Re: BUG: KCSAN: data-race in rtl8169_poll

[Mirsad Todorovac]

On 9/18/23 09:41, Eric Dumazet wrote:
> On Mon, Sep 18, 2023 at 8:15 AM Mirsad Todorovac
> <mirsad.todorovac@xxxxxxxxxxxx> wrote:
> > Hi all,
> >
> > In the vanilla torvalds tree kernel on Ubuntu 22.04, commit 6.6.0-rc1-kcsan-00269-ge789286468a9,
> > KCSAN discovered a data-race in rtl8169_poll():
> >
> > [ 9591.740976] ==================================================================
> > [ 9591.740990] BUG: KCSAN: data-race in rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4430 drivers/net/ethernet/realtek/r8169_main.c:4583) r8169
> >
> > [ 9591.741060] race at unknown origin, with read to 0xffff888109773130 of 4 bytes by interrupt on cpu 21:
> > [ 9591.741073] rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4430 drivers/net/ethernet/realtek/r8169_main.c:4583) r8169
> > [ 9591.741135] __napi_poll (net/core/dev.c:6527)
> > [ 9591.741149] net_rx_action (net/core/dev.c:6596 net/core/dev.c:6727)
> > [ 9591.741161] __do_softirq (kernel/softirq.c:553)
> > [ 9591.741175] __irq_exit_rcu (kernel/softirq.c:427 kernel/softirq.c:632)
> > [ 9591.741185] irq_exit_rcu (kernel/softirq.c:647)
> > [ 9591.741194] common_interrupt (arch/x86/kernel/irq.c:247 (discriminator 14))
> > [ 9591.741206] asm_common_interrupt (./arch/x86/include/asm/idtentry.h:636)
> > [ 9591.741217] cpuidle_enter_state (drivers/cpuidle/cpuidle.c:291)
> > [ 9591.741227] cpuidle_enter (drivers/cpuidle/cpuidle.c:390)
> > [ 9591.741237] call_cpuidle (kernel/sched/idle.c:135)
> > [ 9591.741249] do_idle (kernel/sched/idle.c:219 kernel/sched/idle.c:282)
> > [ 9591.741259] cpu_startup_entry (kernel/sched/idle.c:378 (discriminator 1))
> > [ 9591.741268] start_secondary (arch/x86/kernel/smpboot.c:210 arch/x86/kernel/smpboot.c:294)
> > [ 9591.741281] secondary_startup_64_no_verify (arch/x86/kernel/head_64.S:433)
> >
> > [ 9591.741300] value changed: 0x80003fff -> 0x34044510
> >
> > [ 9591.741314] Reported by Kernel Concurrency Sanitizer on:
> > [ 9591.741322] CPU: 21 PID: 0 Comm: swapper/21 Tainted: G             L     6.6.0-rc1-kcsan-00269-ge789286468a9-dirty #4
> > [ 9591.741334] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
> > [ 9591.741343] ==================================================================
> >
> > (The taint is not from the proprietary module, but triggered from the previous reported and unfixed bug.)
> >
> > Apparently, it is this code:
> >
> > static int rtl8169_poll(struct napi_struct *napi, int budget)
> > {
> >          struct rtl8169_private *tp = container_of(napi, struct rtl8169_private, napi);
> >          struct net_device *dev = tp->dev;
> >          int work_done;
> >
> >          rtl_tx(dev, tp, budget);
> >
> > →       work_done = rtl_rx(dev, tp, budget);
> >
> >          if (work_done < budget && napi_complete_done(napi, work_done))
> >                  rtl_irq_enable(tp);
> >
> >          return work_done;
> > }
> >
> > and
> >
> > static int rtl_rx(struct net_device *dev, struct rtl8169_private *tp, int budget)
> > {
> >          struct device *d = tp_to_dev(tp);
> >          int count;
> >
> >          for (count = 0; count < budget; count++, tp->cur_rx++) {
> >                  unsigned int pkt_size, entry = tp->cur_rx % NUM_RX_DESC;
> >                  struct RxDesc *desc = tp->RxDescArray + entry;
> >                  struct sk_buff *skb;
> >                  const void *rx_buf;
> >                  dma_addr_t addr;
> >                  u32 status;
> >
> > →               status = le32_to_cpu(desc->opts1);
> >                  if (status & DescOwn)
> >                          break;
> >
> >                  /* This barrier is needed to keep us from reading
> >                   * any other fields out of the Rx descriptor until
> >                   * we know the status of DescOwn
> >                   */
> >                  dma_rmb();
> >
> >                  if (unlikely(status & RxRES)) {
> > .
> > .
> > .
> >
> > The reason isn't obvious, so it might be interesting if this is a valid report and whether it caused spurious corruption
> > of the network data on Realtek 8169 compatible cards ...
>
> I think this is pretty much expected.
>
> Driver reads a piece of memory that the hardware can modify.
>
> Adding data_race() annotations could avoid these false positives.
>
> > Hope this helps.
> >
> > Best regards,
> > Mirsad Todorovac

Well, another approach was this quick fix that eliminated all those rtl8169_poll() KCSAN warnings.

If READ_ONCE(desc->opts1) fixed it, then maybe there is more to this than meets the eye?

-------------------------------------------------
 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 6351a2dc13bc..051551ee2a15 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;