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* [syzbot] [sctp?] UBSAN: shift-out-of-bounds in sctp_transport_update_rto
@ 2025-07-21 19:06 syzbot
  0 siblings, 0 replies; 4+ messages in thread
From: syzbot @ 2025-07-21 19:06 UTC (permalink / raw)
  To: davem, edumazet, horms, kuba, linux-kernel, linux-sctp,
	lucien.xin, marcelo.leitner, netdev, pabeni, syzkaller-bugs

Hello,

syzbot found the following issue on:

HEAD commit:    6832a9317eee Merge tag 'net-6.16-rc7' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=137d9d8c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=693e2f5eea496864
dashboard link: https://syzkaller.appspot.com/bug?extid=2e455dd90ca648e48cea
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6a84ef6250f7/disk-6832a931.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/454efb0534f6/vmlinux-6832a931.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7a5841eb7e63/bzImage-6832a931.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2e455dd90ca648e48cea@syzkaller.appspotmail.com

------------[ cut here ]------------
UBSAN: shift-out-of-bounds in net/sctp/transport.c:509:41
shift exponent 237 is too large for 32-bit type 'unsigned int'
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.16.0-rc6-syzkaller-00121-g6832a9317eee #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x16c/0x1f0 lib/dump_stack.c:120
 ubsan_epilogue lib/ubsan.c:233 [inline]
 __ubsan_handle_shift_out_of_bounds+0x27f/0x420 lib/ubsan.c:494
 sctp_transport_update_rto.cold+0x1c/0x34b net/sctp/transport.c:509
 sctp_check_transmitted+0x11c4/0x1c30 net/sctp/outqueue.c:1502
 sctp_outq_sack+0x4ef/0x1b20 net/sctp/outqueue.c:1338
 sctp_cmd_process_sack net/sctp/sm_sideeffect.c:840 [inline]
 sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1372 [inline]
 sctp_side_effects net/sctp/sm_sideeffect.c:1204 [inline]
 sctp_do_sm+0x36df/0x5c80 net/sctp/sm_sideeffect.c:1175
 sctp_assoc_bh_rcv+0x392/0x6f0 net/sctp/associola.c:1034
 sctp_inq_push+0x1db/0x270 net/sctp/inqueue.c:88
 sctp_rcv+0x14d8/0x3c60 net/sctp/input.c:243
 ip_protocol_deliver_rcu+0x447/0x4c0 net/ipv4/ip_input.c:205
 ip_local_deliver_finish+0x316/0x570 net/ipv4/ip_input.c:233
 NF_HOOK include/linux/netfilter.h:317 [inline]
 NF_HOOK include/linux/netfilter.h:311 [inline]
 ip_local_deliver+0x18e/0x1f0 net/ipv4/ip_input.c:254
 dst_input include/net/dst.h:469 [inline]
 ip_rcv_finish net/ipv4/ip_input.c:448 [inline]
 NF_HOOK include/linux/netfilter.h:317 [inline]
 NF_HOOK include/linux/netfilter.h:311 [inline]
 ip_rcv+0x2c3/0x5d0 net/ipv4/ip_input.c:568
 __netif_receive_skb_one_core+0x197/0x1e0 net/core/dev.c:5977
 __netif_receive_skb+0x1d/0x160 net/core/dev.c:6090
 process_backlog+0x442/0x15e0 net/core/dev.c:6442
 __napi_poll.constprop.0+0xb7/0x550 net/core/dev.c:7414
 napi_poll net/core/dev.c:7478 [inline]
 net_rx_action+0xa9f/0xfe0 net/core/dev.c:7605
 handle_softirqs+0x219/0x8e0 kernel/softirq.c:579
 __do_softirq kernel/softirq.c:613 [inline]
 invoke_softirq kernel/softirq.c:453 [inline]
 __irq_exit_rcu+0x109/0x170 kernel/softirq.c:680
 irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
 sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1050
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702
RIP: 0010:pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:82
Code: 0b 6f 02 c3 cc cc cc cc 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d d3 77 25 00 fb f4 <e9> 8c fb 02 00 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 90 90 90
RSP: 0018:ffffffff8e207e08 EFLAGS: 000002c6
RAX: 0000000000448811 RBX: 0000000000000000 RCX: ffffffff8b849c69
RDX: 0000000000000000 RSI: ffffffff8de2d10c RDI: ffffffff8c157960
RBP: fffffbfff1c52ef0 R08: 0000000000000001 R09: ffffed1017086645
R10: ffff8880b843322b R11: 0000000000000001 R12: 0000000000000000
R13: ffffffff8e297780 R14: ffffffff90a9a650 R15: 0000000000000000
 arch_safe_halt arch/x86/include/asm/paravirt.h:107 [inline]
 default_idle+0x13/0x20 arch/x86/kernel/process.c:749
 default_idle_call+0x6d/0xb0 kernel/sched/idle.c:117
 cpuidle_idle_call kernel/sched/idle.c:185 [inline]
 do_idle+0x391/0x510 kernel/sched/idle.c:325
 cpu_startup_entry+0x4f/0x60 kernel/sched/idle.c:423
 rest_init+0x16b/0x2b0 init/main.c:745
 start_kernel+0x3ee/0x4d0 init/main.c:1102
 x86_64_start_reservations+0x18/0x30 arch/x86/kernel/head64.c:307
 x86_64_start_kernel+0x130/0x190 arch/x86/kernel/head64.c:288
 common_startup_64+0x13e/0x148
 </TASK>
---[ end trace ]---
----------------
Code disassembly (best guess):
   0:	0b 6f 02             	or     0x2(%rdi),%ebp
   3:	c3                   	ret
   4:	cc                   	int3
   5:	cc                   	int3
   6:	cc                   	int3
   7:	cc                   	int3
   8:	0f 1f 00             	nopl   (%rax)
   b:	90                   	nop
   c:	90                   	nop
   d:	90                   	nop
   e:	90                   	nop
   f:	90                   	nop
  10:	90                   	nop
  11:	90                   	nop
  12:	90                   	nop
  13:	90                   	nop
  14:	90                   	nop
  15:	90                   	nop
  16:	90                   	nop
  17:	90                   	nop
  18:	90                   	nop
  19:	90                   	nop
  1a:	90                   	nop
  1b:	f3 0f 1e fa          	endbr64
  1f:	66 90                	xchg   %ax,%ax
  21:	0f 00 2d d3 77 25 00 	verw   0x2577d3(%rip)        # 0x2577fb
  28:	fb                   	sti
  29:	f4                   	hlt
* 2a:	e9 8c fb 02 00       	jmp    0x2fbbb <-- trapping instruction
  2f:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
  36:	00 00 00
  39:	66 90                	xchg   %ax,%ax
  3b:	90                   	nop
  3c:	90                   	nop
  3d:	90                   	nop
  3e:	90                   	nop
  3f:	90                   	nop


