[PATCH net 2/2] tcp: fix shift-out-of-bounds in inet_csk_rto_backoff()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

icsk_backoff is bumped on every RTO and zero window probe, and the only
thing bounding it is tcp_retries2 (a u8 sysctl, 0..255).
inet_csk_rto_backoff() then uses it as a shift amount:

	u64 when = (u64)icsk->icsk_rto << icsk->icsk_backoff;

Set tcp_retries2 to 64 or more and keep the peer silent, icsk_backoff
climbs past 63 and the shift is UB. For example an ICMP host unreachable
arriving on such a socket goes through tcp_ld_RTO_revert():

  UBSAN: shift-out-of-bounds in ./include/net/inet_connection_sock.h:258:40
  shift exponent 73 is too large for 64-bit type 'long long unsigned int'
  Call Trace:
   <IRQ>
   dump_stack_lvl+0xc1/0xf0
   dump_stack+0x10/0x20
   __ubsan_handle_shift_out_of_bounds+0x1d1/0x390
   tcp_ld_RTO_revert.cold+0x16/0x92
   tcp_v4_err+0xdb3/0x13e0
   icmp_socket_deliver+0x1a7/0x390
   icmp_unreach+0x2dd/0xe50
   icmp_rcv+0x93e/0xe30
   ip_protocol_deliver_rcu+0x36b/0x3e0
   ip_local_deliver_finish+0x37d/0x620
   ip_local_deliver+0x165/0x1d0
   ip_rcv+0x284/0x320
   __netif_receive_skb_one_core+0x16b/0x1c0
   __netif_receive_skb+0x21/0x150
   process_backlog+0x37e/0x1200
   __napi_poll.constprop.0+0xa5/0x5e0
   net_rx_action+0x4c0/0xf70
   handle_softirqs+0x204/0x9b0
   __irq_exit_rcu+0x13c/0x220
   irq_exit_rcu+0xe/0x20
   sysvec_apic_timer_interrupt+0x92/0xd0
   </IRQ>

The orphan check in tcp_probe_timer() calls the same helper.

tcp_probe0_when() got a similar clamp in 2020, but this helper was left
as is. Cap the shift at 32 like it was suggested back then: icsk_rto is
at least 1 jiffy, so 1 << 32 jiffies is already far above any max_when,
and the min_t() result is unchanged. Unlike an ilog2(RTO_MAX/RTO_MIN)
bound, 32 still holds when tcp_rto_min_us is lowered.

Fixes: fcdd1cf4dd63 ("tcp: avoid possible arithmetic overflows")
Cc: stable@vger.kernel.org
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 include/net/inet_connection_sock.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 903499581db7..562e12033a5d 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -255,9 +255,10 @@ 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, struct proto_accept_arg *arg);
-- 
2.43.0