possible deadlock in send_sigurg

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 7be97138 Merge tag 'xfs-5.7-merge-8' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14952b6de00000
kernel config: https://syzkaller.appspot.com/x/.config?x=d6a1e2f9a9986236
dashboard link: https://syzkaller.appspot.com/bug?extid=f675f964019f884dbd0f
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1643bf2fe00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13ef5733e00000

The bug was bisected to:

commit 7bc3e6e55acf065500a24621f3b313e7e5998acf
Author: Eric W. Biederman <ebie...@xmission.com>
Date: Thu Feb 20 00:22:26 2020 +0000

proc: Use a list of inodes to flush from proc

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=11aa9747e00000
final crash: https://syzkaller.appspot.com/x/report.txt?x=13aa9747e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=15aa9747e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f675f9...@syzkaller.appspotmail.com
Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")

========================================================
WARNING: possible irq lock inversion dependency detected
5.6.0-syzkaller #0 Not tainted
--------------------------------------------------------
swapper/1/0 just changed the state of lock:
ffffffff898090d8 (tasklist_lock){.+.?}-{2:2}, at: send_sigurg+0x9f/0x320 fs/fcntl.c:840
but this lock took another, SOFTIRQ-unsafe lock in the past:
(&pid->wait_pidfd){+.+.}-{2:2}


and interrupts could create inverse lock ordering between them.


other info that might help us debug this:
Possible interrupt unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&pid->wait_pidfd);
local_irq_disable();
lock(tasklist_lock);
lock(&pid->wait_pidfd);
<Interrupt>
lock(tasklist_lock);

*** DEADLOCK ***

4 locks held by swapper/1/0:
#0: ffffffff899bacc0 (rcu_read_lock){....}-{1:2}, at: __write_once_size include/linux/compiler.h:226 [inline]
#0: ffffffff899bacc0 (rcu_read_lock){....}-{1:2}, at: __skb_unlink include/linux/skbuff.h:2078 [inline]
#0: ffffffff899bacc0 (rcu_read_lock){....}-{1:2}, at: __skb_dequeue include/linux/skbuff.h:2093 [inline]
#0: ffffffff899bacc0 (rcu_read_lock){....}-{1:2}, at: process_backlog+0x1ad/0x7a0 net/core/dev.c:6131
#1: ffffffff899bacc0 (rcu_read_lock){....}-{1:2}, at: __skb_pull include/linux/skbuff.h:2309 [inline]
#1: ffffffff899bacc0 (rcu_read_lock){....}-{1:2}, at: ip_local_deliver_finish+0x124/0x360 net/ipv4/ip_input.c:228
#2: ffff888093e42de0 (slock-AF_INET/1){+.-.}-{2:2}, at: tcp_v4_rcv+0x2d09/0x39c0 net/ipv4/tcp_ipv4.c:1997
#3: ffff8880950c23b8 (&f->f_owner.lock){.+.?}-{2:2}, at: send_sigurg+0x1a/0x320 fs/fcntl.c:824

