linux-kernel - Re: [PATCH net] net: ena: fix shift-out-of-bounds in exponential backoff

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Date:   Thu, 13 Jul 2023 10:46:55 +0300
From:   Shay Agroskin <shayagr@...zon.com>
To:     Krister Johansen <kjlx@...pleofstupid.com>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Arthur Kiyanovski" <akiyano@...zon.com>,
        David Arinzon <darinzon@...zon.com>,
        "Noam Dagan" <ndagan@...zon.com>,
        Saeed Bishara <saeedb@...zon.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        "Jakub Kicinski" <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] net: ena: fix shift-out-of-bounds in exponential backoff


Krister Johansen <kjlx@...pleofstupid.com> writes:

>
> On Tue, Jul 11, 2023 at 08:47:32PM +0300, Shay Agroskin wrote:
>>
>> Krister Johansen <kjlx@...pleofstupid.com> writes:
>>
>> > diff --git a/drivers/net/ethernet/amazon/ena/ena_com.c
>> > b/drivers/net/ethernet/amazon/ena/ena_com.c
>> > index 451c3a1b6255..633b321d7fdd 100644
>> > --- a/drivers/net/ethernet/amazon/ena/ena_com.c
>> > +++ b/drivers/net/ethernet/amazon/ena/ena_com.c
>> > @@ -35,6 +35,8 @@
>> >  #define ENA_REGS_ADMIN_INTR_MASK 1
>> > +#define ENA_MAX_BACKOFF_DELAY_EXP 16U
>> > +
>> >  #define ENA_MIN_ADMIN_POLL_US 100
>> >  #define ENA_MAX_ADMIN_POLL_US 5000
>> > @@ -536,6 +538,7 @@ static int 
>> > ena_com_comp_status_to_errno(struct
>> > ena_com_admin_queue *admin_queue,
>> >    static void ena_delay_exponential_backoff_us(u32 exp, u32 
>> >    delay_us)
>> >  {
>> > +   exp = min_t(u32, exp, ENA_MAX_BACKOFF_DELAY_EXP);
>> >     delay_us = max_t(u32, ENA_MIN_ADMIN_POLL_US, delay_us);
>> >     delay_us = min_t(u32, delay_us * (1U << exp), 
>> >     ENA_MAX_ADMIN_POLL_US);
>> >     usleep_range(delay_us, 2 * delay_us);
>>
>> Hi, thanks for submitting this patch (:
>
> Absolutely; thanks for the review!
>
>> Going over the logic here, the driver sleeps for `delay_us` 
>> micro-seconds in
>> each iteration that this function gets called.
>>
>> For an exp = 14 it'd sleep (I added units notation)
>> delay_us * (2 ^ exp) us = 100 * (2 ^ 14) us = (10 * (2 ^ 14)) / 
>> (1000000) s
>> = 1.6 s
>>
>> For an exp = 15 it'd sleep
>> (10 * (2 ^ 15)) / (1000000) = 3.2s
>>
>> To even get close to an overflow value, say exp=29 the driver 
>> would sleep in
>> a single iteration
>> 53687 s = 14.9 hours.
>>
>> The driver should stop trying to get a response from the device 
>> after a
>> timeout period received from the device which is 3 seconds by 
>> default.
>>
>> The point being, it seems very unlikely to hit this 
>> overflow. Did you
>> experience it or was the issue discovered by a static analyzer 
>> ?
>
> No, no use of fuzzing or static analysis.  This was hit on a 
> production
> instance that was having ENA trouble.
>
> I'm apparently reading the code differently.  I thought this 
> line:
>
>> >     delay_us = min_t(u32, delay_us * (1U << exp), 
>> >     ENA_MAX_ADMIN_POLL_US);
>
> Was going to cap that delay_us at (delay_us * (1U << exp)) or
> 5000us, whichever is smaller.  By that measure, if delay_us is 
> 100 and
> ENA_MAX_ADMIN_POLL_US is 5000, this should start getting capped 
> after
> exp = 6, correct?  By my estimate, that puts it at between 160ms 
> and
> 320ms of sleeping before one could hit this problem.
>
> I went and pulled the logs out of the archive and have the 
> following
> timeline.  This is seconds from boot as reported by dmesg:
>
>    11244.226583 - ena warns TX not completed on time, 10112000 
>    usecs since
>     last napi execution, missing tx timeout val of 5000 msec
>
>    11245.190453 - netdev watchdog fires
>
>    11245.190781 - ena records Transmit timeout
>    11245.250739 - ena records Trigger reset on
>
>    11246.812620 - UBSAN message to console
>
>    11248.590441 - ena reports Reset inidication didn't turn off
>    11250.633545 - ena reports failure to reset device
>    12013.529338 - last logline before new boot
>
> While the difference between the panic and the trigger reset is 
> more
> than 320ms, it is definitely on the order of seconds instead of 
> hours.
>

Yup you're right. I was so entangled in my exponent calculations 
that I completely missed the min_t expression there.

That's quite an awkward design to be honest, I hope to submit a 
re-write for it in one of the releases.

Thanks again for the work you've put into it