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From:   Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
To:     Heiner Kallweit <hkallweit1@gmail.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     nic_swsd@realtek.com, "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>,
        Marco Elver <elver@google.com>
Subject: [PATCH RFC v1 2/3] r8169: fix the KCSAN reported data race in rtl_rx while reading desc->opts1
Date:   Wed, 27 Sep 2023 20:41:59 +0200
Message-Id: <20230927184158.243575-2-mirsad.todorovac@alu.unizg.hr>
In-Reply-To: <20230927184158.243575-1-mirsad.todorovac@alu.unizg.hr>
References: <20230927184158.243575-1-mirsad.todorovac@alu.unizg.hr>
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KCSAN reported the following data-race bug:

==================================================================
BUG: KCSAN: data-race in rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4430 drivers/net/ethernet/realtek/r8169_main.c:4583) r8169

race at unknown origin, with read to 0xffff888117e43510 of 4 bytes by interrupt on cpu 21:
rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4430 drivers/net/ethernet/realtek/r8169_main.c:4583) r8169
__napi_poll (net/core/dev.c:6527)
net_rx_action (net/core/dev.c:6596 net/core/dev.c:6727)
__do_softirq (kernel/softirq.c:553)
__irq_exit_rcu (kernel/softirq.c:427 kernel/softirq.c:632)
irq_exit_rcu (kernel/softirq.c:647)
sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1074 (discriminator 14))
asm_sysvec_apic_timer_interrupt (./arch/x86/include/asm/idtentry.h:645)
cpuidle_enter_state (drivers/cpuidle/cpuidle.c:291)
cpuidle_enter (drivers/cpuidle/cpuidle.c:390)
call_cpuidle (kernel/sched/idle.c:135)
do_idle (kernel/sched/idle.c:219 kernel/sched/idle.c:282)
cpu_startup_entry (kernel/sched/idle.c:378 (discriminator 1))
start_secondary (arch/x86/kernel/smpboot.c:210 arch/x86/kernel/smpboot.c:294)
secondary_startup_64_no_verify (arch/x86/kernel/head_64.S:433)

value changed: 0x80003fff -> 0x3402805f

Reported by Kernel Concurrency Sanitizer on:
CPU: 21 PID: 0 Comm: swapper/21 Tainted: G             L     6.6.0-rc2-kcsan-00143-gb5cbe7c00aa0 #41
Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
==================================================================

drivers/net/ethernet/realtek/r8169_main.c:
==========================================
   4429
 → 4430                 status = le32_to_cpu(desc->opts1);
   4431                 if (status & DescOwn)
   4432                         break;
   4433
   4434                 /* This barrier is needed to keep us from reading
   4435                  * any other fields out of the Rx descriptor until
   4436                  * we know the status of DescOwn
   4437                  */
   4438                 dma_rmb();
   4439
   4440                 if (unlikely(status & RxRES)) {
   4441                         if (net_ratelimit())
   4442                                 netdev_warn(dev, "Rx ERROR. status = %08x\n",

Marco Elver explained that dma_rmb() doesn't prevent the compiler to tear up the access to
desc->opts1 which can be written to concurrently. READ_ONCE() should prevent that from
happening:

   4429
 → 4430                 status = le32_to_cpu(READ_ONCE(desc->opts1));
   4431                 if (status & DescOwn)
   4432                         break;
   4433

As the consequence of this fix, this KCSAN warning was eliminated.

Fixes: 6202806e7c03a ("r8169: drop member opts1_mask from struct rtl8169_private")
Suggested-by: Marco Elver <elver@google.com>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: nic_swsd@realtek.com
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/lkml/dc7fc8fa-4ea4-e9a9-30a6-7c83e6b53188@alu.unizg.hr/
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>

Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
---
v1:
 the initial patch proposal. fixes the KCSAN warning.

 drivers/net/ethernet/realtek/r8169_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index 281aaa851847..81be6085a480 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -4427,7 +4427,7 @@ static int rtl_rx(struct net_device *dev, struct rtl8169_private *tp, int budget
 		dma_addr_t addr;
 		u32 status;
 
-		status = le32_to_cpu(desc->opts1);
+		status = le32_to_cpu(READ_ONCE(desc->opts1));
 		if (status & DescOwn)
 			break;
 
-- 
2.34.1