Re: [PATCH] rpc: avoid crash when system time jump back

[Ján Tomko <jtomko@xxxxxxxxxx>]

On a Monday in 2021, BiaoxiangYe wrote:
> From: BiaoXiang Ye <yebiaoxiang@xxxxxxxxxx>
>
> Setting the system time backward would lead to a
> multiplication overflow in function virKeepAliveStart.
> FunctionvirKeepAliveTimerInternal got the same bug too.
>
> Backtrace below:
> #0  0x0000ffffae898470 in raise () from /usr/lib64/libc.so.6
> #1  0x0000ffffae89981c in abort () from /usr/lib64/libc.so.6
> #2  0x0000ffffaf9a36a8 in __mulvsi3 () from /usr/lib64/libvirt.so.0
> #3  0x0000ffffaf8fd9e8 in virKeepAliveStart (ka=0xaaaaf954ce10, interval=interval@entry=0,
>     count=count@entry=0) at ../../src/rpc/virkeepalive.c:283
> #4  0x0000ffffaf908560 in virNetServerClientStartKeepAlive (client=0xaaaaf954cbe0)
>     at ../../src/rpc/virnetserverclient.c:1628
> #5  0x0000aaaac57eb6dc in remoteDispatchConnectSupportsFeature (server=0xaaaaf95309d0,
>     msg=0xaaaaf9549d90, ret=0xffff8c007fc0, args=0xffff8c002e70, rerr=0xffff9ea054a0,
>     client=0xaaaaf954cbe0) at ../../src/remote/remote_daemon_dispatch.c:5063
> #6  remoteDispatchConnectSupportsFeatureHelper (server=0xaaaaf95309d0, client=0xaaaaf954cbe0,
>     msg=0xaaaaf9549d90, rerr=0xffff9ea054a0, args=0xffff8c002e70, ret=0xffff8c007fc0)
>     at ./remote/remote_daemon_dispatch_stubs.h:3503
> #7  0x0000ffffaf9053a4 in virNetServerProgramDispatchCall(msg=0xaaaaf9549d90, client=0xaaaaf954cbe0,
>     server=0x0, prog=0xaaaaf953a170) at ../../src/rpc/virnetserverprogram.c:451
> #8  virNetServerProgramDispatch (prog=0xaaaaf953a170, server=0x0, server@entry=0xaaaaf95309d0,
>     client=0xaaaaf954cbe0, msg=0xaaaaf9549d90) at ../../src/rpc/virnetserverprogram.c:306
> #9  0x0000ffffaf90a6bc in virNetServerProcessMsg (msg=<optimized out>, prog=<optimized out>,
>     client=<optimized out>, srv=0xaaaaf95309d0) at ../../src/rpc/virnetserver.c:137
> #10 virNetServerHandleJob (jobOpaque=0xaaaaf950df80, opaque=0xaaaaf95309d0)
>     at ../../src/rpc/virnetserver.c:154
> #11 0x0000ffffaf812e14 in virThreadPoolWorker (opaque=<optimized out>)
>     at ../../src/util/virthreadpool.c:163
> #12 0x0000ffffaf81237c in virThreadHelper (data=<optimized out>) at ../../src/util/virthread.c:246
> #13 0x0000ffffaea327ac in ?? () from /usr/lib64/libpthread.so.0
> #14 0x0000ffffae93747c in ?? () from /usr/lib64/libc.so.6
> (gdb) frame 3
> #3  0x0000ffffaf8fd9e8 in virKeepAliveStart (ka=0xaaaaf954ce10, interval=interval@entry=0,
>     count=count@entry=0) at ../../src/rpc/virkeepalive.c:283
> 283	        timeout = ka->interval - delay;
> (gdb) list
> 278    now = time(NULL);
> 279    delay = now - ka->lastPacketReceived; <='delay' got a negative value
> 280    if (delay > ka->interval)
> 281        timeout = 0;
> 282    else
> 283        timeout = ka->interval - delay;
> 284    ka->intervalStart = now - (ka->interval - timeout);
> 285    ka->timer = virEventAddTimeout(timeout * 1000, virKeepAliveTimer, <= multiplication overflow
> 286                                   ka, virObjectFreeCallback);
> 287    if (ka->timer < 0)
> (gdb) p now
> $2 = 18288001
> (gdb) p ka->lastPacketReceived
> $3 = 1609430405
>
> Signed-off-by: BiaoXiang Ye <yebiaoxiang@xxxxxxxxxx>
> ---
> src/rpc/virkeepalive.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/src/rpc/virkeepalive.c b/src/rpc/virkeepalive.c
> index 860b91b6b1..e662484ea6 100644
> --- a/src/rpc/virkeepalive.c
> +++ b/src/rpc/virkeepalive.c
> @@ -119,12 +119,25 @@ virKeepAliveTimerInternal(virKeepAlivePtr ka,
>     if (ka->interval <= 0 || ka->intervalStart == 0)
>         return false;
>
> +    if (now < ka->intervalStart) {
> +        VIR_WARN("system time jump back detected. now=%ld,"
> +                 " intervalStart=%ld",
> +                 now, ka->intervalStart);
> +        ka->intervalStart = now;
> +    }
> +

Instead of detecting a jump backwards, we could use a monotonic timer
that does not go backwards.

g_get_monotonic_time () from GLib should do that
https://developer.gnome.org/glib/stable/glib-Date-and-Time-Functions.html#g-get-monotonic-time

(Although they say the timer might not tick while the machine is
suspended - not sure whether we need that here or not)

Jano

>     if (now - ka->intervalStart < ka->interval) {
>         timeval = ka->interval - (now - ka->intervalStart);
>         virEventUpdateTimeout(ka->timer, timeval * 1000);
>         return false;
>     }
>
> +    if (now < ka->lastPacketReceived) {
> +        VIR_WARN("system time jump back detected. now=%ld,"
> +                 " lastPacketReceived=%ld",
> +                 now, ka->lastPacketReceived);
> +        ka->lastPacketReceived = now;
> +    }
>     timeval = now - ka->lastPacketReceived;
>     PROBE(RPC_KEEPALIVE_TIMEOUT,
>           "ka=%p client=%p countToDeath=%d idle=%d",

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