > @@ -237,9 +237,6 @@ make_checksum_hmac_md5(struct krb5_ctx *kctx, char *header, int hdrlen,
> >  
> >         ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
> >  
> > -       err = crypto_ahash_init(req);
> > -       if (err)
> > -               goto out;
> >         err = crypto_ahash_setkey(hmac_md5, cksumkey, kctx->gk5e->keylength);
> >         if (err)
> >                 goto out;
> > 
> > If that's not it, it's also possible that the error is coming from the
> > crypto_ahash_init() in make_checksum().  That can only happen if 'cksumkey' is
> > NULL and the hash algorithm is keyed, which implies a logical error as it
> > doesn't make sense to use a keyed hash algorithm without the key.  The callers
> > do check kctx->gk5e->keyed_cksum which I'd hope would prevent this, though
> > perhaps kctx->cksum can be NULL.
> > 
> > Eric
> 
> The patch fixes the problem.
> 
> 	Michael Young

Okay, thanks for testing!  I'll send a formal patch.

Eric