the shortest dependencies between 2nd lock and 1st lock:
-> (&pid->wait_pidfd){+.+.}-{2:2} {
HARDIRQ-ON-W at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
_raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
spin_lock include/linux/spinlock.h:353 [inline]
proc_pid_make_inode+0x1f9/0x3c0 fs/proc/base.c:1880
proc_pid_instantiate+0x51/0x150 fs/proc/base.c:3285
proc_pid_lookup+0x1da/0x340 fs/proc/base.c:3320
proc_root_lookup+0x20/0x60 fs/proc/root.c:243
__lookup_slow+0x256/0x490 fs/namei.c:1530
lookup_slow fs/namei.c:1547 [inline]
walk_component+0x418/0x6a0 fs/namei.c:1846
link_path_walk.part.0+0x4f1/0xb50 fs/namei.c:2166
link_path_walk fs/namei.c:2098 [inline]
path_openat+0x25a/0x27b0 fs/namei.c:3342
do_filp_open+0x203/0x260 fs/namei.c:3375
do_sys_openat2+0x585/0x770 fs/open.c:1148
do_sys_open+0xc3/0x140 fs/open.c:1164
do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
entry_SYSCALL_64_after_hwframe+0x49/0xb3
SOFTIRQ-ON-W at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
_raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
spin_lock include/linux/spinlock.h:353 [inline]
proc_pid_make_inode+0x1f9/0x3c0 fs/proc/base.c:1880
proc_pid_instantiate+0x51/0x150 fs/proc/base.c:3285
proc_pid_lookup+0x1da/0x340 fs/proc/base.c:3320
proc_root_lookup+0x20/0x60 fs/proc/root.c:243
__lookup_slow+0x256/0x490 fs/namei.c:1530
lookup_slow fs/namei.c:1547 [inline]
walk_component+0x418/0x6a0 fs/namei.c:1846
link_path_walk.part.0+0x4f1/0xb50 fs/namei.c:2166
link_path_walk fs/namei.c:2098 [inline]
path_openat+0x25a/0x27b0 fs/namei.c:3342
do_filp_open+0x203/0x260 fs/namei.c:3375
do_sys_openat2+0x585/0x770 fs/open.c:1148
do_sys_open+0xc3/0x140 fs/open.c:1164
do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
entry_SYSCALL_64_after_hwframe+0x49/0xb3
INITIAL USE at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x8c/0xbf kernel/locking/spinlock.c:159
__wake_up_common_lock+0xb4/0x130 kernel/sched/wait.c:122
do_notify_pidfd kernel/signal.c:1900 [inline]
do_notify_parent+0x19e/0xe60 kernel/signal.c:1927
exit_notify kernel/exit.c:660 [inline]
do_exit+0x238f/0x2dd0 kernel/exit.c:816
call_usermodehelper_exec_async+0x507/0x710 kernel/umh.c:125
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
}
... key at: [<ffffffff8bba4680>] __key.53714+0x0/0x40
... acquired at:
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x8c/0xbf kernel/locking/spinlock.c:159
__wake_up_common_lock+0xb4/0x130 kernel/sched/wait.c:122
do_notify_pidfd kernel/signal.c:1900 [inline]
do_notify_parent+0x19e/0xe60 kernel/signal.c:1927
exit_notify kernel/exit.c:660 [inline]
do_exit+0x238f/0x2dd0 kernel/exit.c:816
call_usermodehelper_exec_async+0x507/0x710 kernel/umh.c:125
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

