[PATCH v4 00/10] use-after-free in svc_process_common()

[Vasily Averin <vvs@virtuozzo.com>]

v4: 
- re-split,
- use direct call of svc_tcp_prep_reply_hdr() in svc_process_common()
- removed unused bc_up
- removed useless svc_tcp_bc_class and svc_rdma_bc_class
- removed unused xpo_prep_reply_hdr

v3:
- first patch was reworked again,
  instead of svc_xprt search svc_process_common() now uses 
  bc_prep_reply_hdr() function pointer saved on per-netns sunrpc_net.
- first patch was splitted into 5 parts.
- comments cleanup

v2:
- first patch was reworked to satisfy Trond's requirements:
  to do not assign rqstp->rq_xprt in svc_process_common() at all,
  provide proper xpt_ops reference as a new parameter,
  adopt functions potentially called from svc_process_common() 
   to properly handle rqstp->rq_xprt = NULL case.


nfsv41+ clients are still not properly net-namespace-filied.

OpenVz got report on crash in svc_process_common() 
abd founf that bc_svc_process() cannot use serv->sv_bc_xprt as a pointer.

serv is global structure, but sv_bc_xprt is assigned per-netnamespace.
If nfsv41+ shares (with the same minorversion) are mounted in several containers together
then bc_svc_process() can use wrong backchannel or even access freed memory.

OpenVz got report on crash svc_process_common(),
and after careful investigations Evgenii Shatokhin have found its reproducer.
Then I've reproduced the problem on last mainline kernel.

In described scenario you need to have: 
- nodeA: VM with 2 interfaces and debug kernel with enabled KASAN.
- nodeB: any other node 
- NFS-SRV: NFSv41+ server (4.2 is used in exaple below) 

1) nodeA: mount nfsv41+ share
# mount -t nfs4 -o vers=4.2 NFS-SRV:/export/ /mnt/ns1
  VvS: here serv->sv_bc_xprt is assigned first time,
       in xs_tcp_bc_up() it is assigned to svc_xprt of mount's backchannel

2) nodeA: create net namespace, and mount the same (or any other) NFSv41+ share
# ip netns add second
# ip link set ens2 netns second
# ip netns exec second bash
(inside netns second) # dhclient ens2 
  VvS: now nets got access to external network
(inside netns second) # mount -t nfs4 -o vers=4.2 NFS-SRV:/export/ /mnt/ns2 
  VvS: now serv->sv_bc_xprt is overwritten by reference to svc_xprt of new mount's backchannel
  NB: you can mount any other NFS share but minorversion must be the same.
  NB2: if hardware allows you can use rdma transport here
  NB3: you can access nothing in mounted share, problem's trigger was enabled already.

3) NodeA, destroy mount inside netns and then netns itself.

(inside netns second) # umount /mnt/ns2
(inside netns second) # ip link set ens2 netns 1
(inside netns second) # exit
   VvS: return to init_net
# ip netns del second
   VvS: now second NFS mount and second net namespace was destroyed.

4) Node A: prepare backchannel event
# echo test1 > /mnt/ns1/test1.txt
# echo test2 > /mnt/ns1/test2.txt
# python
>>> fl=open('/mnt/ns1/test1.txt','r')
>>>

4) Node B: replace file open by NodeA
# mount -t nfs -o vers=4.2 NFS-SRV:/export/ /mnt/
# mv /mnt/test2.txt /mnt/test1.txt

===> KASAN on nodeA detect an access to already freed memory.
(see dmesg example in attach of v1 patch version)

svc_process_common() 
        /* Setup reply header */
        rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr(rqstp); <<< HERE

svc_process_common() uses already freed rqstp->rq_xprt,
it was assigned in bc_svc_process() where it was taken from serv->sv_bc_xprt.

serv->sv_bc_xprt cannot be used as a pointer,
it can be assigned per net-namespace, either in svc_bc_tcp_create() 
or in xprt_rdma_bc_up().

According to Trond, the whole "let's set up rqstp->rq_xprt
for the back channel" is nothing but a giant hack in order
to work around the fact that svc_process_common() uses it
to find the xpt_ops, and perform a couple of (meaningless
for the back channel) tests of xpt_flags.

All we really need in svc_process_common() is to be able to run
rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr()

Bruce J Fields points that this xpo_prep_reply_hdr() call
is an awfully roundabout way just to do "svc_putnl(resv, 0);"
in the tcp case.

To fix the problem svc_process_common() checks svc_rqstp->rq_prot
inherited from incoming request and if required calls 
svc_tcp_prep_reply_hdr() directly.

It was also required to store a pointer to struct net in the
struct svc_rqst so that functions called from inside
svc_process_common() (nfs4_callback_compound(),
svcauth_gss_accept() and some other) can find it.
Some other functions was adopted to handle empty rqstp->rq_xprt

First patch switches svnauth_gss-* function to use SVC_NET()
2nd patch fixes use-after-free itself:
 to adjust reply header svc_process_common() checks prot of incoming request
   and if required calls svc_tcp_prep_reply_hdr() directly
 function called from svc_process_common() are adopted to properly handle
   rqstp->rq_xprt = NULL
3rd patch replaces sv_bc_xprt use in in svc_is_backchannel()
 by simple boolean flag
4rd patch removes unused bc_up calls

5th and 6th patches removes unused fake "transports", svc_tcp/rdma_bc_class
7th patch removes unused xpo_prep_reply_hdr callback
Rest of patches are minor cleanup. 

Vasily Averin (10):
  sunrpc: use SVC_NET() in svcauth_gss_* functions
  sunrpc: use-after-free in svc_process_common()
  sunrpc: replace svc_serv->sv_bc_xprt by boolean flag
  sunrpc: remove unused bc_up operation from rpc_xprt_ops
  sunrpc: remove svc_tcp_bc_class
  sunrpc: remove svc_rdma_bc_class
  sunrpc: remove unused xpo_prep_reply_hdr callback
  sunrpc: make visible processing error in bc_svc_process()
  sunrpc: fix debug message in svc_create_xprt()
  nfs: minor typo in nfs4_callback_up_net()

 fs/nfs/callback.c                        |  10 +-
 include/linux/sunrpc/bc_xprt.h           |  10 +-
 include/linux/sunrpc/svc.h               |   7 +-
 include/linux/sunrpc/svc_rdma.h          |   1 -
 include/linux/sunrpc/svc_xprt.h          |   1 -
 include/linux/sunrpc/xprt.h              |   1 -
 include/trace/events/sunrpc.h            |   6 +-
 net/sunrpc/auth_gss/svcauth_gss.c        |   8 +-
 net/sunrpc/svc.c                         |  24 +++--
 net/sunrpc/svc_xprt.c                    |   9 +-
 net/sunrpc/svcsock.c                     | 120 -----------------------
 net/sunrpc/xprtrdma/backchannel.c        |  20 ----
 net/sunrpc/xprtrdma/svc_rdma.c           |   6 --
 net/sunrpc/xprtrdma/svc_rdma_sendto.c    |   4 -
 net/sunrpc/xprtrdma/svc_rdma_transport.c |  59 -----------
 net/sunrpc/xprtrdma/transport.c          |   1 -
 net/sunrpc/xprtrdma/xprt_rdma.h          |   1 -
 net/sunrpc/xprtsock.c                    |  12 ---
 18 files changed, 46 insertions(+), 254 deletions(-)

-- 
2.17.1