We've been seeing some crashes in testing that look like this:

BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffff8135ce99>] memcpy_orig+0x29/0x110
PGD 212ca2067 PUD 212ca3067 PMD 0
Oops: 0002 [#1] SMP
Modules linked in: rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache ppdev parport_pc i2c_piix4 sg parport i2c_core virtio_balloon pcspkr acpi_cpufreq nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c sd_mod ata_generic pata_acpi virtio_scsi 8139too ata_piix libata 8139cp mii virtio_pci floppy virtio_ring serio_raw virtio
CPU: 1 PID: 1540 Comm: nfsd Not tainted 4.9.0-rc1 #39
Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2007
task: ffff88020d7ed200 task.stack: ffff880211838000
RIP: 0010:[<ffffffff8135ce99>]  [<ffffffff8135ce99>] memcpy_orig+0x29/0x110
RSP: 0018:ffff88021183bdd0  EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff88020d7fa000 RCX: 000000f400000000
RDX: 0000000000000014 RSI: ffff880212927020 RDI: 0000000000000000
RBP: ffff88021183be30 R08: 01000000ef896996 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff880211704ca8
R13: ffff88021473f000 R14: 00000000ef896996 R15: ffff880211704800
FS:  0000000000000000(0000) GS:ffff88021fc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000212ca1000 CR4: 00000000000006e0
Stack:
 ffffffffa01ea087 ffffffff63400001 ffff880215145e00 ffff880211bacd00
 ffff88021473f2b8 0000000000000004 00000000d0679d67 ffff880211bacd00
 ffff88020d7fa000 ffff88021473f000 0000000000000000 ffff88020d7faa30
Call Trace:
 [<ffffffffa01ea087>] ? svc_tcp_recvfrom+0x5a7/0x790 [sunrpc]
 [<ffffffffa01f84d8>] svc_recv+0xad8/0xbd0 [sunrpc]
 [<ffffffffa0262d5e>] nfsd+0xde/0x160 [nfsd]
 [<ffffffffa0262c80>] ? nfsd_destroy+0x60/0x60 [nfsd]
 [<ffffffff810a9418>] kthread+0xd8/0xf0
 [<ffffffff816dbdbf>] ret_from_fork+0x1f/0x40
 [<ffffffff810a9340>] ? kthread_park+0x60/0x60
Code: 00 00 48 89 f8 48 83 fa 20 72 7e 40 38 fe 7c 35 48 83 ea 20 48 83 ea 20 4c 8b 06 4c 8b 4e 08 4c 8b 56 10 4c 8b 5e 18 48 8d 76 20 <4c> 89 07 4c 89 4f 08 4c 89 57 10 4c 89 5f 18 48 8d 7f 20 73 d4
RIP  [<ffffffff8135ce99>] memcpy_orig+0x29/0x110
 RSP <ffff88021183bdd0>
CR2: 0000000000000000

Both Bruce and Eryu ran a bisect here and found that the problematic
patch was 68778945 (SUNRPC: Separate buffer pointers for RPC Call and
Reply messages).

That patch changed rpc_xdr_encode to use a new rq_rbuffer pointer to
set up the receive buffer, but didn't change all of the necessary
codepaths to set it properly. In particular the backchannel setup was
missing.

We need to set rq_rbuffer whenever rq_buffer is set. Ensure that it is.
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Chuck Lever <chuck.lever@oracle.com>
Reported-by: Eryu Guan <guaneryu@gmail.com>
Tested-by: Eryu Guan <guaneryu@gmail.com>
Fixes: 68778945 "SUNRPC: Separate buffer pointers..."
Reported-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>