NFSD: Protect against send buffer overflow in NFSv3 READ

Since before the git era, NFSD has conserved the number of pages
held by each nfsd thread by combining the RPC receive and send
buffers into a single array of pages. This works because there are
no cases where an operation needs a large RPC Call message and a
large RPC Reply at the same time.

Once an RPC Call has been received, svc_process() updates
svc_rqst::rq_res to describe the part of rq_pages that can be
used for constructing the Reply. This means that the send buffer
(rq_res) shrinks when the received RPC record containing the RPC
Call is large.

A client can force this shrinkage on TCP by sending a correctly-
formed RPC Call header contained in an RPC record that is
excessively large. The full maximum payload size cannot be
constructed in that case.

Cc: <stable@vger.kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
index 7a159785499a6d8060ea5f60a79ef03fa206bddc..5b1e771238b352f0724249f360431b6352f597c3 100644 (file)
--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -150,7 +150,6 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
 {
        struct nfsd3_readargs *argp = rqstp->rq_argp;
        struct nfsd3_readres *resp = rqstp->rq_resp;
-       u32 max_blocksize = svc_max_payload(rqstp);
        unsigned int len;
        int v;
 
@@ -159,7 +158,8 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
                                (unsigned long) argp->count,
                                (unsigned long long) argp->offset);
 
-       argp->count = min_t(u32, argp->count, max_blocksize);
+       argp->count = min_t(u32, argp->count, svc_max_payload(rqstp));
+       argp->count = min_t(u32, argp->count, rqstp->rq_res.buflen);
        if (argp->offset > (u64)OFFSET_MAX)
                argp->offset = (u64)OFFSET_MAX;
        if (argp->offset + argp->count > (u64)OFFSET_MAX)