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^ permalink raw reply	[flat|nested] 4+ messages in thread
* [syzbot] [sctp?] UBSAN: shift-out-of-bounds in sctp_transport_update_rto
@ 2025-07-22 19:46 Moon Hee Lee
  2025-07-22 19:47 ` syzbot
  0 siblings, 1 reply; 4+ messages in thread
From: Moon Hee Lee @ 2025-07-22 19:46 UTC (permalink / raw)
  To: syzbot+2e455dd90ca648e48cea
  Cc: syzkaller-bugs, linux-kernel, linux-sctp, netdev

[-- Attachment #1: Type: text/plain, Size: 76 bytes --]

#syz test git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main

[-- Attachment #2: 0001-sctp-guard-rto_alpha-and-rto_beta-against-unsafe-shi.patch --]
[-- Type: text/x-patch, Size: 2517 bytes --]

From 7c472b40b901a12cfc8dc00ca0a99ac2d992c338 Mon Sep 17 00:00:00 2001
From: Moon Hee Lee <moonhee.lee.ca@gmail.com>
Date: Tue, 22 Jul 2025 12:31:16 -0700
Subject: [PATCH net] sctp: guard rto_alpha and rto_beta against unsafe shift
 values

rto_alpha and rto_beta are used as shift amounts in the RTT smoothing
calculation, where they represent inverse powers of two (e.g. 3 means 1/8).

Currently, the code uses net->sctp.rto_alpha and rto_beta directly in shift
expressions without validating them. If user-controlled or corrupted values
exceed valid shift bounds for 32-bit integers (e.g. 237), this leads to
undefined behavior and runtime faults.

syzbot reported such a case via UBSAN:

  UBSAN: shift-out-of-bounds in net/sctp/transport.c:509:41
  shift exponent 237 is too large for 32-bit type 'unsigned int'

This patch ensures both values are within the safe shift range [0, 31].
If not, the code falls back to the default constants SCTP_RTO_ALPHA and
SCTP_RTO_BETA to ensure correctness and system stability.

This preserves SCTP tunability while preventing undefined behavior.

Signed-off-by: Moon Hee Lee <moonhee.lee.ca@gmail.com>
---
 net/sctp/transport.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 6946c1462793..848311bb7a9f 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -495,6 +495,8 @@ void sctp_transport_update_rto(struct sctp_transport *tp, __u32 rtt)
 
 	if (tp->rttvar || tp->srtt) {
 		struct net *net = tp->asoc->base.net;
+		int rto_alpha = net->sctp.rto_alpha;
+		int rto_beta = net->sctp.rto_beta;
 		/* 6.3.1 C3) When a new RTT measurement R' is made, set
 		 * RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'|
 		 * SRTT <- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'
@@ -505,7 +507,16 @@ void sctp_transport_update_rto(struct sctp_transport *tp, __u32 rtt)
 		 * of two.
 		 * For example, assuming the default value of RTO.Alpha of
 		 * 1/8, rto_alpha would be expressed as 3.
+		 *
+		 * Guard rto_alpha and rto_beta to ensure they are within
+		 * valid shift bounds [0, 31] to avoid undefined behavior.
 		 */
+		if (unlikely(rto_alpha < 0 || rto_alpha >= 32))
+			rto_alpha = SCTP_RTO_ALPHA;
+
+		if (unlikely(rto_beta < 0 || rto_beta >= 32))
+			rto_beta = SCTP_RTO_BETA;
+
 		tp->rttvar = tp->rttvar - (tp->rttvar >> net->sctp.rto_beta)
 			+ (((__u32)abs((__s64)tp->srtt - (__s64)rtt)) >> net->sctp.rto_beta);
 		tp->srtt = tp->srtt - (tp->srtt >> net->sctp.rto_alpha)
-- 
2.43.0


^ permalink raw reply related	[flat|nested] 4+ messages in thread
[parent not found: <87ldlz2ps2.fsf@gmail.com>]
end of thread, other threads:[~2025-09-28  3:56 UTC | newest]

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