Re: [PATCH 1/4] nfs: use-after-free in svc_process_common()

[Vasily Averin <vvs@virtuozzo.com>]

On 12/18/18 3:49 PM, Trond Myklebust wrote:
> On Tue, 2018-12-18 at 09:45 +0300, Vasily Averin wrote:
>> On 12/18/18 12:50 AM, J. Bruce Fields wrote:
>>> On Mon, Dec 17, 2018 at 07:23:54PM +0300, Vasily Averin wrote:
>>>> if node have NFSv41+ mounts inside several net namespaces
>>>> it can lead to use-after-free in svc_process_common()
>>>>
>>>> svc_process_common() 
>>>>         /* Setup reply header */
>>>>         rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr(rqstp); <<<
>>>> HERE
>>>>
>>>> svc_process_common() can use already freed rqstp->rq_xprt,
>>>> it was assigned in bc_svc_process() where it was taken from serv-
>>>>> sv_bc_xprt.
>>>>
>>>> serv is global structure but sv_bc_xprt is assigned per-
>>>> netnamespace,
>>>> so if nfsv41+ shares are mounted in several containers together
>>>> bc_svc_process() can use wrong backchannel or even access freed
>>>> memory.
>>>>
>>>> To find correct svc_xprt of client-related backchannel
>>>> bc_svc_process() now calls new .bc_get_xprt callback
>>>> that executes svc_find_xprt() with proper xprt name.
>>>
>>> This stuff is confusing and I need to stare at it some more before
>>> I
>>> understand, but it's weird that we'd need to search for the right
>>> xprt.
>>
>> All NFS clients in all net namespaces used the same minorversion 
>> shares common nfs_callback_data taken from global nfs_callback_info
>> array.
>>
>> Moreover these clients can use either rdma or nfs transport,
>> however only one of them can be used in one net namespace.
>>
>> Each net namespace must have own backchannel, 
>> it cannot depend on other net namespaces, 
>> because at least they can use different transports.
>>
>> So one svc_serv should be able to handle several (per-netns)
>> backchannels.
>>
>> Frankly speaking If you prefer I can easily convert global
>> nfs_callback_info to per net-namespace.
>> I've checked, it works too. However current solution looks better for
>> me.
>>
>>> We know which connection the backchannel request came over, and
>>> there
>>> should only be one backchannel using that connection, why can't we
>>> find
>>> it by just chasing pointers the right way?
>>
>> it is allocated by using follwing calltrace:
>> nfs_callback_up
>>  nfs_callback_up_net
>>   xprt->ops->bc_up(serv, net) -> xs_tcp_bc_up
>>    svc_create_xprt(serv, "tcp-bc")
>>     __svc_xpo_create
>>      svc_bc_tcp_create
>>       svc_bc_create_socket
>>
>> Here backchannel's svc_sock/svc/xprt is created.
>> It is per-netns and therefore it cannot be saved as pointer on global
>> svc_serv.
>>
>> It could be saved on some xprt related to forechannel,
>> I've expected it was done already -- but it was not done.
>> I've tried to find any way to do it -- but without success,
>> according structures seems are not accessible in svc_bc_tcp_create.
>>
>> Finally I've found that backchannel's xprt is added into serv-
>>> sv_permsocks
>> and svc_find_xprt can find it by name.
>>
>> It would be great if you can advise some more simple way.  
>>
>>> OK, I do need to look at it more.
>>
>> It is quite important for containers so I think this patch (or any
>> alternative solution)
>> should be pushed in stable@.
>>
> 
> 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.
> 
> What say we just pass in the xpt_ops as a parameter to
> svc_process_common(), and make those xpt_flags tests check for whether
> or not rqstp->rq_xprt is actually non-NULL?

To access proper xpt_flags inside svc_process_common() 
we need to pass svc_xprt instead of xpt_ops.

Do you mean something like following?

--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -1148,7 +1148,7 @@ static __printf(2,3) void svc_printk(struct svc_rqst *rqstp, const char *fmt, ..
  * Common routine for processing the RPC request.
  */
 static int
-svc_process_common(struct svc_rqst *rqstp, struct kvec *argv, struct kvec *resv)
+svc_process_common(struct svc_rqst *rqstp, struct kvec *argv, struct kvec *resv, struct svc_xprt *s_xprt)
 {
        struct svc_program      *progp;
        const struct svc_version *versp = NULL; /* compiler food */
@@ -1172,7 +1172,7 @@ svc_process_common(struct svc_rqst *rqstp, struct kvec *argv, struct kvec *resv)
        clear_bit(RQ_DROPME, &rqstp->rq_flags);
 
        /* Setup reply header */
-       rqstp->rq_xprt->xpt_ops->xpo_prep_reply_hdr(rqstp);
+       s_xprt->xpt_ops->xpo_prep_reply_hdr(rqstp);
 
        svc_putu32(resv, rqstp->rq_xid);
 
@@ -1245,7 +1245,7 @@ svc_process_common(struct svc_rqst *rqstp, struct kvec *argv, struct kvec *resv)
         * fit.
         */
        if (versp->vs_need_cong_ctrl &&
-           !test_bit(XPT_CONG_CTRL, &rqstp->rq_xprt->xpt_flags))
+           !test_bit(XPT_CONG_CTRL, &s_xprt->xpt_flags))


@@ -1336,8 +1336,8 @@ svc_process_common(struct svc_rqst *rqstp, struct kvec *argv, struct kvec *resv)
        return 0;
 
  close:
-       if (test_bit(XPT_TEMP, &rqstp->rq_xprt->xpt_flags))
-               svc_close_xprt(rqstp->rq_xprt);
+       if (test_bit(XPT_TEMP, &s_xprt->xpt_flags))
+               svc_close_xprt(s_xprt);
        dprintk("svc: svc_process close\n");
        return 0;


> It probably also requires us to store a pointer to struct net in the
> struct svc_rqst so that nfs4_callback_compound() and
> svcauth_gss_accept() can find it, but that should be OK since the
> transport already has that referenced.
> 
> Cheers,
>   Trond
>