On (06/25/18 16:19), syzbot wrote:

Hello,

syzbot found the following crash on:

HEAD commit:    77072ca59fdd Merge tag 'for-linus-20180623' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=169c7c04400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=befbcd7305e41bb0
dashboard link: https://syzkaller.appspot.com/bug?extid=d29d18215e477cfbfbdd
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1585147f800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d29d18215e477cfbfbdd@syzkaller.appspotmail.com

llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
llcp: nfc_llcp_send_ui_frame: Could not allocate PDU

Hi,

Thanks for the report.

I'll Cc networking people on this.

I've a strong feeling that we saw it before. The kernel is
CONFIG_PREEMPT_VOLUNTARY=y, llcp_sock_sendmsg() stuck in a error
reporting loop:

	do {
	...
		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
					 frag_len + LLCP_HEADER_SIZE, &err);
		if (pdu == NULL) {
			pr_err("Could not allocate PDU\n");
			continue;
		}
	...
	} while (remaining_len > 0);

[ 1004.674843] llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
[ 1004.681035] llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
...
[ 1098.508526] llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
[ 1098.514698] llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
[ 1098.520844] INFO: rcu_sched self-detected stall on CPU

94 seconds worth of heavy printing, no preemption:

INFO: rcu_sched self-detected stall on CPU
	1-....: (20918 ticks this GP) idle=55a/1/4611686018427387906
softirq=11347/11347 fqs=20240
	 (t=125005 jiffies g=5572 c=5571 q=149)
NMI backtrace for cpu 1
CPU: 1 PID: 4811 Comm: syz-executor0 Not tainted 4.18.0-rc1+ #115
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
 nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
 nmi_trigger_cpumask_backtrace+0x151/0x192 lib/nmi_backtrace.c:62
 arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
 trigger_single_cpu_backtrace include/linux/nmi.h:156 [inline]
 rcu_dump_cpu_stacks+0x175/0x1c2 kernel/rcu/tree.c:1336
 print_cpu_stall kernel/rcu/tree.c:1485 [inline]
 check_cpu_stall.isra.60.cold.78+0x36c/0x5a6 kernel/rcu/tree.c:1553
 __rcu_pending kernel/rcu/tree.c:3244 [inline]
 rcu_pending kernel/rcu/tree.c:3291 [inline]
 rcu_check_callbacks+0x23f/0xcd0 kernel/rcu/tree.c:2646
 update_process_times+0x2d/0x70 kernel/time/timer.c:1636
 tick_sched_handle+0x9f/0x180 kernel/time/tick-sched.c:164
 tick_sched_timer+0x45/0x130 kernel/time/tick-sched.c:1274
 __run_hrtimer kernel/time/hrtimer.c:1398 [inline]
 __hrtimer_run_queues+0x3eb/0x10c0 kernel/time/hrtimer.c:1460
 hrtimer_interrupt+0x2f3/0x750 kernel/time/hrtimer.c:1518
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1025 [inline]
 smp_apic_timer_interrupt+0x165/0x730 arch/x86/kernel/apic/apic.c:1050
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:863
 </IRQ>
RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783
[inline]
RIP: 0010:console_unlock+0xc84/0x10b0 kernel/printk/printk.c:2397
Code: c1 e8 03 42 80 3c 38 00 0f 85 bd 03 00 00 48 83 3d 38 f7 8e 07 00 0f
84 69 02 00 00 e8 45 56 19 00 48 8b bd b0 fe ff ff 57 9d <0f> 1f 44 00 00 e9
96 f5 ff ff e8 2d 56 19 00 48 8b 7d 08 e8 94 cf
RSP: 0018:ffff8801aab0f358 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
RAX: ffff8801aa2802c0 RBX: 0000000000000200 RCX: 1ffff10035450163
RDX: 0000000000000000 RSI: ffffffff8162b8fb RDI: 0000000000000293
RBP: ffff8801aab0f4c0 R08: ffff8801aa280af8 R09: 0000000000000006
R10: ffff8801aa2802c0 R11: 0000000000000000 R12: 0000000000000000
R13: ffffffff84ea9880 R14: 0000000000000001 R15: dffffc0000000000
 vprintk_emit+0x6c6/0xdf0 kernel/printk/printk.c:1907
 vprintk_default+0x28/0x30 kernel/printk/printk.c:1948
 vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:382
 printk+0xa7/0xcf kernel/printk/printk.c:1981
 nfc_llcp_send_ui_frame.cold.9+0x18/0x1f net/nfc/llcp_commands.c:758
 llcp_sock_sendmsg+0x278/0x350 net/nfc/llcp_sock.c:786
 sock_sendmsg_nosec net/socket.c:645 [inline]
 sock_sendmsg+0xd5/0x120 net/socket.c:655
 ___sys_sendmsg+0x51d/0x930 net/socket.c:2161
 __sys_sendmmsg+0x240/0x6f0 net/socket.c:2256
 __do_sys_sendmmsg net/socket.c:2285 [inline]
 __se_sys_sendmmsg net/socket.c:2282 [inline]
 __x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2282
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

So we can try switching to ratelimited error reporting
[that would be option A]:

---

diff --git a/net/nfc/llcp_commands.c b/net/nfc/llcp_commands.c
index 2ceefa183cee..2f3becb709b8 100644
--- a/net/nfc/llcp_commands.c
+++ b/net/nfc/llcp_commands.c

@@ -755,7 +755,7 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
 					 frag_len + LLCP_HEADER_SIZE, &err);
 		if (pdu == NULL) {
-			pr_err("Could not allocate PDU\n");
+			pr_err_ratelimited("Could not allocate PDU\n");
 			continue;
 		}
 
---

Or ratelimited error reporting and cond_resched()
[that would be option B]:

---

diff --git a/net/nfc/llcp_commands.c b/net/nfc/llcp_commands.c
index 2ceefa183cee..61741db4c4e6 100644
--- a/net/nfc/llcp_commands.c
+++ b/net/nfc/llcp_commands.c

@@ -755,7 +755,8 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
 					 frag_len + LLCP_HEADER_SIZE, &err);
 		if (pdu == NULL) {
-			pr_err("Could not allocate PDU\n");
+			pr_err_ratelimited("Could not allocate PDU\n");
+			cond_resched();
 			continue;
 		}
 
---

	-ss