Message-ID: <CANn89iKyLdvSF11aHvg-Ytr+HbnHb4QXMie2N5GpZhxSHx-XtA@mail.gmail.com>
Date: Mon, 18 Sep 2023 12:39:22 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Mirsad Todorovac <mirsad.todorovac@....unizg.hr>, Marco Elver <elver@...gle.com>
Cc: nic_swsd@...ltek.com, Heiner Kallweit <hkallweit1@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: BUG: KCSAN: data-race in rtl8169_poll

On Mon, Sep 18, 2023 at 11:43 AM Mirsad Todorovac
<mirsad.todorovac@....unizg.hr> wrote:
>
> On 9/18/23 09:41, Eric Dumazet wrote:
> > On Mon, Sep 18, 2023 at 8:15 AM Mirsad Todorovac
> > <mirsad.todorovac@....unizg.hr> 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;
>

This is also working, but in this case we already have barriers (
dma_rmb() here)
to synchronize host side and hardware (when flipping DescOwn) bit.

tools/memory-model/Documentation/access-marking.txt is saying that
data_race() would be ok.

It also says that in theory, if we want to both restrict compiler
optimizations and disable
KCSAN diagnostics, we would need data_race(READ_ONCE(a))

Let us ask Marco Elver what would be the most sensical annotation(s) here.

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