-> (tasklist_lock){.+.?}-{2:2} {
HARDIRQ-ON-R at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_read_lock include/linux/rwlock_api_smp.h:149 [inline]
_raw_read_lock+0x2d/0x40 kernel/locking/spinlock.c:223
do_wait+0x3b9/0xa00 kernel/exit.c:1436
kernel_wait4+0x14c/0x260 kernel/exit.c:1611
call_usermodehelper_exec_sync kernel/umh.c:150 [inline]
call_usermodehelper_exec_work+0x172/0x260 kernel/umh.c:187
process_one_work+0x965/0x16a0 kernel/workqueue.c:2266
worker_thread+0x96/0xe20 kernel/workqueue.c:2412
kthread+0x388/0x470 kernel/kthread.c:268
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
IN-SOFTIRQ-R at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_read_lock include/linux/rwlock_api_smp.h:149 [inline]
_raw_read_lock+0x2d/0x40 kernel/locking/spinlock.c:223
send_sigurg+0x9f/0x320 fs/fcntl.c:840
sk_send_sigurg+0x76/0x300 net/core/sock.c:2855
tcp_check_urg net/ipv4/tcp_input.c:5353 [inline]
tcp_urg+0x38c/0xb80 net/ipv4/tcp_input.c:5394
tcp_rcv_established+0x8f3/0x1d90 net/ipv4/tcp_input.c:5724
tcp_v4_do_rcv+0x605/0x8b0 net/ipv4/tcp_ipv4.c:1621
tcp_v4_rcv+0x2f60/0x39c0 net/ipv4/tcp_ipv4.c:2003
ip_protocol_deliver_rcu+0x57/0x880 net/ipv4/ip_input.c:204
ip_local_deliver_finish+0x220/0x360 net/ipv4/ip_input.c:231
NF_HOOK include/linux/netfilter.h:307 [inline]
NF_HOOK include/linux/netfilter.h:301 [inline]
ip_local_deliver+0x1c8/0x4e0 net/ipv4/ip_input.c:252
dst_input include/net/dst.h:441 [inline]
ip_rcv_finish+0x1da/0x2f0 net/ipv4/ip_input.c:428
NF_HOOK include/linux/netfilter.h:307 [inline]
NF_HOOK include/linux/netfilter.h:301 [inline]
ip_rcv+0xd0/0x3c0 net/ipv4/ip_input.c:539
__netif_receive_skb_one_core+0xf5/0x160 net/core/dev.c:5187
__netif_receive_skb+0x27/0x1c0 net/core/dev.c:5301
process_backlog+0x21e/0x7a0 net/core/dev.c:6133
napi_poll net/core/dev.c:6571 [inline]
net_rx_action+0x4c2/0x1070 net/core/dev.c:6639
__do_softirq+0x26c/0x9f7 kernel/softirq.c:292
invoke_softirq kernel/softirq.c:373 [inline]
irq_exit+0x192/0x1d0 kernel/softirq.c:413
exiting_irq arch/x86/include/asm/apic.h:546 [inline]
smp_apic_timer_interrupt+0x19e/0x600 arch/x86/kernel/apic/apic.c:1140
apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60
arch_safe_halt arch/x86/include/asm/paravirt.h:144 [inline]
default_idle+0x49/0x350 arch/x86/kernel/process.c:697
cpuidle_idle_call kernel/sched/idle.c:154 [inline]
do_idle+0x393/0x690 kernel/sched/idle.c:269
cpu_startup_entry+0x14/0x20 kernel/sched/idle.c:361
start_secondary+0x2f3/0x400 arch/x86/kernel/smpboot.c:268
secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:242
SOFTIRQ-ON-R at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_read_lock include/linux/rwlock_api_smp.h:149 [inline]
_raw_read_lock+0x2d/0x40 kernel/locking/spinlock.c:223
do_wait+0x3b9/0xa00 kernel/exit.c:1436
kernel_wait4+0x14c/0x260 kernel/exit.c:1611
call_usermodehelper_exec_sync kernel/umh.c:150 [inline]
call_usermodehelper_exec_work+0x172/0x260 kernel/umh.c:187
