[PATCH 6.18.y 2/2] svcrdma: Reject Read lists that exceed the page budget

[Sasha Levin <sashal@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

[ Upstream commit 0ca487abb3bdf581851664b5db21f364caf57682 ]

Individual Read segment lengths are validated at decode time, but
nothing prevents a requester from sending multiple segments whose
cumulative length exceeds the rq_pages array budget. When one
segment fills the page array exactly, the runtime guard in
svc_rdma_build_read_segment() is bypassed because len reaches zero.
A subsequent segment then accesses the NULL sentinel slot at
rq_pages[rq_maxpages], resulting in a NULL pointer dereference during
DMA mapping.

Accumulate pages across all Read segments and reject the message at
decode time when the total would overflow the page budget.

Fixes: 026d958b38c6 ("svcrdma: Add recvfrom helpers to svc_rdma_rw.c")
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index 30157636288eb..1075bfd9fed61 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -430,11 +430,14 @@ static void svc_rdma_build_arg_xdr(struct svc_rqst *rqstp,
  *	    to the first byte past the Read list. rc_read_pcl and
  *	    rc_call_pcl cl_count fields are set to the number of
  *	    Read segments in the list.
- *  %false: Read list is corrupt. @rctxt's xdr_stream is left in an
- *	    unknown state.
+ *  %false: Read list is corrupt or exceeds the page budget. @rctxt's
+ *	    xdr_stream is left in an unknown state.
  */
 static bool xdr_count_read_segments(struct svc_rdma_recv_ctxt *rctxt, __be32 *p)
 {
+	unsigned int maxlen = rctxt->rc_maxpages << PAGE_SHIFT;
+	unsigned int total_len = 0;
+
 	rctxt->rc_call_pcl.cl_count = 0;
 	rctxt->rc_read_pcl.cl_count = 0;
 	while (xdr_item_is_present(p)) {
@@ -448,7 +451,10 @@ static bool xdr_count_read_segments(struct svc_rdma_recv_ctxt *rctxt, __be32 *p)
 
 		xdr_decode_read_segment(p, &position, &handle,
 					    &length, &offset);
-		if (length > rctxt->rc_maxpages << PAGE_SHIFT)
+		if (length > maxlen)
+			return false;
+		total_len += length;
+		if (PAGE_ALIGN(total_len) > maxlen)
 			return false;
 		if (position) {
 			if (position & 3)
-- 
2.53.0