nfsd: check get_user() return when reading princhashlen

commit e186fa1 upstream.

In __cld_pipe_inprogress_downcall(), the get_user() that reads
princhashlen from the userspace cld_msg_v2 buffer does not check its
return value. A failing copy leaves princhashlen with uninitialised
stack contents, which are then used to drive memdup_user() and stored
as princhash.len on the resulting reclaim record. The other get_user()
calls in this function all check the return; only this one is missed,
which is most likely a copy-paste oversight from when v2 upcalls were
introduced.

Mirror the existing pattern used a few lines above for namelen.
namecopy is declared with __free(kfree) so the early return cleans up
the already-allocated buffer automatically.

Fixes: 6ee95d1 ("nfsd: add support for upcall version 2")
Cc: stable@vger.kernel.org
Signed-off-by: Dominik Woźniak <stalion@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: glivter

fs/nfsd/nfs4recover.c

@@ -800,7 +800,8 @@ __cld_pipe_inprogress_downcall(const struct cld_msg_v2 __user *cmsg,
 			if (IS_ERR(name.data))
 				return PTR_ERR(name.data);
 			name.len = namelen;
-			get_user(princhashlen, &ci->cc_princhash.cp_len);
+			if (get_user(princhashlen, &ci->cc_princhash.cp_len))
+				return -EFAULT;
 			if (princhashlen > 0) {
 				princhash.data = memdup_user(
 						&ci->cc_princhash.cp_data,