process_one_work+0x965/0x16a0 kernel/workqueue.c:2266
worker_thread+0x96/0xe20 kernel/workqueue.c:2412
kthread+0x388/0x470 kernel/kthread.c:268
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
INITIAL USE at:
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_write_lock_irq include/linux/rwlock_api_smp.h:196 [inline]
_raw_write_lock_irq+0x5b/0x80 kernel/locking/spinlock.c:311
copy_process+0x3430/0x72c0 kernel/fork.c:2204
_do_fork+0x12d/0x1010 kernel/fork.c:2431
kernel_thread+0xb1/0xf0 kernel/fork.c:2518
rest_init+0x23/0x365 init/main.c:626
start_kernel+0x867/0x8a1 init/main.c:998
secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:242
}
... key at: [<ffffffff898090d8>] tasklist_lock+0x18/0x40
... acquired at:
mark_lock_irq kernel/locking/lockdep.c:3585 [inline]
mark_lock+0x624/0xf10 kernel/locking/lockdep.c:3935
mark_usage kernel/locking/lockdep.c:3826 [inline]
__lock_acquire+0x1ed9/0x4e00 kernel/locking/lockdep.c:4298
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_read_lock include/linux/rwlock_api_smp.h:149 [inline]
_raw_read_lock+0x2d/0x40 kernel/locking/spinlock.c:223
send_sigurg+0x9f/0x320 fs/fcntl.c:840
sk_send_sigurg+0x76/0x300 net/core/sock.c:2855
tcp_check_urg net/ipv4/tcp_input.c:5353 [inline]
tcp_urg+0x38c/0xb80 net/ipv4/tcp_input.c:5394
tcp_rcv_established+0x8f3/0x1d90 net/ipv4/tcp_input.c:5724
tcp_v4_do_rcv+0x605/0x8b0 net/ipv4/tcp_ipv4.c:1621
tcp_v4_rcv+0x2f60/0x39c0 net/ipv4/tcp_ipv4.c:2003
ip_protocol_deliver_rcu+0x57/0x880 net/ipv4/ip_input.c:204
ip_local_deliver_finish+0x220/0x360 net/ipv4/ip_input.c:231
NF_HOOK include/linux/netfilter.h:307 [inline]
NF_HOOK include/linux/netfilter.h:301 [inline]
ip_local_deliver+0x1c8/0x4e0 net/ipv4/ip_input.c:252
dst_input include/net/dst.h:441 [inline]
ip_rcv_finish+0x1da/0x2f0 net/ipv4/ip_input.c:428
NF_HOOK include/linux/netfilter.h:307 [inline]
NF_HOOK include/linux/netfilter.h:301 [inline]
ip_rcv+0xd0/0x3c0 net/ipv4/ip_input.c:539
__netif_receive_skb_one_core+0xf5/0x160 net/core/dev.c:5187
__netif_receive_skb+0x27/0x1c0 net/core/dev.c:5301
process_backlog+0x21e/0x7a0 net/core/dev.c:6133
napi_poll net/core/dev.c:6571 [inline]
net_rx_action+0x4c2/0x1070 net/core/dev.c:6639
__do_softirq+0x26c/0x9f7 kernel/softirq.c:292
invoke_softirq kernel/softirq.c:373 [inline]
irq_exit+0x192/0x1d0 kernel/softirq.c:413
exiting_irq arch/x86/include/asm/apic.h:546 [inline]
smp_apic_timer_interrupt+0x19e/0x600 arch/x86/kernel/apic/apic.c:1140
apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60
arch_safe_halt arch/x86/include/asm/paravirt.h:144 [inline]
default_idle+0x49/0x350 arch/x86/kernel/process.c:697
cpuidle_idle_call kernel/sched/idle.c:154 [inline]
do_idle+0x393/0x690 kernel/sched/idle.c:269
cpu_startup_entry+0x14/0x20 kernel/sched/idle.c:361
start_secondary+0x2f3/0x400 arch/x86/kernel/smpboot.c:268
secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:242


