netdev - Re: [PATCH net-next] net: Limit logical shift left of TCP probe0 timeout

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CANn89i+=QRibXsm6tw-eVwhB3wjOtgF-MtVu0G--K5=oWP97+A@mail.gmail.com>
Date:   Tue, 15 Dec 2020 12:06:57 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Cambda Zhu <cambda@...ux.alibaba.com>,
        netdev <netdev@...r.kernel.org>,
        Dust Li <dust.li@...ux.alibaba.com>,
        Tony Lu <tonylu@...ux.alibaba.com>
Subject: Re: [PATCH net-next] net: Limit logical shift left of TCP probe0 timeout

On Tue, Dec 15, 2020 at 3:08 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sun, 13 Dec 2020 21:59:45 +0800 Cambda Zhu wrote:
> > > On Dec 13, 2020, at 06:32, Jakub Kicinski <kuba@...nel.org> wrote:
> > > On Tue,  8 Dec 2020 17:19:10 +0800 Cambda Zhu wrote:
> > >> For each TCP zero window probe, the icsk_backoff is increased by one and
> > >> its max value is tcp_retries2. If tcp_retries2 is greater than 63, the
> > >> probe0 timeout shift may exceed its max bits. On x86_64/ARMv8/MIPS, the
> > >> shift count would be masked to range 0 to 63. And on ARMv7 the result is
> > >> zero. If the shift count is masked, only several probes will be sent
> > >> with timeout shorter than TCP_RTO_MAX. But if the timeout is zero, it
> > >> needs tcp_retries2 times probes to end this false timeout. Besides,
> > >> bitwise shift greater than or equal to the width is an undefined
> > >> behavior.
> > >
> > > If icsk_backoff can reach 64, can it not also reach 256 and wrap?
> >
> > If tcp_retries2 is set greater than 255, it can be wrapped. But for TCP probe0,
> > it seems to be not a serious problem. The timeout will be icsk_rto and backoff
> > again. And considering icsk_backoff is 8 bits, not only it may always be lesser
> > than tcp_retries2, but also may always be lesser than tcp_orphan_retries. And
> > the icsk_probes_out is 8 bits too. So if the max_probes is greater than 255,
> > the connection won’t abort even if it’s an orphan sock in some cases.
> >
> > We can change the type of icsk_backoff/icsk_probes_out to fix these problems.
> > But I think maybe the retries greater than 255 have no sense indeed and the RFC
> > only requires the timeout(R2) greater than 100s at least. Could it be better to
> > limit the min/max ranges of their sysctls?
>
> All right, I think the patch is good as is, applied for 5.11, thank you!

It looks like we can remove the (u64) casts then.

Also if we _really_ care about icsk_backoff approaching 63, we also
need to change inet_csk_rto_backoff() ?

Was your patch based on a real world use, or some fuzzer UBSAN report ?

diff --git a/include/net/inet_connection_sock.h
b/include/net/inet_connection_sock.h
index 7338b3865a2a3d278dc27c0167bba1b966bbda9f..a2a145e3b062c0230935c293fc1900df095937d4
100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -242,9 +242,10 @@ static inline unsigned long
 inet_csk_rto_backoff(const struct inet_connection_sock *icsk,
                     unsigned long max_when)
 {
-        u64 when = (u64)icsk->icsk_rto << icsk->icsk_backoff;
+       u8 backoff = min_t(u8, 32U, icsk->icsk_backoff);
+       u64 when = (u64)icsk->icsk_rto << backoff;

-        return (unsigned long)min_t(u64, when, max_when);
+       return (unsigned long)min_t(u64, when, max_when);
 }

 struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern);
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 78d13c88720fda50e3f1880ac741cea1985ef3e9..fc6e4d40fd94a717d24ebd8aef7f7930a4551fe9
100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1328,9 +1328,8 @@ static inline unsigned long
tcp_probe0_when(const struct sock *sk,
 {
        u8 backoff = min_t(u8, ilog2(TCP_RTO_MAX / TCP_RTO_MIN) + 1,
                           inet_csk(sk)->icsk_backoff);
-       u64 when = (u64)tcp_probe0_base(sk) << backoff;

-       return (unsigned long)min_t(u64, when, max_when);
+       return min(tcp_probe0_base(sk) << backoff, max_when);
 }

 static inline void tcp_check_probe_timer(struct sock *sk)