net/rds: Fix UBSAN: array-index-out-of-bounds in rds_cmsg_recv

Syzcaller UBSAN crash occurs in rds_cmsg_recv(),
which reads inc->i_rx_lat_trace[j + 1] with index 4 (3 + 1),
but with array size of 4 (RDS_RX_MAX_TRACES).
Here 'j' is assigned from rs->rs_rx_trace[i] and in-turn from
trace.rx_trace_pos[i] in rds_recv_track_latency(),
with both arrays sized 3 (RDS_MSG_RX_DGRAM_TRACE_MAX). So fix the
off-by-one bounds check in rds_recv_track_latency() to prevent
a potential crash in rds_cmsg_recv().

Found by syzcaller:
=================================================================
UBSAN: array-index-out-of-bounds in net/rds/recv.c:585:39
index 4 is out of range for type 'u64 [4]'
CPU: 1 PID: 8058 Comm: syz-executor228 Not tainted 6.6.0-gd2f51b3516da #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.15.0-1 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x136/0x150 lib/dump_stack.c:106
 ubsan_epilogue lib/ubsan.c:217 [inline]
 __ubsan_handle_out_of_bounds+0xd5/0x130 lib/ubsan.c:348
 rds_cmsg_recv+0x60d/0x700 net/rds/recv.c:585
 rds_recvmsg+0x3fb/0x1610 net/rds/recv.c:716
 sock_recvmsg_nosec net/socket.c:1044 [inline]
 sock_recvmsg+0xe2/0x160 net/socket.c:1066
 __sys_recvfrom+0x1b6/0x2f0 net/socket.c:2246
 __do_sys_recvfrom net/socket.c:2264 [inline]
 __se_sys_recvfrom net/socket.c:2260 [inline]
 __x64_sys_recvfrom+0xe0/0x1b0 net/socket.c:2260
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x40/0x110 arch/x86/entry/common.c:82
 entry_SYSCALL_64_after_hwframe+0x63/0x6b
==================================================================

Fixes: 3289025 ("RDS: add receive message trace used by application")
Reported-by: Chenyuan Yang <chenyuan0y@gmail.com>
Closes: https://lore.kernel.org/linux-rdma/CALGdzuoVdq-wtQ4Az9iottBqC5cv9ZhcE5q8N7LfYFvkRsOVcw@mail.gmail.com/
Signed-off-by: Sharath Srinivasan <sharath.srinivasan@oracle.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/af_rds.c

@@ -419,7 +419,7 @@ static int rds_recv_track_latency(struct rds_sock *rs, sockptr_t optval,
 
 	rs->rs_rx_traces = trace.rx_traces;
 	for (i = 0; i < rs->rs_rx_traces; i++) {
-		if (trace.rx_trace_pos[i] > RDS_MSG_RX_DGRAM_TRACE_MAX) {
+		if (trace.rx_trace_pos[i] >= RDS_MSG_RX_DGRAM_TRACE_MAX) {
 			rs->rs_rx_traces = 0;
 			return -EFAULT;
 		}