stack backtrace:
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-syzkaller #0
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+0x188/0x20d lib/dump_stack.c:118
print_irq_inversion_bug kernel/locking/lockdep.c:3448 [inline]
check_usage_forwards.cold+0x20/0x29 kernel/locking/lockdep.c:3472
mark_lock_irq kernel/locking/lockdep.c:3585 [inline]
mark_lock+0x624/0xf10 kernel/locking/lockdep.c:3935
mark_usage kernel/locking/lockdep.c:3826 [inline]
__lock_acquire+0x1ed9/0x4e00 kernel/locking/lockdep.c:4298
lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
__raw_read_lock include/linux/rwlock_api_smp.h:149 [inline]
_raw_read_lock+0x2d/0x40 kernel/locking/spinlock.c:223
send_sigurg+0x9f/0x320 fs/fcntl.c:840
sk_send_sigurg+0x76/0x300 net/core/sock.c:2855
tcp_check_urg net/ipv4/tcp_input.c:5353 [inline]
tcp_urg+0x38c/0xb80 net/ipv4/tcp_input.c:5394
tcp_rcv_established+0x8f3/0x1d90 net/ipv4/tcp_input.c:5724
tcp_v4_do_rcv+0x605/0x8b0 net/ipv4/tcp_ipv4.c:1621
tcp_v4_rcv+0x2f60/0x39c0 net/ipv4/tcp_ipv4.c:2003
ip_protocol_deliver_rcu+0x57/0x880 net/ipv4/ip_input.c:204
ip_local_deliver_finish+0x220/0x360 net/ipv4/ip_input.c:231
NF_HOOK include/linux/netfilter.h:307 [inline]
NF_HOOK include/linux/netfilter.h:301 [inline]
ip_local_deliver+0x1c8/0x4e0 net/ipv4/ip_input.c:252
dst_input include/net/dst.h:441 [inline]
ip_rcv_finish+0x1da/0x2f0 net/ipv4/ip_input.c:428
NF_HOOK include/linux/netfilter.h:307 [inline]
NF_HOOK include/linux/netfilter.h:301 [inline]
ip_rcv+0xd0/0x3c0 net/ipv4/ip_input.c:539
__netif_receive_skb_one_core+0xf5/0x160 net/core/dev.c:5187
__netif_receive_skb+0x27/0x1c0 net/core/dev.c:5301
process_backlog+0x21e/0x7a0 net/core/dev.c:6133
napi_poll net/core/dev.c:6571 [inline]
net_rx_action+0x4c2/0x1070 net/core/dev.c:6639
__do_softirq+0x26c/0x9f7 kernel/softirq.c:292
invoke_softirq kernel/softirq.c:373 [inline]
irq_exit+0x192/0x1d0 kernel/softirq.c:413
exiting_irq arch/x86/include/asm/apic.h:546 [inline]
smp_apic_timer_interrupt+0x19e/0x600 arch/x86/kernel/apic/apic.c:1140
apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
</IRQ>
RIP: 0010:native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:61
Code: cc cc cc cc cc cc cc cc cc cc cc cc e9 07 00 00 00 0f 00 2d 44 ae 5e 00 f4 c3 66 90 e9 07 00 00 00 0f 00 2d 34 ae 5e 00 fb f4 <c3> cc 41 56 41 55 41 54 55 53 e8 c3 07 97 f9 e8 9e 72 cb fb 0f 1f
RSP: 0018:ffffc90000d3fdb8 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff13
RAX: 1ffffffff13291af RBX: ffff8880a95f2340 RCX: 0000000000000000
RDX: dffffc0000000000 RSI: 0000000000000006 RDI: ffff8880a95f2c04
RBP: dffffc0000000000 R08: ffff8880a95f2340 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffed10152be468
R13: 0000000000000001 R14: ffffffff8a883540 R15: 0000000000000000
arch_safe_halt arch/x86/include/asm/paravirt.h:144 [inline]
default_idle+0x49/0x350 arch/x86/kernel/process.c:697
cpuidle_idle_call kernel/sched/idle.c:154 [inline]
do_idle+0x393/0x690 kernel/sched/idle.c:269
cpu_startup_entry+0x14/0x20 kernel/sched/idle.c:361
start_secondary+0x2f3/0x400 arch/x86/kernel/smpboot.c:268
secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:242


---
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syzbot can test patches for this bug, for details see:
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[Oleg Nesterov]

On 04/02, syzbot wrote:
>
> lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
> __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
> _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
> spin_lock include/linux/spinlock.h:353 [inline]
> proc_pid_make_inode+0x1f9/0x3c0 fs/proc/base.c:1880

Yes, spin_lock(wait_pidfd.lock) is not safe...

Eric, at first glance the fix is simple.

Oleg.


diff --git a/fs/proc/base.c b/fs/proc/base.c
index 74f948a6b621..9ec8c114aa60 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1839,9 +1839,9 @@ void proc_pid_evict_inode(struct proc_inode *ei)
struct pid *pid = ei->pid;

if (S_ISDIR(ei->vfs_inode.i_mode)) {
- spin_lock(&pid->wait_pidfd.lock);
+ spin_lock_irq(&pid->wait_pidfd.lock);
hlist_del_init_rcu(&ei->sibling_inodes);
- spin_unlock(&pid->wait_pidfd.lock);
+ spin_unlock_irq(&pid->wait_pidfd.lock);
}

put_pid(pid);
@@ -1877,9 +1877,9 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
/* Let the pid remember us for quick removal */
ei->pid = pid;
if (S_ISDIR(mode)) {
- spin_lock(&pid->wait_pidfd.lock);
+ spin_lock_irq(&pid->wait_pidfd.lock);
hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
- spin_unlock(&pid->wait_pidfd.lock);
+ spin_unlock_irq(&pid->wait_pidfd.lock);
}

task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 1e730ea1dcd6..6b7ee76e1b36 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -123,9 +123,9 @@ void proc_invalidate_siblings_dcache(struct hlist_head *inodes, spinlock_t *lock
if (!node)
break;
ei = hlist_entry(node, struct proc_inode, sibling_inodes);
- spin_lock(lock);
+ spin_lock_irq(lock);
hlist_del_init_rcu(&ei->sibling_inodes);
- spin_unlock(lock);
+ spin_unlock_irq(lock);

inode = &ei->vfs_inode;
sb = inode->i_sb;

[Christian Brauner]

On Fri, Apr 03, 2020 at 11:11:35AM +0200, Oleg Nesterov wrote:
> On 04/02, syzbot wrote:
> >
> > lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
> > __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
> > _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
> > spin_lock include/linux/spinlock.h:353 [inline]
> > proc_pid_make_inode+0x1f9/0x3c0 fs/proc/base.c:1880
>
> Yes, spin_lock(wait_pidfd.lock) is not safe...
>
> Eric, at first glance the fix is simple.
>
> Oleg.
>
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c

Um, when did this lock get added to proc/base.c in the first place and
why has it been abused for this?
People just recently complained loudly about this in the
cred_guard_mutex thread that abusing locks for things they weren't
intended for is a bad idea...

[Hillf Danton]

On Thu, 02 Apr 2020 23:15:12 -0700

Add a new lock in bid to avoid abusing waitqueue lock.

--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -63,6 +63,7 @@ struct pid
/* lists of tasks that use this pid */
struct hlist_head tasks[PIDTYPE_MAX];
struct hlist_head inodes;
+ spinlock_t inodes_lock;
/* wait queue for pidfd notifications */
wait_queue_head_t wait_pidfd;
struct rcu_head rcu;
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -261,6 +261,7 @@ struct pid *alloc_pid(struct pid_namespa

init_waitqueue_head(&pid->wait_pidfd);
INIT_HLIST_HEAD(&pid->inodes);
+ spin_lock_init(&pid->inodes_lock);

upid = pid->numbers + ns->level;
spin_lock_irq(&pidmap_lock);
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1839,9 +1839,11 @@ void proc_pid_evict_inode(struct proc_in

struct pid *pid = ei->pid;

if (S_ISDIR(ei->vfs_inode.i_mode)) {
- spin_lock(&pid->wait_pidfd.lock);

+ unsigned long flags;
+
+ spin_lock_irqsave(&pid->inodes_lock, flags);

hlist_del_init_rcu(&ei->sibling_inodes);
- spin_unlock(&pid->wait_pidfd.lock);

+ spin_unlock_irqrestore(&pid->inodes_lock, flags);
}

put_pid(pid);
@@ -1877,9 +1879,11 @@ struct inode *proc_pid_make_inode(struct

/* Let the pid remember us for quick removal */
ei->pid = pid;
if (S_ISDIR(mode)) {
- spin_lock(&pid->wait_pidfd.lock);

+ unsigned long flags;
+
+ spin_lock_irqsave(&pid->inodes_lock, flags);

hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
- spin_unlock(&pid->wait_pidfd.lock);

+ spin_unlock_irqrestore(&pid->inodes_lock, flags);

}

task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);

@@ -3273,7 +3277,7 @@ static const struct inode_operations pro

void proc_flush_pid(struct pid *pid)
{
- proc_invalidate_siblings_dcache(&pid->inodes, &pid->wait_pidfd.lock);
+ proc_invalidate_siblings_dcache(&pid->inodes, &pid->inodes_lock);
put_pid(pid);
}

--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -118,14 +118,15 @@ void proc_invalidate_siblings_dcache(str

rcu_read_lock();
for (;;) {
+ unsigned long flags;
struct super_block *sb;
node = hlist_first_rcu(inodes);

if (!node)
break;
ei = hlist_entry(node, struct proc_inode, sibling_inodes);
- spin_lock(lock);

+ spin_lock_irqsave(lock, flags);
hlist_del_init_rcu(&ei->sibling_inodes);
- spin_unlock(lock);
+ spin_unlock_irqrestore(lock, flags);

[Eric W. Biederman]

Christian Brauner <christia...@ubuntu.com> writes:

> On Fri, Apr 03, 2020 at 11:11:35AM +0200, Oleg Nesterov wrote:
>> On 04/02, syzbot wrote:
>> >
>> > lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
>> > __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>> > _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
>> > spin_lock include/linux/spinlock.h:353 [inline]
>> > proc_pid_make_inode+0x1f9/0x3c0 fs/proc/base.c:1880
>>
>> Yes, spin_lock(wait_pidfd.lock) is not safe...
>>
>> Eric, at first glance the fix is simple.
>>
>> Oleg.
>>
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>
> Um, when did this lock get added to proc/base.c in the first place and
> why has it been abused for this?

Because struct pid is too bloated already.

> People just recently complained loudly about this in the
> cred_guard_mutex thread that abusing locks for things they weren't
> intended for is a bad idea...

The problem there is/was holding locks over places they shouldn't.
It looks like I made an equally dump mistake with struct pid.

That said can you take a look at calling putting do_notify_pidfd
someplace sane. I can't see how it makes sense to call that in
the same set of circumstances where we notify the parent.

Reparenting should not be a concern, nor should ptracing. Which I think
means that do_notify_pid can potentially get called many times more
than it needs to be.

Not to mention it is being called a bit too soon when called from
do_notify_parent. Which I saw earlier is causing problems. Signal
sending can call do_notify_parent early because everything just queues
up and no action is taken. Wake-ups on the other hand trigger more
immediate action.

There is no connection to the current bug except this discussion
just remimded me about do_notify_pidfd and I figured I should say
something before I forget again.

Eric

[Eric W. Biederman]

syzbot wrote:
> ========================================================
> WARNING: possible irq lock inversion dependency detected
> 5.6.0-syzkaller #0 Not tainted
> --------------------------------------------------------
> swapper/1/0 just changed the state of lock:
> ffffffff898090d8 (tasklist_lock){.+.?}-{2:2}, at: send_sigurg+0x9f/0x320 fs/fcntl.c:840
> but this lock took another, SOFTIRQ-unsafe lock in the past:
> (&pid->wait_pidfd){+.+.}-{2:2}
>
>
> and interrupts could create inverse lock ordering between them.
>
>
> other info that might help us debug this:
> Possible interrupt unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&pid->wait_pidfd);
> local_irq_disable();
> lock(tasklist_lock);
> lock(&pid->wait_pidfd);
> <Interrupt>
> lock(tasklist_lock);
>
> *** DEADLOCK ***
>
> 4 locks held by swapper/1/0:

The problem is that because wait_pidfd.lock is taken under the tasklist
lock. It must always be taken with irqs disabled as tasklist_lock can be
taken from interrupt context and if wait_pidfd.lock was already taken this
would create a lock order inversion.

Oleg suggested just disabling irqs where I have added extra calls to
wait_pidfd.lock. That should be safe and I think the code will eventually
do that. It was rightly pointed out by Christian that sharing the
wait_pidfd.lock was a premature optimization.

It is also true that my pre-merge window testing was insufficient. So
remove the premature optimization and give struct pid a dedicated lock of
it's own for struct pid things. I have verified that lockdep sees all 3
paths where we take the new pid->lock and lockdep does not complain.

It is my current day dream that one day pid->lock can be used to guard the
task lists as well and then the tasklist_lock won't need to be held to
deliver signals. That will require taking pid->lock with irqs disabled.

Link: https://lore.kernel.org/lkml/00000000000011...@google.com/
Cc: Oleg Nesterov <ol...@redhat.com>
Cc: Christian Brauner <christia...@ubuntu.com>
Reported-by: syzbot+343f75...@syzkaller.appspotmail.com
Reported-by: syzbot+832aab...@syzkaller.appspotmail.com
Reported-by: syzbot+f675f9...@syzkaller.appspotmail.com
Reported-by: syzbot+a9fb14...@syzkaller.appspotmail.com

Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")

Signed-off-by: "Eric W. Biederman" <ebie...@xmission.com>
---

If anyone sees an issue please holer otherwise I plan on sending
this fix to Linus.

fs/proc/base.c | 10 +++++-----
include/linux/pid.h | 1 +
kernel/pid.c | 1 +
3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 74f948a6b621..6042b646ab27 100644

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1839,9 +1839,9 @@ void proc_pid_evict_inode(struct proc_inode *ei)
struct pid *pid = ei->pid;

if (S_ISDIR(ei->vfs_inode.i_mode)) {
- spin_lock(&pid->wait_pidfd.lock);

+ spin_lock(&pid->lock);

hlist_del_init_rcu(&ei->sibling_inodes);
- spin_unlock(&pid->wait_pidfd.lock);

+ spin_unlock(&pid->lock);

}

put_pid(pid);
@@ -1877,9 +1877,9 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
/* Let the pid remember us for quick removal */
ei->pid = pid;
if (S_ISDIR(mode)) {
- spin_lock(&pid->wait_pidfd.lock);

+ spin_lock(&pid->lock);

hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
- spin_unlock(&pid->wait_pidfd.lock);

+ spin_unlock(&pid->lock);

}

task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);

@@ -3273,7 +3273,7 @@ static const struct inode_operations proc_tgid_base_inode_operations = {

void proc_flush_pid(struct pid *pid)
{
- proc_invalidate_siblings_dcache(&pid->inodes, &pid->wait_pidfd.lock);

+ proc_invalidate_siblings_dcache(&pid->inodes, &pid->lock);
put_pid(pid);
}

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 01a0d4e28506..cc896f0fc4e3 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -60,6 +60,7 @@ struct pid
{
refcount_t count;
unsigned int level;
+ spinlock_t lock;

/* lists of tasks that use this pid */
struct hlist_head tasks[PIDTYPE_MAX];
struct hlist_head inodes;

diff --git a/kernel/pid.c b/kernel/pid.c
index efd34874b3d1..517d0855d4cf 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -246,6 +246,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,

get_pid_ns(ns);
refcount_set(&pid->count, 1);
+ spin_lock_init(&pid->lock);
for (type = 0; type < PIDTYPE_MAX; ++type)
INIT_HLIST_HEAD(&pid->tasks[type]);

--
2.20.1

Eric

[Christian Brauner]

Thanks, Eric.
Acked-by: Christian Brauner <christia...@ubuntu.com>

Christian